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APFIM and AEMinvestigation ofCF8 and CF8Mpril11arycoolantpipe steels

M K MillerJ Bentley

Atom probe field ion microscopy analytical electron microscopy and opticalmicroscopy have been used to investigate the changes that occur in themicrostructure of cast CF8 and CF8M primary coolant pipe steels after longterm thermal aging These cast steels have a duplex microstructure consisting ofaustenite with approximately 15 vol - ferrite In material aged at 300 or 400degCfor up to 70 000 h the ferrite had spinodally decomposed into a modulated finescale interconnected network consisting of an iron rich a phase and a chromiumenriched a phase with a periodicity of between 2 and 9 nm Roughly sphericalG phase precipitates 2 to 10 nm in diameter were also observed at concentrationsof more than 1021 m-3bull The degradation in mechanical properties of thesematerials is a consequence of the spinodal decomposition and G phaseprecipitation in the ferrite MST 1184

copy 1990 The Institute of Metals The authors are in the Metals and CeramicsDivision Oak Ridge National Laboratory Oak Ridge TN USA

Introduction

The long term mechanical integrity of pipes used to carrythe primary cooling water in pressurised water nuclear reac-tors is of the utmost importance for safe operation Thecoolant pipes are designed for a service life of 40 yearsHowever it is well known that the mechanical properties ofthe cast stainless steel pipes that are used for this applic-ation are degraded by aging at temperatures in the range300-400degC The pipes and other related components arefabricated from stainless steel to reduce corrosion and asso-ciated problems at the service temperature of 300degC Thesecoolant pipes are up to 1 m in diameter and 100 mm thickand because of their large size are made by welding castsections The type of cast stainless steel that is used has aduplex microstructure of austenite with approximately 15-20 vol- ferrite The ferrite is a necessary component ofthese steels since it improves the properties of the pipes byincreasing the yield strength of the cast material and byreducing the susceptibility to hot cracking during solidifica-tion However long term thermal aging produces anincrease in hardness and tensile properties together with adecrease in the impact properties ductility and toughnessAlthough an in-service failure that is controlled by theimpact properties is considered unlikely these materialscan suffer a dramatic loss in impact properties decreasingto almost 15 of the initial value after prolonged aging 1Some microstructural characterisations of several heats ofCF8 and other similar steels have been presentedI-3 Inthese materials the ferrite decomposes into an iron rich aphase and a chromium enriched a phase In addition acomplex nickel silicide known as G phase is observed in theferrite These fine scale phase transformations are consid-ered to be responsible for the degradation in mechaniCalproperties In come cases M23C6carbide formation occursat the ferriteaustenite interface

In this paper a review is presented of a combined atomprobe field ion microscopy (APFIM) and analyticalelectron microscopy (AEM) study that was performed to

Table 1 Nominal bulk composition of CF8 and CF8M steelswtmiddot

C Si Mn S Cr Mo Ni N Fe

CF80middot038 1middot0 0middot28 0middot019 20middot2 0middot13 8middot3 0middot027 Sal

CF8M0middot04 0middot81 0middot79 0middot021 20middot8 2middot5 10middot6 0middot042 Sal

characterise and compare the microstructures of cast CF8and CF8M stainless steels and to determine the changesthat occur during long term low temperature thermalaging4-10These types of steel have also been studied usingthe atom probe by Godfrey and co-workersll-13

Experimental

The nominal compositions of the CF8 (heat 278 from GeorgFischer Co Switzerland) and the CF8M alloys used in thisinvestigation are given in Table 1 The major differencebetween these two materials is that the CF8M alloy hashigher molybdenum and nickel levels than the CF8 steelThe CF8M steel was examined in the as cast unaged con-dition and also after aging for 7500 h at 400degC whereas theCFS steel was examined after laboratory aging cast materialfor 70 000 hat 300350 and 400degC It should be noted thatthe 400degC aging temperature is approximately 100 K higherthan the normal service temperature and was used toaccelerate the microstructural changes that may occur dur-ing service14 Since these pipes are external to the reactorthey are not exposed to any significant levels of radiationthat may influence the aging behaviour

Atom probe analyses were conducted primarily on theORNL energy compensated atom probe 15although initialexperiments on the CF8M steel were performed on thestraight time of flight atom probe at the University ofOxford Details of these instruments and the types of ana-lysis that may be performed are described elsewhere 1617Field ion micrographs (FIM) were recorded using neon asthe image gas and a specimen temperature between 70 and90 K

Atom probe composition profiles were analysed using avariety of statistical techniques including chi-squared fre-quency distribution tests17 autocorrelation functions17Johnson and Klotz Markov chain analysis 1718Hetherington and Miller mean separation method 1719andthe sample distribution analysis methodll-13 developed bythe atom probe group at the University of Oxford For thefrequency distribution tests the autocorrelation analysisand the sample distribution analysis the atom probe datawere divided in blocks each containing 50 ions and theircomposition determined The standard chi-squared test ofthe frequency distribution examines whether the observed

Oak Ridge National Laboratory

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286 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

a

b1 Duplex microstructure in CF8 steel aged for 70000 h at

a 300degC and b 400degC reversion of ferrite and carbideprecipitation is evident in material aged at 400degC (TEM)

distribution of these composition blocks differs from thebinomial distribution associated with a random solid solu-tion The autocorrelation function examines pairs of com-position blocks a fixed distance or lag apart and sums theresults over the entire composition profile If the compos-itions of these blocks deviate from the mean composition inthe same sense then a positive correlation is determinedThe autocorrelation of adjacent composition blocks in theprofilel provides a technique to detect phase separation orclustering The sample distribution analysis attempts toquantify the extent of phase separation by calculating thecompositional range 2Pa of the observed distributionll-13

Both the Johnson and Klotz and the Hetherington andMiller methods examine the atom-by-atom data chain inorder to detect phase separation or measure the extent ofclustering The Johnson and Klotz method determines anordering parameter ()from the number of AA AB and BBpairs of atoms in the data chain and compares the resultwith that expected from a random solid solutionI718 TheHetherington and Miller mean separation method com-pares the variance of the distances between similar typeatoms in the data chain with that expected from a randomdistribution 17 19

Materials Science and Technology March 1990 Vol 6

a

ba weak beam TEM of M23C6 particle located at originalferriteaustenite interface b field ion micrograph (FIM) ofbrightly imaging M23C6 carbide and ferrite (arrows indicateinterface between M23C6 particle and ferrite)

2 Carbide precipitates in CF8 steel aged for 70 000 h at 400degC

Analyses using AEM were performed on PhilipsEM400T PEG and EM430T analytical electron micro-scopes both equipped with EDAX 910070 energy disper-sive X-ray spectrometers (EDS) and Gatan 607 electronenergy loss spectrometer (EELS) systems The elementalcompositions were obtained from EDS data by standard-less procedures It should be noted that the chromiumcontents are probably 1-2 overestimated because offluo-rescence effects and these compositions are averages of themicrostructure including any decomposition that hadoccurred This is particularly relevant in the ferrite asshown below

Results

GENERAL MICROSTRUCTURE

Optical microscopy indicated that the cast and agedmaterials consisted of a duplex microstructure of austenitewith approximately 15 0 ferrite Analyses of the compos-

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 287

3 Austenite and ferrite in CF8M steel aged for 7500 h at400degC (TEM))

itions of the ferrite and austenite by EDS in the unagedCF8M material revealed that the ferrite was enriched inchromium silicon and molybdenum and depleted in nickeland manganese as can be seen in Table 2 Similar trendswere obtained in the CF8 material after the extended agingtreatment at 400degC (see Table 2)

A comparison of the duplex microstructure of CF8materials aged at 300 and 400degC is shown in the transmissionelectron micrographs (TEM) in Fig 1 In material aged for70000 h at 400degC the ferrite had undergone a small amountof reversion to austenite at the ferriteaustenite interfaceto a depth of approximately 4 ~m with the precipitation ofsome carbides (Fig 1b) Most of these precipitates werepreferentially located at the original ferriteaustenite inter-face (Fig 2a) These precipitates imaged brightly in thefield ion micrographs as shown in Fig 2b No markedreversion was observed in the CF8M material (Fig 3) butthe aging time was an order of magnitude shorter Electrondiffraction and analyses by EDS and EELS revealed thatthe precipitates were chromium rich M23C6carbides Atomprobe selected area analyses also confirmed that theseprecipitates were alloy carbides

By contrast no carbides or reversion of the ferrite wereobserved in the CF8 alloy aged for 70 000 h at 300degC(Fig 1a) The ferrite had slightly reverted in the CF8material aged for 70000 h at 350degC but no carbides wereobserved at the ferriteaustenite interface Unfortunatelyno material aged at 300degC was available heat treated to thesame equivalent time3 as the 400degC aging treatment so itwas not possible to ascertain whether reversion eventuallyoccurs at much longer aging times at 300degC

Aging produced a change in the relative microhardnessof the austenite and ferrite phases The hardness of theferrite increased significantly whereas the hardness of theaustenite remained essentially constant Transmissionelectron micrographs revealed that dislocations in the fer-rite in the CF8 steel aged at 300 and 400degC were pinned asshown in Fig 4

Table 2 EDS analysis of compositions of austenite andferrite wt-

Phase Cr Ni Mn Mo Si Fe

CF8M unagedAustenite 20middot8 11middot4 1middot1 1middot8 0middot8 SalFerrite 26middot7 6middot4 0middot8 4middot8 1middot0 Sal

CF8 aged for 70000 h at 400degCAustenite 20middot2 8middot4 0middot22 0middot26 0middot99 SalFerrite 28middot6 3middot7 0middot08 0middot36 1middot42 Sal

a

b4 Pinned dislocations in ferrite phase in CF8 steel aged for

70 000 h at a 300degC and b 400degC (TEM)

Any conclusions based on results of accelerated tests thatare performed at 400degC should be carefully examined sincethe microstructure that develops is distinctly different fromthat at 300degC This difference in the microstructure wasindicated by the reversion of the ferrite into austenite andthe precipitation of M23C6that occurred at 400degC14 but notat the lower temperatures of 350 or 300degC The presence ofthese carbides could be a factor in the fracture process andthereby influence the mechanical properties

G PHASE PRECIPITATESTwo marked changes in the microstructure and micro-chemistry of the ferrite were found to accompany lowtemperature aging in both alloys Numerous small roughlyspherical precipitates 10 nm in diameter which exhibitedbrightly imaging contrast in the field ion micrographs (egFig 5a) were found distributed throughout the ferritephase in the CF8M alloy These precipitates were notobserved in field ion micrographs of the austenite (Fig 5b)The bright spots in the field ion micrograph of austenite areprobably due to the presence of molybdenum and siliconand are typical for micrographs of austenite containinglarge amounts of solute Precipitates that imaged similarly

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288 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

a

b

a

ba ferrite with brightly imaging G phase precipitates b austenite

5 CF8M steel aged for 7500 h at 400degC (FIM)

to those in the aged CF8M steel were also observed in theferrite of the CF8 steel aged at 300 and 400degC as shown inFig 6 The size (1-1middot5 nm) and number density(1023 m-3) of these precipitates was approximately similarfor the two aging conditions with a slightly larger size and alower number density for the 400degC aging treatment Somevariation in the distribution of these precipitates wasobserved from one region of the ferrite to another The sizeof these precipitates was smaller than observed in theCF8M steel

The precipitates in the ferrite were also imaged usingTEM A precipitate dark field image is shown in Fig 7 inthe aged CF8M steel where the precipitates were of uni-form size 10nm in diameter and were present at anumber density of 1023 m -3 giving a volume fraction of10 in the ferrite These results are in agreement with thefield ion microscopy observations However in CF8material aged at 300 or 400degC distinct bimodal size distribu-tions of the precipitates were observed in the TEM (Fig 8)The smaller precipitates were randomly distributed in theferrite matrix whereas the larger precipitates were associ-ated with dislocations (see Fig 9) The TEM revealed thatthe finer precipitates were approximately 1middot5 and 2 nm in

Materials Science and Technology March 1990 Vol 6

6 Ferrite of CF8 steel aged for 70 000 h at a 300degC and b 400degC(FIM)

diameter and present at number densities of gt1024 and1021 m-3 in the CF8 materials aged at 300 and 400degCrespectively The discrepancy in the size and number den-sity measured in the FIM and the TEM is a consequence ofthe small size of the precipitates since the TEM is notsensitive to the smallest precipitates which results in anunderestimation of the number density and an over-estimation of the size This also illustrates the difficulty inresolving and measuring the extent of precipitates whentheir size approaches unit cell dimensions

The precipitates were 4 to 5 times larger on the disloca-tions than in the matrix presumably as a result of enhancednucleation and growth because of assistance of pipe diffu-sion along the dislocation core G phase precipitates werenot observed in the as cast un aged material or in theaustenite in the aged material

Atom probe analysis and EDS analysis of extractionreplicas revealed that the precipitates in the aged CF8Mferrite were alloy silicides as shown in Table 3 Selectedarea electron diffraction patterns of these silicide precipi-tates (Fig 7b) were consistent with a fcc crystal structurewith some additional features Lattice parameters of 1middot09and 1middot11 nm were measured for the precipitates in the

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 289

a

ba dark field TEM b electron diffraction pattern

7 G phase precipitates in ferrite of CF8M steel after aging for7500 h at 400degC

CF8M and CF8 alloys respectively The diffraction pat-terns also revealed that the precipitates had a cube on cubeorientation relationship with the ferrite matrix These smallsilicide precipitates in the ferrite phase were identified as G

Table 3 APFIM and EDS analysis of composition of G phaseprecipitates in CF8M steel aged for 7500 h at 400degCat-

Si Ni Fe Mo Cr C

APFIM27middot7 plusmn 3middot4 24middot0 plusmn 3middot2 20middot6 plusmn 3middot1 13middot0 plusmn 2middot5 12middot0 plusmn 2middot5 1middot0 plusmn 0middot7EDS20middot9 plusmn 2middot0 31middot1plusmn2middot2 10middot5 plusmn 1middot5 19middot9 plusmn 1middot1 17middot8 plusmn 2middot1 ND

ND not detected

phase The composition the fcc crystal structure with theweak or absent 220 and 400 and strong 333 reflections in theelectron diffraction patterns the cube on cube orientationrelationship with the ferrite matrix and the lattice para-meter all support this identification G phase is regarded asa complex silicide with a fcc crystal structure containing 116atoms per unit cell The model of G phase is based onNi16ShTi6 with various elements such as Cr Fe Mo Mn VNb Ta Hf and Zr substituting for the titanium andnicke12021 This substitution results in a series of G phaseswith variable composition and lattice parameter It shouldbe emphasised that without the characterisation of theCF8M steel the G phase in the CF8 alloy could have beenoverlooked because of its small size and weak contribution to the diffraction patterns

SPINODAL DECOMPOSITIONMore detailed analyses of the ferrite matrix in the CF8 steelby AEM and APFIM revealed that it had decomposedduring thermal aging Phase contrast electron micrographsof the structure in material aged at 300 and 400degC are shownin Fig lOa b The scale of this two phase modulated micro-structure was measured frqm these electron micrographs as4 and 9 nm in the 300 and 400degC aged materials respect-ively A field ion micrograph of the same two phase micro-structure in the material aged at 400degC is shown in Fig 10c

a b

8 Bimodal distribution of G phase precipitates in ferrite of CF8 steel aged at a 300degC and h C 400degC (TEM)

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290 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

abca 333 precipitate-reflection dark field TEM b weak beam dark field TEM c superposition of a and b

9 Coarser G phase precipitates associated with dislocations of CF8 steel aged at 400degC

ca b TEM c FIM showing brightly imaging a and darklyimaging a phases

10 Spinodal decomposition into a and a phases of CF8 steelaged at a 300degC and b C 400degC

Materials Science and Technology March 1990 Vol 6

where the darkly imaging a and the brightly imaging aphases are evident The periodicity of the modulations ofthe two phases was measured from FIM micrographs to be7 nm Field evaporation sequences revealed that themodulated microstructure was interconnected indicativeof phase separation by isotropic spinodal decomposition

Atom probe composition profiles through the ferrite alsoindicated that the ferrite had decomposed into a chromiumenriched a phase and an iron rich a phase A short sectionof an atom probe composition profile through the ferrite ofthe CF8 steel aged for 70 000 h at 400degC is shown in Fig 11The large amplitude fluctuations that are evident is indica-tive of phase separation on a fine scale Extended compos-ition profiles for both austenite and ferrite phases in theCF8 material were subjected to statistical analyses Theresults of these analyses are summarised in Table 4 Thesample distribution analysis autocorrelation function rbthe Johnson and Klotz (JK) Markov chain ordering para-meter () and the Hetherington and Miller mean separationmethod all indicated that the ferrite had phase separatedinto iron rich and chromium enriched regions and the chi-squared tests of the frequency distributions indicate that thesolute was not randomly distributed With the exception ofthe Pa sample distribution analysis the statistical analysis ofthe austenite indicated a random distribution of chromiumThe significance of Pa was much smaller for the austenite(3middot1) than for the ferrite (22)

Atom probe composition profiles were obtained from theferrite phase in the CF8M steel avoiding the silicide pre-cipitates and also revealed decomposition into the a and aphases as shown in Fig 12 The absolute compositions ofthe two phases are probably more extreme than those indi-cated from the composition profile since the probe aper-ture was larger than the extent of the chromium enrichedregions and therefore some averaging of the composition ofthe two phases occurred The phase separation was notresolved in the field ion micrographs partly because of theextremely fine scale (2 nm) and partly because the silicideprecipitates altered the local imaging conditions

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 291

2 3 4 5 6 7DISTANCE (nm)

12 Atom probe composition profile in ferrite phase of agedCF8M steel showing phase separation into iron rich a andchromium enriched a phases

70 60

60 AGED 70 OOOh AT 400degC AGED 7500h AT 4000C50

50fl ~

40

~40 E~ 2 30~30

E0

I 00 r

20u20

10 10

0 o 100 200 300DISTANCE - 50 ION BLOCKS

11 Section of atom probe composition profile throughferrite phase of CF8 steel aged for 70 000 h at 400degCshowing phase separation into iron rich a andchromium enriched a phases

Discussion

The microstructures of the CF8 and CF8M steels weresimilar In both types of steel the ferrite spino dally decom-posed into an isotropic network of a and a phases and Gphase precipitates The major difference between the twotypes of steel was the size and volume fraction of the Gphase precipitates In the CF8M material aged for 7500 hat400degC the G phase precipitates exhibited a much largervolume fraction compared with the CF8 steel that was alsoaged at 400degC but for almost 10 times longer This largervolume of G phase is related to the differences in initialcomposition between the two alloys The G phase silicide isrich in nickel and molybdenum which were present athigher levels in the CF8M steel than in the CF8 steelAlthough a small fraction of the G phase precipitates wasobserved pinning dislocations it should be noted that theseresidual dislocations will not be of the same type or behavein the similar manner as those generated during furtherdeformation G phase precipitates on dislocations in ferritehave also been observed by Vitek22 in similar steels Areduction in the levels of silicon nickel and perhapsmolybdenum in the ferrite would reduce or even suppressthe amount of G phase that is precipitated

The composition of the ferrite phase is in the range wherea miscibility gap exists at low temperatures Phase separa-tion of the ferrite into a chromium enriched phase and aniron rich phase is similar to that observed in Fe-Cr andmany Fe-Cr-X systems which undergo isotropic spinodaldecomposition within a miscibility gap under certain condi-tions Relatively small changes in the chromium and molyb-denum levels in the ferrite will alter the position in themiscibility gap and therefore the volume fraction of the ironrich and chromium enriched phases This could affect themorphology of the transformation products and hence alterthe mechanical properties and aging behaviour However

changing the chromium content of the alloy does not neces-sarily change the composition of the ferrite it may merelyalter the quantity of ferrite

The fine scale spinodal decomposition and the G phaseprecipitation in the ferrite bo~h contribute to the changes inmechanical properties that occur during aging Howeversince the volume of G phase was much lower in the CF8than inmiddot the CF8M steel and the behaviour of the steelsis similar the degradation in mechanical properties isprimarily due to the spinodal decomposition of the ferriteduring aging In addition the observation that the increasein hardness in the CF8 and CF8M steels is similar to thatpreviously observed in a spino dally decomposed Fe-30Cralloy which did not contain G phase23 also suggests thatspinodal decomposition is the primary factor influencingmechanical properties The results presented here arespecific to the material and heat treatment and may varyconsiderably in each individual casting depending on theprecise alloy composition and casting conditions

Conclusions

The results of this study have pointed out the complexityand the very fine scale of the decomposition processes thatoccur at low temperatures in CF8 and CF8M type stainlesssteels This study has also shown that the near atomicresolution of the atom probe was able to detect and quantifythe extremely fine scale phase separation that occurredBoth APFIM and AEM results indicate that the chromiumenriched ferrite had decomposed into a very fine network ofchromium enriched a and iron rich a phases as a result ofisotropic spinodal decomposition Coarse M23C6 precipi-tates were observed at the ferriteaustenite interface in theCF8 steel aged at 400degC Very fine G phase silicide precipi-tates were observed in the ferrite A comparison betweenthe results from the CF8 and CF8M steels indicates that

Table 4 Summary of statistical analysis of atom probe data for CF8 stainless steel aged for 70 000 h at400degC

Sample J and K Markov Frequencydistribution Autocorrelation chain Mean distributionanalysis function separation

Phase Pa r () Sig Sig X2 DF

Ferrite 0middot066 plusmn 0middot003 0middot467 plusmn 0middot06 1middot129 4middot26 6middot35 43middot0 22Austenite 0middot028 plusmn 0middot009 0middot24 plusmn 0middot09 1middot011 0middot34 1middot65 19middot1 13

Sig significance DF degrees of freedom

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292 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

relatively small differences in the alloy compositions signifi-cantly alter the quantity of G phase present in the micro-structure The degradation in mechanical properties is aconsequence of the spinodal decomposition of the ferritetogether with G phase precipitation that occurs duringaging

Acknowledgments

This research was sponsored by the Division of MaterialsSciences US Department of Energy under contractDE-AC05-840R21400 with Martin Marietta EnergySystems Inc The authors would like to thankDr H M Chung and Dr O K Chopra of ArgonneNational Laboratory for supplying the aged CF8 steelsDr J A Spitznagel of Westinghouse RampD CenterPittsburgh PA for supplying the CF8M alloyDr M G Hetherington of the University of Oxfordfor providing the sample distribution softwareDr G D W Smith of the University of Oxford for useof the atom probe for some of the analyses andMs K F Russell for her technical assistance

References

1 o K CHOPRAand H M CHUNGin Proc 13th Water ReactorSafety Research Information Meeting 1985 GaithersburgMD US National Bureau of Standards

2 H M CHUNGand o K CHOPRAin Proc 2nd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (ed J T Roberts et al) 287-2921985 Monterey CA American Nuclear Society

3 H M CHUNGand o K CHOPRAProperties of stainless steels inelevated temperature service (ed M Prager) PVP-ASMEVol 132 MPC Vol 26 17-34 1987 New YorkASMEjMaterials Properties Council

4 M K MILLERJ BENTLEYS s BRENNERand J A SPITZNAGELJPhys 1984 45-C9 385-390

5 M K MILLERand 1 BENTLEYJ Phys 1986 47-C7 239-244

6 M K MILLER1 BENTLEYS s BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 326-327 1985 SanFrancisco San Francisco Press

7 1 BENTLEYM K MILLERS S BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 328-329 1985 SanFrancisco San Francisco Press

8 1 BENTLEYand M K MILLERin Analytical electron micros-copy (ed D C Joy) 73-75 1987 San Francisco San Fran-cisco Press

9 M K MILLERand 1 BENTLEYin Proc 3rd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (eds G J Theus and J R Weeks)341-349 1988 Pittsburgh PA TMS

10 1 BENTLEYand M K MILLERin MRS Symp Characterizationof defects in materials Vol 82 (eds R W Siegeletal) 163-168 1987 Pittsburgh PA Materials Research Society

11 T J GODFREYand G D W SMITHJ Phys 1986 47-C7 217-222

12 T1 GODFREYM G HETHERINGTON1 M SASSENand G D WSMITHJ Phys 1988 49-C6 421-426

13 1 M SASSENM G HETHERINGTONT 1 GODFREYG D W SMITHP H PUMPHREYand K N AKHURSTin Properties of stainlesssteels in elevated temperature service (ed M Prager) PVP-ASME Vol 132 MPC Vol 26 65-78 1987 New YorkASMEjMaterials Properties Council

14 G SLAMA P PETREQUINS H MASSONand T MAGERinStructural Mechanics in Reactor Technology (SMIRT) Post-conference Seminar 6 Assuring structural integrity ofsteel reactor pressure boundary components MontereyCA August 1983

15 M K MILLERJ Phys 1986 47-C2 493-498 and 499-50416 M K MILLERInt Mater Rev 198732221-24017 M K MILLERand G D W SMITHAtom probe microanalysis

principles and applications to materials problems 1989Pittsburgh PA Materials Research Society

18 c A JOHNSONand 1 H KLOTZTeehnometries 1974 16 483-493

19 M G HETHERINGTONand M K MILLERJ Phys 1987 48-C6559-561

20 F X SPIEGELD BARDOSand P A BECKTrans AIME 1963227 575

21 E H LEEP 1 MAZIASZandA F ROWCLIFFEin Phase stabilityduring irradiation (ed J R Holland et al) 191-218 1981Warrendale PA The Metallurgical Society of AIME

22 1 M VITEKMetall Trans 1987 18A 154-15623 s S BRENNERM K MILLERand w A SOFFASer Metall 1982

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286 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

a

b1 Duplex microstructure in CF8 steel aged for 70000 h at

a 300degC and b 400degC reversion of ferrite and carbideprecipitation is evident in material aged at 400degC (TEM)

distribution of these composition blocks differs from thebinomial distribution associated with a random solid solu-tion The autocorrelation function examines pairs of com-position blocks a fixed distance or lag apart and sums theresults over the entire composition profile If the compos-itions of these blocks deviate from the mean composition inthe same sense then a positive correlation is determinedThe autocorrelation of adjacent composition blocks in theprofilel provides a technique to detect phase separation orclustering The sample distribution analysis attempts toquantify the extent of phase separation by calculating thecompositional range 2Pa of the observed distributionll-13

Both the Johnson and Klotz and the Hetherington andMiller methods examine the atom-by-atom data chain inorder to detect phase separation or measure the extent ofclustering The Johnson and Klotz method determines anordering parameter ()from the number of AA AB and BBpairs of atoms in the data chain and compares the resultwith that expected from a random solid solutionI718 TheHetherington and Miller mean separation method com-pares the variance of the distances between similar typeatoms in the data chain with that expected from a randomdistribution 17 19

Materials Science and Technology March 1990 Vol 6

a

ba weak beam TEM of M23C6 particle located at originalferriteaustenite interface b field ion micrograph (FIM) ofbrightly imaging M23C6 carbide and ferrite (arrows indicateinterface between M23C6 particle and ferrite)

2 Carbide precipitates in CF8 steel aged for 70 000 h at 400degC

Analyses using AEM were performed on PhilipsEM400T PEG and EM430T analytical electron micro-scopes both equipped with EDAX 910070 energy disper-sive X-ray spectrometers (EDS) and Gatan 607 electronenergy loss spectrometer (EELS) systems The elementalcompositions were obtained from EDS data by standard-less procedures It should be noted that the chromiumcontents are probably 1-2 overestimated because offluo-rescence effects and these compositions are averages of themicrostructure including any decomposition that hadoccurred This is particularly relevant in the ferrite asshown below

Results

GENERAL MICROSTRUCTURE

Optical microscopy indicated that the cast and agedmaterials consisted of a duplex microstructure of austenitewith approximately 15 0 ferrite Analyses of the compos-

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 287

3 Austenite and ferrite in CF8M steel aged for 7500 h at400degC (TEM))

itions of the ferrite and austenite by EDS in the unagedCF8M material revealed that the ferrite was enriched inchromium silicon and molybdenum and depleted in nickeland manganese as can be seen in Table 2 Similar trendswere obtained in the CF8 material after the extended agingtreatment at 400degC (see Table 2)

A comparison of the duplex microstructure of CF8materials aged at 300 and 400degC is shown in the transmissionelectron micrographs (TEM) in Fig 1 In material aged for70000 h at 400degC the ferrite had undergone a small amountof reversion to austenite at the ferriteaustenite interfaceto a depth of approximately 4 ~m with the precipitation ofsome carbides (Fig 1b) Most of these precipitates werepreferentially located at the original ferriteaustenite inter-face (Fig 2a) These precipitates imaged brightly in thefield ion micrographs as shown in Fig 2b No markedreversion was observed in the CF8M material (Fig 3) butthe aging time was an order of magnitude shorter Electrondiffraction and analyses by EDS and EELS revealed thatthe precipitates were chromium rich M23C6carbides Atomprobe selected area analyses also confirmed that theseprecipitates were alloy carbides

By contrast no carbides or reversion of the ferrite wereobserved in the CF8 alloy aged for 70 000 h at 300degC(Fig 1a) The ferrite had slightly reverted in the CF8material aged for 70000 h at 350degC but no carbides wereobserved at the ferriteaustenite interface Unfortunatelyno material aged at 300degC was available heat treated to thesame equivalent time3 as the 400degC aging treatment so itwas not possible to ascertain whether reversion eventuallyoccurs at much longer aging times at 300degC

Aging produced a change in the relative microhardnessof the austenite and ferrite phases The hardness of theferrite increased significantly whereas the hardness of theaustenite remained essentially constant Transmissionelectron micrographs revealed that dislocations in the fer-rite in the CF8 steel aged at 300 and 400degC were pinned asshown in Fig 4

Table 2 EDS analysis of compositions of austenite andferrite wt-

Phase Cr Ni Mn Mo Si Fe

CF8M unagedAustenite 20middot8 11middot4 1middot1 1middot8 0middot8 SalFerrite 26middot7 6middot4 0middot8 4middot8 1middot0 Sal

CF8 aged for 70000 h at 400degCAustenite 20middot2 8middot4 0middot22 0middot26 0middot99 SalFerrite 28middot6 3middot7 0middot08 0middot36 1middot42 Sal

a

b4 Pinned dislocations in ferrite phase in CF8 steel aged for

70 000 h at a 300degC and b 400degC (TEM)

Any conclusions based on results of accelerated tests thatare performed at 400degC should be carefully examined sincethe microstructure that develops is distinctly different fromthat at 300degC This difference in the microstructure wasindicated by the reversion of the ferrite into austenite andthe precipitation of M23C6that occurred at 400degC14 but notat the lower temperatures of 350 or 300degC The presence ofthese carbides could be a factor in the fracture process andthereby influence the mechanical properties

G PHASE PRECIPITATESTwo marked changes in the microstructure and micro-chemistry of the ferrite were found to accompany lowtemperature aging in both alloys Numerous small roughlyspherical precipitates 10 nm in diameter which exhibitedbrightly imaging contrast in the field ion micrographs (egFig 5a) were found distributed throughout the ferritephase in the CF8M alloy These precipitates were notobserved in field ion micrographs of the austenite (Fig 5b)The bright spots in the field ion micrograph of austenite areprobably due to the presence of molybdenum and siliconand are typical for micrographs of austenite containinglarge amounts of solute Precipitates that imaged similarly

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288 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

a

b

a

ba ferrite with brightly imaging G phase precipitates b austenite

5 CF8M steel aged for 7500 h at 400degC (FIM)

to those in the aged CF8M steel were also observed in theferrite of the CF8 steel aged at 300 and 400degC as shown inFig 6 The size (1-1middot5 nm) and number density(1023 m-3) of these precipitates was approximately similarfor the two aging conditions with a slightly larger size and alower number density for the 400degC aging treatment Somevariation in the distribution of these precipitates wasobserved from one region of the ferrite to another The sizeof these precipitates was smaller than observed in theCF8M steel

The precipitates in the ferrite were also imaged usingTEM A precipitate dark field image is shown in Fig 7 inthe aged CF8M steel where the precipitates were of uni-form size 10nm in diameter and were present at anumber density of 1023 m -3 giving a volume fraction of10 in the ferrite These results are in agreement with thefield ion microscopy observations However in CF8material aged at 300 or 400degC distinct bimodal size distribu-tions of the precipitates were observed in the TEM (Fig 8)The smaller precipitates were randomly distributed in theferrite matrix whereas the larger precipitates were associ-ated with dislocations (see Fig 9) The TEM revealed thatthe finer precipitates were approximately 1middot5 and 2 nm in

Materials Science and Technology March 1990 Vol 6

6 Ferrite of CF8 steel aged for 70 000 h at a 300degC and b 400degC(FIM)

diameter and present at number densities of gt1024 and1021 m-3 in the CF8 materials aged at 300 and 400degCrespectively The discrepancy in the size and number den-sity measured in the FIM and the TEM is a consequence ofthe small size of the precipitates since the TEM is notsensitive to the smallest precipitates which results in anunderestimation of the number density and an over-estimation of the size This also illustrates the difficulty inresolving and measuring the extent of precipitates whentheir size approaches unit cell dimensions

The precipitates were 4 to 5 times larger on the disloca-tions than in the matrix presumably as a result of enhancednucleation and growth because of assistance of pipe diffu-sion along the dislocation core G phase precipitates werenot observed in the as cast un aged material or in theaustenite in the aged material

Atom probe analysis and EDS analysis of extractionreplicas revealed that the precipitates in the aged CF8Mferrite were alloy silicides as shown in Table 3 Selectedarea electron diffraction patterns of these silicide precipi-tates (Fig 7b) were consistent with a fcc crystal structurewith some additional features Lattice parameters of 1middot09and 1middot11 nm were measured for the precipitates in the

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 289

a

ba dark field TEM b electron diffraction pattern

7 G phase precipitates in ferrite of CF8M steel after aging for7500 h at 400degC

CF8M and CF8 alloys respectively The diffraction pat-terns also revealed that the precipitates had a cube on cubeorientation relationship with the ferrite matrix These smallsilicide precipitates in the ferrite phase were identified as G

Table 3 APFIM and EDS analysis of composition of G phaseprecipitates in CF8M steel aged for 7500 h at 400degCat-

Si Ni Fe Mo Cr C

APFIM27middot7 plusmn 3middot4 24middot0 plusmn 3middot2 20middot6 plusmn 3middot1 13middot0 plusmn 2middot5 12middot0 plusmn 2middot5 1middot0 plusmn 0middot7EDS20middot9 plusmn 2middot0 31middot1plusmn2middot2 10middot5 plusmn 1middot5 19middot9 plusmn 1middot1 17middot8 plusmn 2middot1 ND

ND not detected

phase The composition the fcc crystal structure with theweak or absent 220 and 400 and strong 333 reflections in theelectron diffraction patterns the cube on cube orientationrelationship with the ferrite matrix and the lattice para-meter all support this identification G phase is regarded asa complex silicide with a fcc crystal structure containing 116atoms per unit cell The model of G phase is based onNi16ShTi6 with various elements such as Cr Fe Mo Mn VNb Ta Hf and Zr substituting for the titanium andnicke12021 This substitution results in a series of G phaseswith variable composition and lattice parameter It shouldbe emphasised that without the characterisation of theCF8M steel the G phase in the CF8 alloy could have beenoverlooked because of its small size and weak contribution to the diffraction patterns

SPINODAL DECOMPOSITIONMore detailed analyses of the ferrite matrix in the CF8 steelby AEM and APFIM revealed that it had decomposedduring thermal aging Phase contrast electron micrographsof the structure in material aged at 300 and 400degC are shownin Fig lOa b The scale of this two phase modulated micro-structure was measured frqm these electron micrographs as4 and 9 nm in the 300 and 400degC aged materials respect-ively A field ion micrograph of the same two phase micro-structure in the material aged at 400degC is shown in Fig 10c

a b

8 Bimodal distribution of G phase precipitates in ferrite of CF8 steel aged at a 300degC and h C 400degC (TEM)

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290 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

abca 333 precipitate-reflection dark field TEM b weak beam dark field TEM c superposition of a and b

9 Coarser G phase precipitates associated with dislocations of CF8 steel aged at 400degC

ca b TEM c FIM showing brightly imaging a and darklyimaging a phases

10 Spinodal decomposition into a and a phases of CF8 steelaged at a 300degC and b C 400degC

Materials Science and Technology March 1990 Vol 6

where the darkly imaging a and the brightly imaging aphases are evident The periodicity of the modulations ofthe two phases was measured from FIM micrographs to be7 nm Field evaporation sequences revealed that themodulated microstructure was interconnected indicativeof phase separation by isotropic spinodal decomposition

Atom probe composition profiles through the ferrite alsoindicated that the ferrite had decomposed into a chromiumenriched a phase and an iron rich a phase A short sectionof an atom probe composition profile through the ferrite ofthe CF8 steel aged for 70 000 h at 400degC is shown in Fig 11The large amplitude fluctuations that are evident is indica-tive of phase separation on a fine scale Extended compos-ition profiles for both austenite and ferrite phases in theCF8 material were subjected to statistical analyses Theresults of these analyses are summarised in Table 4 Thesample distribution analysis autocorrelation function rbthe Johnson and Klotz (JK) Markov chain ordering para-meter () and the Hetherington and Miller mean separationmethod all indicated that the ferrite had phase separatedinto iron rich and chromium enriched regions and the chi-squared tests of the frequency distributions indicate that thesolute was not randomly distributed With the exception ofthe Pa sample distribution analysis the statistical analysis ofthe austenite indicated a random distribution of chromiumThe significance of Pa was much smaller for the austenite(3middot1) than for the ferrite (22)

Atom probe composition profiles were obtained from theferrite phase in the CF8M steel avoiding the silicide pre-cipitates and also revealed decomposition into the a and aphases as shown in Fig 12 The absolute compositions ofthe two phases are probably more extreme than those indi-cated from the composition profile since the probe aper-ture was larger than the extent of the chromium enrichedregions and therefore some averaging of the composition ofthe two phases occurred The phase separation was notresolved in the field ion micrographs partly because of theextremely fine scale (2 nm) and partly because the silicideprecipitates altered the local imaging conditions

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 291

2 3 4 5 6 7DISTANCE (nm)

12 Atom probe composition profile in ferrite phase of agedCF8M steel showing phase separation into iron rich a andchromium enriched a phases

70 60

60 AGED 70 OOOh AT 400degC AGED 7500h AT 4000C50

50fl ~

40

~40 E~ 2 30~30

E0

I 00 r

20u20

10 10

0 o 100 200 300DISTANCE - 50 ION BLOCKS

11 Section of atom probe composition profile throughferrite phase of CF8 steel aged for 70 000 h at 400degCshowing phase separation into iron rich a andchromium enriched a phases

Discussion

The microstructures of the CF8 and CF8M steels weresimilar In both types of steel the ferrite spino dally decom-posed into an isotropic network of a and a phases and Gphase precipitates The major difference between the twotypes of steel was the size and volume fraction of the Gphase precipitates In the CF8M material aged for 7500 hat400degC the G phase precipitates exhibited a much largervolume fraction compared with the CF8 steel that was alsoaged at 400degC but for almost 10 times longer This largervolume of G phase is related to the differences in initialcomposition between the two alloys The G phase silicide isrich in nickel and molybdenum which were present athigher levels in the CF8M steel than in the CF8 steelAlthough a small fraction of the G phase precipitates wasobserved pinning dislocations it should be noted that theseresidual dislocations will not be of the same type or behavein the similar manner as those generated during furtherdeformation G phase precipitates on dislocations in ferritehave also been observed by Vitek22 in similar steels Areduction in the levels of silicon nickel and perhapsmolybdenum in the ferrite would reduce or even suppressthe amount of G phase that is precipitated

The composition of the ferrite phase is in the range wherea miscibility gap exists at low temperatures Phase separa-tion of the ferrite into a chromium enriched phase and aniron rich phase is similar to that observed in Fe-Cr andmany Fe-Cr-X systems which undergo isotropic spinodaldecomposition within a miscibility gap under certain condi-tions Relatively small changes in the chromium and molyb-denum levels in the ferrite will alter the position in themiscibility gap and therefore the volume fraction of the ironrich and chromium enriched phases This could affect themorphology of the transformation products and hence alterthe mechanical properties and aging behaviour However

changing the chromium content of the alloy does not neces-sarily change the composition of the ferrite it may merelyalter the quantity of ferrite

The fine scale spinodal decomposition and the G phaseprecipitation in the ferrite bo~h contribute to the changes inmechanical properties that occur during aging Howeversince the volume of G phase was much lower in the CF8than inmiddot the CF8M steel and the behaviour of the steelsis similar the degradation in mechanical properties isprimarily due to the spinodal decomposition of the ferriteduring aging In addition the observation that the increasein hardness in the CF8 and CF8M steels is similar to thatpreviously observed in a spino dally decomposed Fe-30Cralloy which did not contain G phase23 also suggests thatspinodal decomposition is the primary factor influencingmechanical properties The results presented here arespecific to the material and heat treatment and may varyconsiderably in each individual casting depending on theprecise alloy composition and casting conditions

Conclusions

The results of this study have pointed out the complexityand the very fine scale of the decomposition processes thatoccur at low temperatures in CF8 and CF8M type stainlesssteels This study has also shown that the near atomicresolution of the atom probe was able to detect and quantifythe extremely fine scale phase separation that occurredBoth APFIM and AEM results indicate that the chromiumenriched ferrite had decomposed into a very fine network ofchromium enriched a and iron rich a phases as a result ofisotropic spinodal decomposition Coarse M23C6 precipi-tates were observed at the ferriteaustenite interface in theCF8 steel aged at 400degC Very fine G phase silicide precipi-tates were observed in the ferrite A comparison betweenthe results from the CF8 and CF8M steels indicates that

Table 4 Summary of statistical analysis of atom probe data for CF8 stainless steel aged for 70 000 h at400degC

Sample J and K Markov Frequencydistribution Autocorrelation chain Mean distributionanalysis function separation

Phase Pa r () Sig Sig X2 DF

Ferrite 0middot066 plusmn 0middot003 0middot467 plusmn 0middot06 1middot129 4middot26 6middot35 43middot0 22Austenite 0middot028 plusmn 0middot009 0middot24 plusmn 0middot09 1middot011 0middot34 1middot65 19middot1 13

Sig significance DF degrees of freedom

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292 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

relatively small differences in the alloy compositions signifi-cantly alter the quantity of G phase present in the micro-structure The degradation in mechanical properties is aconsequence of the spinodal decomposition of the ferritetogether with G phase precipitation that occurs duringaging

Acknowledgments

This research was sponsored by the Division of MaterialsSciences US Department of Energy under contractDE-AC05-840R21400 with Martin Marietta EnergySystems Inc The authors would like to thankDr H M Chung and Dr O K Chopra of ArgonneNational Laboratory for supplying the aged CF8 steelsDr J A Spitznagel of Westinghouse RampD CenterPittsburgh PA for supplying the CF8M alloyDr M G Hetherington of the University of Oxfordfor providing the sample distribution softwareDr G D W Smith of the University of Oxford for useof the atom probe for some of the analyses andMs K F Russell for her technical assistance

References

1 o K CHOPRAand H M CHUNGin Proc 13th Water ReactorSafety Research Information Meeting 1985 GaithersburgMD US National Bureau of Standards

2 H M CHUNGand o K CHOPRAin Proc 2nd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (ed J T Roberts et al) 287-2921985 Monterey CA American Nuclear Society

3 H M CHUNGand o K CHOPRAProperties of stainless steels inelevated temperature service (ed M Prager) PVP-ASMEVol 132 MPC Vol 26 17-34 1987 New YorkASMEjMaterials Properties Council

4 M K MILLERJ BENTLEYS s BRENNERand J A SPITZNAGELJPhys 1984 45-C9 385-390

5 M K MILLERand 1 BENTLEYJ Phys 1986 47-C7 239-244

6 M K MILLER1 BENTLEYS s BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 326-327 1985 SanFrancisco San Francisco Press

7 1 BENTLEYM K MILLERS S BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 328-329 1985 SanFrancisco San Francisco Press

8 1 BENTLEYand M K MILLERin Analytical electron micros-copy (ed D C Joy) 73-75 1987 San Francisco San Fran-cisco Press

9 M K MILLERand 1 BENTLEYin Proc 3rd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (eds G J Theus and J R Weeks)341-349 1988 Pittsburgh PA TMS

10 1 BENTLEYand M K MILLERin MRS Symp Characterizationof defects in materials Vol 82 (eds R W Siegeletal) 163-168 1987 Pittsburgh PA Materials Research Society

11 T J GODFREYand G D W SMITHJ Phys 1986 47-C7 217-222

12 T1 GODFREYM G HETHERINGTON1 M SASSENand G D WSMITHJ Phys 1988 49-C6 421-426

13 1 M SASSENM G HETHERINGTONT 1 GODFREYG D W SMITHP H PUMPHREYand K N AKHURSTin Properties of stainlesssteels in elevated temperature service (ed M Prager) PVP-ASME Vol 132 MPC Vol 26 65-78 1987 New YorkASMEjMaterials Properties Council

14 G SLAMA P PETREQUINS H MASSONand T MAGERinStructural Mechanics in Reactor Technology (SMIRT) Post-conference Seminar 6 Assuring structural integrity ofsteel reactor pressure boundary components MontereyCA August 1983

15 M K MILLERJ Phys 1986 47-C2 493-498 and 499-50416 M K MILLERInt Mater Rev 198732221-24017 M K MILLERand G D W SMITHAtom probe microanalysis

principles and applications to materials problems 1989Pittsburgh PA Materials Research Society

18 c A JOHNSONand 1 H KLOTZTeehnometries 1974 16 483-493

19 M G HETHERINGTONand M K MILLERJ Phys 1987 48-C6559-561

20 F X SPIEGELD BARDOSand P A BECKTrans AIME 1963227 575

21 E H LEEP 1 MAZIASZandA F ROWCLIFFEin Phase stabilityduring irradiation (ed J R Holland et al) 191-218 1981Warrendale PA The Metallurgical Society of AIME

22 1 M VITEKMetall Trans 1987 18A 154-15623 s S BRENNERM K MILLERand w A SOFFASer Metall 1982

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 287

3 Austenite and ferrite in CF8M steel aged for 7500 h at400degC (TEM))

itions of the ferrite and austenite by EDS in the unagedCF8M material revealed that the ferrite was enriched inchromium silicon and molybdenum and depleted in nickeland manganese as can be seen in Table 2 Similar trendswere obtained in the CF8 material after the extended agingtreatment at 400degC (see Table 2)

A comparison of the duplex microstructure of CF8materials aged at 300 and 400degC is shown in the transmissionelectron micrographs (TEM) in Fig 1 In material aged for70000 h at 400degC the ferrite had undergone a small amountof reversion to austenite at the ferriteaustenite interfaceto a depth of approximately 4 ~m with the precipitation ofsome carbides (Fig 1b) Most of these precipitates werepreferentially located at the original ferriteaustenite inter-face (Fig 2a) These precipitates imaged brightly in thefield ion micrographs as shown in Fig 2b No markedreversion was observed in the CF8M material (Fig 3) butthe aging time was an order of magnitude shorter Electrondiffraction and analyses by EDS and EELS revealed thatthe precipitates were chromium rich M23C6carbides Atomprobe selected area analyses also confirmed that theseprecipitates were alloy carbides

By contrast no carbides or reversion of the ferrite wereobserved in the CF8 alloy aged for 70 000 h at 300degC(Fig 1a) The ferrite had slightly reverted in the CF8material aged for 70000 h at 350degC but no carbides wereobserved at the ferriteaustenite interface Unfortunatelyno material aged at 300degC was available heat treated to thesame equivalent time3 as the 400degC aging treatment so itwas not possible to ascertain whether reversion eventuallyoccurs at much longer aging times at 300degC

Aging produced a change in the relative microhardnessof the austenite and ferrite phases The hardness of theferrite increased significantly whereas the hardness of theaustenite remained essentially constant Transmissionelectron micrographs revealed that dislocations in the fer-rite in the CF8 steel aged at 300 and 400degC were pinned asshown in Fig 4

Table 2 EDS analysis of compositions of austenite andferrite wt-

Phase Cr Ni Mn Mo Si Fe

CF8M unagedAustenite 20middot8 11middot4 1middot1 1middot8 0middot8 SalFerrite 26middot7 6middot4 0middot8 4middot8 1middot0 Sal

CF8 aged for 70000 h at 400degCAustenite 20middot2 8middot4 0middot22 0middot26 0middot99 SalFerrite 28middot6 3middot7 0middot08 0middot36 1middot42 Sal

a

b4 Pinned dislocations in ferrite phase in CF8 steel aged for

70 000 h at a 300degC and b 400degC (TEM)

Any conclusions based on results of accelerated tests thatare performed at 400degC should be carefully examined sincethe microstructure that develops is distinctly different fromthat at 300degC This difference in the microstructure wasindicated by the reversion of the ferrite into austenite andthe precipitation of M23C6that occurred at 400degC14 but notat the lower temperatures of 350 or 300degC The presence ofthese carbides could be a factor in the fracture process andthereby influence the mechanical properties

G PHASE PRECIPITATESTwo marked changes in the microstructure and micro-chemistry of the ferrite were found to accompany lowtemperature aging in both alloys Numerous small roughlyspherical precipitates 10 nm in diameter which exhibitedbrightly imaging contrast in the field ion micrographs (egFig 5a) were found distributed throughout the ferritephase in the CF8M alloy These precipitates were notobserved in field ion micrographs of the austenite (Fig 5b)The bright spots in the field ion micrograph of austenite areprobably due to the presence of molybdenum and siliconand are typical for micrographs of austenite containinglarge amounts of solute Precipitates that imaged similarly

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288 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

a

b

a

ba ferrite with brightly imaging G phase precipitates b austenite

5 CF8M steel aged for 7500 h at 400degC (FIM)

to those in the aged CF8M steel were also observed in theferrite of the CF8 steel aged at 300 and 400degC as shown inFig 6 The size (1-1middot5 nm) and number density(1023 m-3) of these precipitates was approximately similarfor the two aging conditions with a slightly larger size and alower number density for the 400degC aging treatment Somevariation in the distribution of these precipitates wasobserved from one region of the ferrite to another The sizeof these precipitates was smaller than observed in theCF8M steel

The precipitates in the ferrite were also imaged usingTEM A precipitate dark field image is shown in Fig 7 inthe aged CF8M steel where the precipitates were of uni-form size 10nm in diameter and were present at anumber density of 1023 m -3 giving a volume fraction of10 in the ferrite These results are in agreement with thefield ion microscopy observations However in CF8material aged at 300 or 400degC distinct bimodal size distribu-tions of the precipitates were observed in the TEM (Fig 8)The smaller precipitates were randomly distributed in theferrite matrix whereas the larger precipitates were associ-ated with dislocations (see Fig 9) The TEM revealed thatthe finer precipitates were approximately 1middot5 and 2 nm in

Materials Science and Technology March 1990 Vol 6

6 Ferrite of CF8 steel aged for 70 000 h at a 300degC and b 400degC(FIM)

diameter and present at number densities of gt1024 and1021 m-3 in the CF8 materials aged at 300 and 400degCrespectively The discrepancy in the size and number den-sity measured in the FIM and the TEM is a consequence ofthe small size of the precipitates since the TEM is notsensitive to the smallest precipitates which results in anunderestimation of the number density and an over-estimation of the size This also illustrates the difficulty inresolving and measuring the extent of precipitates whentheir size approaches unit cell dimensions

The precipitates were 4 to 5 times larger on the disloca-tions than in the matrix presumably as a result of enhancednucleation and growth because of assistance of pipe diffu-sion along the dislocation core G phase precipitates werenot observed in the as cast un aged material or in theaustenite in the aged material

Atom probe analysis and EDS analysis of extractionreplicas revealed that the precipitates in the aged CF8Mferrite were alloy silicides as shown in Table 3 Selectedarea electron diffraction patterns of these silicide precipi-tates (Fig 7b) were consistent with a fcc crystal structurewith some additional features Lattice parameters of 1middot09and 1middot11 nm were measured for the precipitates in the

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 289

a

ba dark field TEM b electron diffraction pattern

7 G phase precipitates in ferrite of CF8M steel after aging for7500 h at 400degC

CF8M and CF8 alloys respectively The diffraction pat-terns also revealed that the precipitates had a cube on cubeorientation relationship with the ferrite matrix These smallsilicide precipitates in the ferrite phase were identified as G

Table 3 APFIM and EDS analysis of composition of G phaseprecipitates in CF8M steel aged for 7500 h at 400degCat-

Si Ni Fe Mo Cr C

APFIM27middot7 plusmn 3middot4 24middot0 plusmn 3middot2 20middot6 plusmn 3middot1 13middot0 plusmn 2middot5 12middot0 plusmn 2middot5 1middot0 plusmn 0middot7EDS20middot9 plusmn 2middot0 31middot1plusmn2middot2 10middot5 plusmn 1middot5 19middot9 plusmn 1middot1 17middot8 plusmn 2middot1 ND

ND not detected

phase The composition the fcc crystal structure with theweak or absent 220 and 400 and strong 333 reflections in theelectron diffraction patterns the cube on cube orientationrelationship with the ferrite matrix and the lattice para-meter all support this identification G phase is regarded asa complex silicide with a fcc crystal structure containing 116atoms per unit cell The model of G phase is based onNi16ShTi6 with various elements such as Cr Fe Mo Mn VNb Ta Hf and Zr substituting for the titanium andnicke12021 This substitution results in a series of G phaseswith variable composition and lattice parameter It shouldbe emphasised that without the characterisation of theCF8M steel the G phase in the CF8 alloy could have beenoverlooked because of its small size and weak contribution to the diffraction patterns

SPINODAL DECOMPOSITIONMore detailed analyses of the ferrite matrix in the CF8 steelby AEM and APFIM revealed that it had decomposedduring thermal aging Phase contrast electron micrographsof the structure in material aged at 300 and 400degC are shownin Fig lOa b The scale of this two phase modulated micro-structure was measured frqm these electron micrographs as4 and 9 nm in the 300 and 400degC aged materials respect-ively A field ion micrograph of the same two phase micro-structure in the material aged at 400degC is shown in Fig 10c

a b

8 Bimodal distribution of G phase precipitates in ferrite of CF8 steel aged at a 300degC and h C 400degC (TEM)

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290 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

abca 333 precipitate-reflection dark field TEM b weak beam dark field TEM c superposition of a and b

9 Coarser G phase precipitates associated with dislocations of CF8 steel aged at 400degC

ca b TEM c FIM showing brightly imaging a and darklyimaging a phases

10 Spinodal decomposition into a and a phases of CF8 steelaged at a 300degC and b C 400degC

Materials Science and Technology March 1990 Vol 6

where the darkly imaging a and the brightly imaging aphases are evident The periodicity of the modulations ofthe two phases was measured from FIM micrographs to be7 nm Field evaporation sequences revealed that themodulated microstructure was interconnected indicativeof phase separation by isotropic spinodal decomposition

Atom probe composition profiles through the ferrite alsoindicated that the ferrite had decomposed into a chromiumenriched a phase and an iron rich a phase A short sectionof an atom probe composition profile through the ferrite ofthe CF8 steel aged for 70 000 h at 400degC is shown in Fig 11The large amplitude fluctuations that are evident is indica-tive of phase separation on a fine scale Extended compos-ition profiles for both austenite and ferrite phases in theCF8 material were subjected to statistical analyses Theresults of these analyses are summarised in Table 4 Thesample distribution analysis autocorrelation function rbthe Johnson and Klotz (JK) Markov chain ordering para-meter () and the Hetherington and Miller mean separationmethod all indicated that the ferrite had phase separatedinto iron rich and chromium enriched regions and the chi-squared tests of the frequency distributions indicate that thesolute was not randomly distributed With the exception ofthe Pa sample distribution analysis the statistical analysis ofthe austenite indicated a random distribution of chromiumThe significance of Pa was much smaller for the austenite(3middot1) than for the ferrite (22)

Atom probe composition profiles were obtained from theferrite phase in the CF8M steel avoiding the silicide pre-cipitates and also revealed decomposition into the a and aphases as shown in Fig 12 The absolute compositions ofthe two phases are probably more extreme than those indi-cated from the composition profile since the probe aper-ture was larger than the extent of the chromium enrichedregions and therefore some averaging of the composition ofthe two phases occurred The phase separation was notresolved in the field ion micrographs partly because of theextremely fine scale (2 nm) and partly because the silicideprecipitates altered the local imaging conditions

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2 3 4 5 6 7DISTANCE (nm)

12 Atom probe composition profile in ferrite phase of agedCF8M steel showing phase separation into iron rich a andchromium enriched a phases

70 60

60 AGED 70 OOOh AT 400degC AGED 7500h AT 4000C50

50fl ~

40

~40 E~ 2 30~30

E0

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0 o 100 200 300DISTANCE - 50 ION BLOCKS

11 Section of atom probe composition profile throughferrite phase of CF8 steel aged for 70 000 h at 400degCshowing phase separation into iron rich a andchromium enriched a phases

Discussion

The microstructures of the CF8 and CF8M steels weresimilar In both types of steel the ferrite spino dally decom-posed into an isotropic network of a and a phases and Gphase precipitates The major difference between the twotypes of steel was the size and volume fraction of the Gphase precipitates In the CF8M material aged for 7500 hat400degC the G phase precipitates exhibited a much largervolume fraction compared with the CF8 steel that was alsoaged at 400degC but for almost 10 times longer This largervolume of G phase is related to the differences in initialcomposition between the two alloys The G phase silicide isrich in nickel and molybdenum which were present athigher levels in the CF8M steel than in the CF8 steelAlthough a small fraction of the G phase precipitates wasobserved pinning dislocations it should be noted that theseresidual dislocations will not be of the same type or behavein the similar manner as those generated during furtherdeformation G phase precipitates on dislocations in ferritehave also been observed by Vitek22 in similar steels Areduction in the levels of silicon nickel and perhapsmolybdenum in the ferrite would reduce or even suppressthe amount of G phase that is precipitated

The composition of the ferrite phase is in the range wherea miscibility gap exists at low temperatures Phase separa-tion of the ferrite into a chromium enriched phase and aniron rich phase is similar to that observed in Fe-Cr andmany Fe-Cr-X systems which undergo isotropic spinodaldecomposition within a miscibility gap under certain condi-tions Relatively small changes in the chromium and molyb-denum levels in the ferrite will alter the position in themiscibility gap and therefore the volume fraction of the ironrich and chromium enriched phases This could affect themorphology of the transformation products and hence alterthe mechanical properties and aging behaviour However

changing the chromium content of the alloy does not neces-sarily change the composition of the ferrite it may merelyalter the quantity of ferrite

The fine scale spinodal decomposition and the G phaseprecipitation in the ferrite bo~h contribute to the changes inmechanical properties that occur during aging Howeversince the volume of G phase was much lower in the CF8than inmiddot the CF8M steel and the behaviour of the steelsis similar the degradation in mechanical properties isprimarily due to the spinodal decomposition of the ferriteduring aging In addition the observation that the increasein hardness in the CF8 and CF8M steels is similar to thatpreviously observed in a spino dally decomposed Fe-30Cralloy which did not contain G phase23 also suggests thatspinodal decomposition is the primary factor influencingmechanical properties The results presented here arespecific to the material and heat treatment and may varyconsiderably in each individual casting depending on theprecise alloy composition and casting conditions

Conclusions

The results of this study have pointed out the complexityand the very fine scale of the decomposition processes thatoccur at low temperatures in CF8 and CF8M type stainlesssteels This study has also shown that the near atomicresolution of the atom probe was able to detect and quantifythe extremely fine scale phase separation that occurredBoth APFIM and AEM results indicate that the chromiumenriched ferrite had decomposed into a very fine network ofchromium enriched a and iron rich a phases as a result ofisotropic spinodal decomposition Coarse M23C6 precipi-tates were observed at the ferriteaustenite interface in theCF8 steel aged at 400degC Very fine G phase silicide precipi-tates were observed in the ferrite A comparison betweenthe results from the CF8 and CF8M steels indicates that

Table 4 Summary of statistical analysis of atom probe data for CF8 stainless steel aged for 70 000 h at400degC

Sample J and K Markov Frequencydistribution Autocorrelation chain Mean distributionanalysis function separation

Phase Pa r () Sig Sig X2 DF

Ferrite 0middot066 plusmn 0middot003 0middot467 plusmn 0middot06 1middot129 4middot26 6middot35 43middot0 22Austenite 0middot028 plusmn 0middot009 0middot24 plusmn 0middot09 1middot011 0middot34 1middot65 19middot1 13

Sig significance DF degrees of freedom

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relatively small differences in the alloy compositions signifi-cantly alter the quantity of G phase present in the micro-structure The degradation in mechanical properties is aconsequence of the spinodal decomposition of the ferritetogether with G phase precipitation that occurs duringaging

Acknowledgments

This research was sponsored by the Division of MaterialsSciences US Department of Energy under contractDE-AC05-840R21400 with Martin Marietta EnergySystems Inc The authors would like to thankDr H M Chung and Dr O K Chopra of ArgonneNational Laboratory for supplying the aged CF8 steelsDr J A Spitznagel of Westinghouse RampD CenterPittsburgh PA for supplying the CF8M alloyDr M G Hetherington of the University of Oxfordfor providing the sample distribution softwareDr G D W Smith of the University of Oxford for useof the atom probe for some of the analyses andMs K F Russell for her technical assistance

References

1 o K CHOPRAand H M CHUNGin Proc 13th Water ReactorSafety Research Information Meeting 1985 GaithersburgMD US National Bureau of Standards

2 H M CHUNGand o K CHOPRAin Proc 2nd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (ed J T Roberts et al) 287-2921985 Monterey CA American Nuclear Society

3 H M CHUNGand o K CHOPRAProperties of stainless steels inelevated temperature service (ed M Prager) PVP-ASMEVol 132 MPC Vol 26 17-34 1987 New YorkASMEjMaterials Properties Council

4 M K MILLERJ BENTLEYS s BRENNERand J A SPITZNAGELJPhys 1984 45-C9 385-390

5 M K MILLERand 1 BENTLEYJ Phys 1986 47-C7 239-244

6 M K MILLER1 BENTLEYS s BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 326-327 1985 SanFrancisco San Francisco Press

7 1 BENTLEYM K MILLERS S BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 328-329 1985 SanFrancisco San Francisco Press

8 1 BENTLEYand M K MILLERin Analytical electron micros-copy (ed D C Joy) 73-75 1987 San Francisco San Fran-cisco Press

9 M K MILLERand 1 BENTLEYin Proc 3rd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (eds G J Theus and J R Weeks)341-349 1988 Pittsburgh PA TMS

10 1 BENTLEYand M K MILLERin MRS Symp Characterizationof defects in materials Vol 82 (eds R W Siegeletal) 163-168 1987 Pittsburgh PA Materials Research Society

11 T J GODFREYand G D W SMITHJ Phys 1986 47-C7 217-222

12 T1 GODFREYM G HETHERINGTON1 M SASSENand G D WSMITHJ Phys 1988 49-C6 421-426

13 1 M SASSENM G HETHERINGTONT 1 GODFREYG D W SMITHP H PUMPHREYand K N AKHURSTin Properties of stainlesssteels in elevated temperature service (ed M Prager) PVP-ASME Vol 132 MPC Vol 26 65-78 1987 New YorkASMEjMaterials Properties Council

14 G SLAMA P PETREQUINS H MASSONand T MAGERinStructural Mechanics in Reactor Technology (SMIRT) Post-conference Seminar 6 Assuring structural integrity ofsteel reactor pressure boundary components MontereyCA August 1983

15 M K MILLERJ Phys 1986 47-C2 493-498 and 499-50416 M K MILLERInt Mater Rev 198732221-24017 M K MILLERand G D W SMITHAtom probe microanalysis

principles and applications to materials problems 1989Pittsburgh PA Materials Research Society

18 c A JOHNSONand 1 H KLOTZTeehnometries 1974 16 483-493

19 M G HETHERINGTONand M K MILLERJ Phys 1987 48-C6559-561

20 F X SPIEGELD BARDOSand P A BECKTrans AIME 1963227 575

21 E H LEEP 1 MAZIASZandA F ROWCLIFFEin Phase stabilityduring irradiation (ed J R Holland et al) 191-218 1981Warrendale PA The Metallurgical Society of AIME

22 1 M VITEKMetall Trans 1987 18A 154-15623 s S BRENNERM K MILLERand w A SOFFASer Metall 1982

16 831-836

IRONMAKING AND STEELMAKING(alternate-monthly)

Provides international coverage of all aspects of iron and steelmaking including therolling and application of ferrous products1990 Subscription Rates pound9500 US$22500 Members pound5400 US$10800

Orders with remittance to The Institute of Metals Sales amp Marketing Dept 1 CarltonHouse Terrace London SW1Y 5DB Tel 071-976 1338 Fax 071-839 2078

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288 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

a

b

a

ba ferrite with brightly imaging G phase precipitates b austenite

5 CF8M steel aged for 7500 h at 400degC (FIM)

to those in the aged CF8M steel were also observed in theferrite of the CF8 steel aged at 300 and 400degC as shown inFig 6 The size (1-1middot5 nm) and number density(1023 m-3) of these precipitates was approximately similarfor the two aging conditions with a slightly larger size and alower number density for the 400degC aging treatment Somevariation in the distribution of these precipitates wasobserved from one region of the ferrite to another The sizeof these precipitates was smaller than observed in theCF8M steel

The precipitates in the ferrite were also imaged usingTEM A precipitate dark field image is shown in Fig 7 inthe aged CF8M steel where the precipitates were of uni-form size 10nm in diameter and were present at anumber density of 1023 m -3 giving a volume fraction of10 in the ferrite These results are in agreement with thefield ion microscopy observations However in CF8material aged at 300 or 400degC distinct bimodal size distribu-tions of the precipitates were observed in the TEM (Fig 8)The smaller precipitates were randomly distributed in theferrite matrix whereas the larger precipitates were associ-ated with dislocations (see Fig 9) The TEM revealed thatthe finer precipitates were approximately 1middot5 and 2 nm in

Materials Science and Technology March 1990 Vol 6

6 Ferrite of CF8 steel aged for 70 000 h at a 300degC and b 400degC(FIM)

diameter and present at number densities of gt1024 and1021 m-3 in the CF8 materials aged at 300 and 400degCrespectively The discrepancy in the size and number den-sity measured in the FIM and the TEM is a consequence ofthe small size of the precipitates since the TEM is notsensitive to the smallest precipitates which results in anunderestimation of the number density and an over-estimation of the size This also illustrates the difficulty inresolving and measuring the extent of precipitates whentheir size approaches unit cell dimensions

The precipitates were 4 to 5 times larger on the disloca-tions than in the matrix presumably as a result of enhancednucleation and growth because of assistance of pipe diffu-sion along the dislocation core G phase precipitates werenot observed in the as cast un aged material or in theaustenite in the aged material

Atom probe analysis and EDS analysis of extractionreplicas revealed that the precipitates in the aged CF8Mferrite were alloy silicides as shown in Table 3 Selectedarea electron diffraction patterns of these silicide precipi-tates (Fig 7b) were consistent with a fcc crystal structurewith some additional features Lattice parameters of 1middot09and 1middot11 nm were measured for the precipitates in the

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a

ba dark field TEM b electron diffraction pattern

7 G phase precipitates in ferrite of CF8M steel after aging for7500 h at 400degC

CF8M and CF8 alloys respectively The diffraction pat-terns also revealed that the precipitates had a cube on cubeorientation relationship with the ferrite matrix These smallsilicide precipitates in the ferrite phase were identified as G

Table 3 APFIM and EDS analysis of composition of G phaseprecipitates in CF8M steel aged for 7500 h at 400degCat-

Si Ni Fe Mo Cr C

APFIM27middot7 plusmn 3middot4 24middot0 plusmn 3middot2 20middot6 plusmn 3middot1 13middot0 plusmn 2middot5 12middot0 plusmn 2middot5 1middot0 plusmn 0middot7EDS20middot9 plusmn 2middot0 31middot1plusmn2middot2 10middot5 plusmn 1middot5 19middot9 plusmn 1middot1 17middot8 plusmn 2middot1 ND

ND not detected

phase The composition the fcc crystal structure with theweak or absent 220 and 400 and strong 333 reflections in theelectron diffraction patterns the cube on cube orientationrelationship with the ferrite matrix and the lattice para-meter all support this identification G phase is regarded asa complex silicide with a fcc crystal structure containing 116atoms per unit cell The model of G phase is based onNi16ShTi6 with various elements such as Cr Fe Mo Mn VNb Ta Hf and Zr substituting for the titanium andnicke12021 This substitution results in a series of G phaseswith variable composition and lattice parameter It shouldbe emphasised that without the characterisation of theCF8M steel the G phase in the CF8 alloy could have beenoverlooked because of its small size and weak contribution to the diffraction patterns

SPINODAL DECOMPOSITIONMore detailed analyses of the ferrite matrix in the CF8 steelby AEM and APFIM revealed that it had decomposedduring thermal aging Phase contrast electron micrographsof the structure in material aged at 300 and 400degC are shownin Fig lOa b The scale of this two phase modulated micro-structure was measured frqm these electron micrographs as4 and 9 nm in the 300 and 400degC aged materials respect-ively A field ion micrograph of the same two phase micro-structure in the material aged at 400degC is shown in Fig 10c

a b

8 Bimodal distribution of G phase precipitates in ferrite of CF8 steel aged at a 300degC and h C 400degC (TEM)

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290 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

abca 333 precipitate-reflection dark field TEM b weak beam dark field TEM c superposition of a and b

9 Coarser G phase precipitates associated with dislocations of CF8 steel aged at 400degC

ca b TEM c FIM showing brightly imaging a and darklyimaging a phases

10 Spinodal decomposition into a and a phases of CF8 steelaged at a 300degC and b C 400degC

Materials Science and Technology March 1990 Vol 6

where the darkly imaging a and the brightly imaging aphases are evident The periodicity of the modulations ofthe two phases was measured from FIM micrographs to be7 nm Field evaporation sequences revealed that themodulated microstructure was interconnected indicativeof phase separation by isotropic spinodal decomposition

Atom probe composition profiles through the ferrite alsoindicated that the ferrite had decomposed into a chromiumenriched a phase and an iron rich a phase A short sectionof an atom probe composition profile through the ferrite ofthe CF8 steel aged for 70 000 h at 400degC is shown in Fig 11The large amplitude fluctuations that are evident is indica-tive of phase separation on a fine scale Extended compos-ition profiles for both austenite and ferrite phases in theCF8 material were subjected to statistical analyses Theresults of these analyses are summarised in Table 4 Thesample distribution analysis autocorrelation function rbthe Johnson and Klotz (JK) Markov chain ordering para-meter () and the Hetherington and Miller mean separationmethod all indicated that the ferrite had phase separatedinto iron rich and chromium enriched regions and the chi-squared tests of the frequency distributions indicate that thesolute was not randomly distributed With the exception ofthe Pa sample distribution analysis the statistical analysis ofthe austenite indicated a random distribution of chromiumThe significance of Pa was much smaller for the austenite(3middot1) than for the ferrite (22)

Atom probe composition profiles were obtained from theferrite phase in the CF8M steel avoiding the silicide pre-cipitates and also revealed decomposition into the a and aphases as shown in Fig 12 The absolute compositions ofthe two phases are probably more extreme than those indi-cated from the composition profile since the probe aper-ture was larger than the extent of the chromium enrichedregions and therefore some averaging of the composition ofthe two phases occurred The phase separation was notresolved in the field ion micrographs partly because of theextremely fine scale (2 nm) and partly because the silicideprecipitates altered the local imaging conditions

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2 3 4 5 6 7DISTANCE (nm)

12 Atom probe composition profile in ferrite phase of agedCF8M steel showing phase separation into iron rich a andchromium enriched a phases

70 60

60 AGED 70 OOOh AT 400degC AGED 7500h AT 4000C50

50fl ~

40

~40 E~ 2 30~30

E0

I 00 r

20u20

10 10

0 o 100 200 300DISTANCE - 50 ION BLOCKS

11 Section of atom probe composition profile throughferrite phase of CF8 steel aged for 70 000 h at 400degCshowing phase separation into iron rich a andchromium enriched a phases

Discussion

The microstructures of the CF8 and CF8M steels weresimilar In both types of steel the ferrite spino dally decom-posed into an isotropic network of a and a phases and Gphase precipitates The major difference between the twotypes of steel was the size and volume fraction of the Gphase precipitates In the CF8M material aged for 7500 hat400degC the G phase precipitates exhibited a much largervolume fraction compared with the CF8 steel that was alsoaged at 400degC but for almost 10 times longer This largervolume of G phase is related to the differences in initialcomposition between the two alloys The G phase silicide isrich in nickel and molybdenum which were present athigher levels in the CF8M steel than in the CF8 steelAlthough a small fraction of the G phase precipitates wasobserved pinning dislocations it should be noted that theseresidual dislocations will not be of the same type or behavein the similar manner as those generated during furtherdeformation G phase precipitates on dislocations in ferritehave also been observed by Vitek22 in similar steels Areduction in the levels of silicon nickel and perhapsmolybdenum in the ferrite would reduce or even suppressthe amount of G phase that is precipitated

The composition of the ferrite phase is in the range wherea miscibility gap exists at low temperatures Phase separa-tion of the ferrite into a chromium enriched phase and aniron rich phase is similar to that observed in Fe-Cr andmany Fe-Cr-X systems which undergo isotropic spinodaldecomposition within a miscibility gap under certain condi-tions Relatively small changes in the chromium and molyb-denum levels in the ferrite will alter the position in themiscibility gap and therefore the volume fraction of the ironrich and chromium enriched phases This could affect themorphology of the transformation products and hence alterthe mechanical properties and aging behaviour However

changing the chromium content of the alloy does not neces-sarily change the composition of the ferrite it may merelyalter the quantity of ferrite

The fine scale spinodal decomposition and the G phaseprecipitation in the ferrite bo~h contribute to the changes inmechanical properties that occur during aging Howeversince the volume of G phase was much lower in the CF8than inmiddot the CF8M steel and the behaviour of the steelsis similar the degradation in mechanical properties isprimarily due to the spinodal decomposition of the ferriteduring aging In addition the observation that the increasein hardness in the CF8 and CF8M steels is similar to thatpreviously observed in a spino dally decomposed Fe-30Cralloy which did not contain G phase23 also suggests thatspinodal decomposition is the primary factor influencingmechanical properties The results presented here arespecific to the material and heat treatment and may varyconsiderably in each individual casting depending on theprecise alloy composition and casting conditions

Conclusions

The results of this study have pointed out the complexityand the very fine scale of the decomposition processes thatoccur at low temperatures in CF8 and CF8M type stainlesssteels This study has also shown that the near atomicresolution of the atom probe was able to detect and quantifythe extremely fine scale phase separation that occurredBoth APFIM and AEM results indicate that the chromiumenriched ferrite had decomposed into a very fine network ofchromium enriched a and iron rich a phases as a result ofisotropic spinodal decomposition Coarse M23C6 precipi-tates were observed at the ferriteaustenite interface in theCF8 steel aged at 400degC Very fine G phase silicide precipi-tates were observed in the ferrite A comparison betweenthe results from the CF8 and CF8M steels indicates that

Table 4 Summary of statistical analysis of atom probe data for CF8 stainless steel aged for 70 000 h at400degC

Sample J and K Markov Frequencydistribution Autocorrelation chain Mean distributionanalysis function separation

Phase Pa r () Sig Sig X2 DF

Ferrite 0middot066 plusmn 0middot003 0middot467 plusmn 0middot06 1middot129 4middot26 6middot35 43middot0 22Austenite 0middot028 plusmn 0middot009 0middot24 plusmn 0middot09 1middot011 0middot34 1middot65 19middot1 13

Sig significance DF degrees of freedom

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292 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

relatively small differences in the alloy compositions signifi-cantly alter the quantity of G phase present in the micro-structure The degradation in mechanical properties is aconsequence of the spinodal decomposition of the ferritetogether with G phase precipitation that occurs duringaging

Acknowledgments

This research was sponsored by the Division of MaterialsSciences US Department of Energy under contractDE-AC05-840R21400 with Martin Marietta EnergySystems Inc The authors would like to thankDr H M Chung and Dr O K Chopra of ArgonneNational Laboratory for supplying the aged CF8 steelsDr J A Spitznagel of Westinghouse RampD CenterPittsburgh PA for supplying the CF8M alloyDr M G Hetherington of the University of Oxfordfor providing the sample distribution softwareDr G D W Smith of the University of Oxford for useof the atom probe for some of the analyses andMs K F Russell for her technical assistance

References

1 o K CHOPRAand H M CHUNGin Proc 13th Water ReactorSafety Research Information Meeting 1985 GaithersburgMD US National Bureau of Standards

2 H M CHUNGand o K CHOPRAin Proc 2nd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (ed J T Roberts et al) 287-2921985 Monterey CA American Nuclear Society

3 H M CHUNGand o K CHOPRAProperties of stainless steels inelevated temperature service (ed M Prager) PVP-ASMEVol 132 MPC Vol 26 17-34 1987 New YorkASMEjMaterials Properties Council

4 M K MILLERJ BENTLEYS s BRENNERand J A SPITZNAGELJPhys 1984 45-C9 385-390

5 M K MILLERand 1 BENTLEYJ Phys 1986 47-C7 239-244

6 M K MILLER1 BENTLEYS s BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 326-327 1985 SanFrancisco San Francisco Press

7 1 BENTLEYM K MILLERS S BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 328-329 1985 SanFrancisco San Francisco Press

8 1 BENTLEYand M K MILLERin Analytical electron micros-copy (ed D C Joy) 73-75 1987 San Francisco San Fran-cisco Press

9 M K MILLERand 1 BENTLEYin Proc 3rd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (eds G J Theus and J R Weeks)341-349 1988 Pittsburgh PA TMS

10 1 BENTLEYand M K MILLERin MRS Symp Characterizationof defects in materials Vol 82 (eds R W Siegeletal) 163-168 1987 Pittsburgh PA Materials Research Society

11 T J GODFREYand G D W SMITHJ Phys 1986 47-C7 217-222

12 T1 GODFREYM G HETHERINGTON1 M SASSENand G D WSMITHJ Phys 1988 49-C6 421-426

13 1 M SASSENM G HETHERINGTONT 1 GODFREYG D W SMITHP H PUMPHREYand K N AKHURSTin Properties of stainlesssteels in elevated temperature service (ed M Prager) PVP-ASME Vol 132 MPC Vol 26 65-78 1987 New YorkASMEjMaterials Properties Council

14 G SLAMA P PETREQUINS H MASSONand T MAGERinStructural Mechanics in Reactor Technology (SMIRT) Post-conference Seminar 6 Assuring structural integrity ofsteel reactor pressure boundary components MontereyCA August 1983

15 M K MILLERJ Phys 1986 47-C2 493-498 and 499-50416 M K MILLERInt Mater Rev 198732221-24017 M K MILLERand G D W SMITHAtom probe microanalysis

principles and applications to materials problems 1989Pittsburgh PA Materials Research Society

18 c A JOHNSONand 1 H KLOTZTeehnometries 1974 16 483-493

19 M G HETHERINGTONand M K MILLERJ Phys 1987 48-C6559-561

20 F X SPIEGELD BARDOSand P A BECKTrans AIME 1963227 575

21 E H LEEP 1 MAZIASZandA F ROWCLIFFEin Phase stabilityduring irradiation (ed J R Holland et al) 191-218 1981Warrendale PA The Metallurgical Society of AIME

22 1 M VITEKMetall Trans 1987 18A 154-15623 s S BRENNERM K MILLERand w A SOFFASer Metall 1982

16 831-836

IRONMAKING AND STEELMAKING(alternate-monthly)

Provides international coverage of all aspects of iron and steelmaking including therolling and application of ferrous products1990 Subscription Rates pound9500 US$22500 Members pound5400 US$10800

Orders with remittance to The Institute of Metals Sales amp Marketing Dept 1 CarltonHouse Terrace London SW1Y 5DB Tel 071-976 1338 Fax 071-839 2078

Materials Science and Technology March 1990 Vol 6

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 289

a

ba dark field TEM b electron diffraction pattern

7 G phase precipitates in ferrite of CF8M steel after aging for7500 h at 400degC

CF8M and CF8 alloys respectively The diffraction pat-terns also revealed that the precipitates had a cube on cubeorientation relationship with the ferrite matrix These smallsilicide precipitates in the ferrite phase were identified as G

Table 3 APFIM and EDS analysis of composition of G phaseprecipitates in CF8M steel aged for 7500 h at 400degCat-

Si Ni Fe Mo Cr C

APFIM27middot7 plusmn 3middot4 24middot0 plusmn 3middot2 20middot6 plusmn 3middot1 13middot0 plusmn 2middot5 12middot0 plusmn 2middot5 1middot0 plusmn 0middot7EDS20middot9 plusmn 2middot0 31middot1plusmn2middot2 10middot5 plusmn 1middot5 19middot9 plusmn 1middot1 17middot8 plusmn 2middot1 ND

ND not detected

phase The composition the fcc crystal structure with theweak or absent 220 and 400 and strong 333 reflections in theelectron diffraction patterns the cube on cube orientationrelationship with the ferrite matrix and the lattice para-meter all support this identification G phase is regarded asa complex silicide with a fcc crystal structure containing 116atoms per unit cell The model of G phase is based onNi16ShTi6 with various elements such as Cr Fe Mo Mn VNb Ta Hf and Zr substituting for the titanium andnicke12021 This substitution results in a series of G phaseswith variable composition and lattice parameter It shouldbe emphasised that without the characterisation of theCF8M steel the G phase in the CF8 alloy could have beenoverlooked because of its small size and weak contribution to the diffraction patterns

SPINODAL DECOMPOSITIONMore detailed analyses of the ferrite matrix in the CF8 steelby AEM and APFIM revealed that it had decomposedduring thermal aging Phase contrast electron micrographsof the structure in material aged at 300 and 400degC are shownin Fig lOa b The scale of this two phase modulated micro-structure was measured frqm these electron micrographs as4 and 9 nm in the 300 and 400degC aged materials respect-ively A field ion micrograph of the same two phase micro-structure in the material aged at 400degC is shown in Fig 10c

a b

8 Bimodal distribution of G phase precipitates in ferrite of CF8 steel aged at a 300degC and h C 400degC (TEM)

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290 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

abca 333 precipitate-reflection dark field TEM b weak beam dark field TEM c superposition of a and b

9 Coarser G phase precipitates associated with dislocations of CF8 steel aged at 400degC

ca b TEM c FIM showing brightly imaging a and darklyimaging a phases

10 Spinodal decomposition into a and a phases of CF8 steelaged at a 300degC and b C 400degC

Materials Science and Technology March 1990 Vol 6

where the darkly imaging a and the brightly imaging aphases are evident The periodicity of the modulations ofthe two phases was measured from FIM micrographs to be7 nm Field evaporation sequences revealed that themodulated microstructure was interconnected indicativeof phase separation by isotropic spinodal decomposition

Atom probe composition profiles through the ferrite alsoindicated that the ferrite had decomposed into a chromiumenriched a phase and an iron rich a phase A short sectionof an atom probe composition profile through the ferrite ofthe CF8 steel aged for 70 000 h at 400degC is shown in Fig 11The large amplitude fluctuations that are evident is indica-tive of phase separation on a fine scale Extended compos-ition profiles for both austenite and ferrite phases in theCF8 material were subjected to statistical analyses Theresults of these analyses are summarised in Table 4 Thesample distribution analysis autocorrelation function rbthe Johnson and Klotz (JK) Markov chain ordering para-meter () and the Hetherington and Miller mean separationmethod all indicated that the ferrite had phase separatedinto iron rich and chromium enriched regions and the chi-squared tests of the frequency distributions indicate that thesolute was not randomly distributed With the exception ofthe Pa sample distribution analysis the statistical analysis ofthe austenite indicated a random distribution of chromiumThe significance of Pa was much smaller for the austenite(3middot1) than for the ferrite (22)

Atom probe composition profiles were obtained from theferrite phase in the CF8M steel avoiding the silicide pre-cipitates and also revealed decomposition into the a and aphases as shown in Fig 12 The absolute compositions ofthe two phases are probably more extreme than those indi-cated from the composition profile since the probe aper-ture was larger than the extent of the chromium enrichedregions and therefore some averaging of the composition ofthe two phases occurred The phase separation was notresolved in the field ion micrographs partly because of theextremely fine scale (2 nm) and partly because the silicideprecipitates altered the local imaging conditions

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 291

2 3 4 5 6 7DISTANCE (nm)

12 Atom probe composition profile in ferrite phase of agedCF8M steel showing phase separation into iron rich a andchromium enriched a phases

70 60

60 AGED 70 OOOh AT 400degC AGED 7500h AT 4000C50

50fl ~

40

~40 E~ 2 30~30

E0

I 00 r

20u20

10 10

0 o 100 200 300DISTANCE - 50 ION BLOCKS

11 Section of atom probe composition profile throughferrite phase of CF8 steel aged for 70 000 h at 400degCshowing phase separation into iron rich a andchromium enriched a phases

Discussion

The microstructures of the CF8 and CF8M steels weresimilar In both types of steel the ferrite spino dally decom-posed into an isotropic network of a and a phases and Gphase precipitates The major difference between the twotypes of steel was the size and volume fraction of the Gphase precipitates In the CF8M material aged for 7500 hat400degC the G phase precipitates exhibited a much largervolume fraction compared with the CF8 steel that was alsoaged at 400degC but for almost 10 times longer This largervolume of G phase is related to the differences in initialcomposition between the two alloys The G phase silicide isrich in nickel and molybdenum which were present athigher levels in the CF8M steel than in the CF8 steelAlthough a small fraction of the G phase precipitates wasobserved pinning dislocations it should be noted that theseresidual dislocations will not be of the same type or behavein the similar manner as those generated during furtherdeformation G phase precipitates on dislocations in ferritehave also been observed by Vitek22 in similar steels Areduction in the levels of silicon nickel and perhapsmolybdenum in the ferrite would reduce or even suppressthe amount of G phase that is precipitated

The composition of the ferrite phase is in the range wherea miscibility gap exists at low temperatures Phase separa-tion of the ferrite into a chromium enriched phase and aniron rich phase is similar to that observed in Fe-Cr andmany Fe-Cr-X systems which undergo isotropic spinodaldecomposition within a miscibility gap under certain condi-tions Relatively small changes in the chromium and molyb-denum levels in the ferrite will alter the position in themiscibility gap and therefore the volume fraction of the ironrich and chromium enriched phases This could affect themorphology of the transformation products and hence alterthe mechanical properties and aging behaviour However

changing the chromium content of the alloy does not neces-sarily change the composition of the ferrite it may merelyalter the quantity of ferrite

The fine scale spinodal decomposition and the G phaseprecipitation in the ferrite bo~h contribute to the changes inmechanical properties that occur during aging Howeversince the volume of G phase was much lower in the CF8than inmiddot the CF8M steel and the behaviour of the steelsis similar the degradation in mechanical properties isprimarily due to the spinodal decomposition of the ferriteduring aging In addition the observation that the increasein hardness in the CF8 and CF8M steels is similar to thatpreviously observed in a spino dally decomposed Fe-30Cralloy which did not contain G phase23 also suggests thatspinodal decomposition is the primary factor influencingmechanical properties The results presented here arespecific to the material and heat treatment and may varyconsiderably in each individual casting depending on theprecise alloy composition and casting conditions

Conclusions

The results of this study have pointed out the complexityand the very fine scale of the decomposition processes thatoccur at low temperatures in CF8 and CF8M type stainlesssteels This study has also shown that the near atomicresolution of the atom probe was able to detect and quantifythe extremely fine scale phase separation that occurredBoth APFIM and AEM results indicate that the chromiumenriched ferrite had decomposed into a very fine network ofchromium enriched a and iron rich a phases as a result ofisotropic spinodal decomposition Coarse M23C6 precipi-tates were observed at the ferriteaustenite interface in theCF8 steel aged at 400degC Very fine G phase silicide precipi-tates were observed in the ferrite A comparison betweenthe results from the CF8 and CF8M steels indicates that

Table 4 Summary of statistical analysis of atom probe data for CF8 stainless steel aged for 70 000 h at400degC

Sample J and K Markov Frequencydistribution Autocorrelation chain Mean distributionanalysis function separation

Phase Pa r () Sig Sig X2 DF

Ferrite 0middot066 plusmn 0middot003 0middot467 plusmn 0middot06 1middot129 4middot26 6middot35 43middot0 22Austenite 0middot028 plusmn 0middot009 0middot24 plusmn 0middot09 1middot011 0middot34 1middot65 19middot1 13

Sig significance DF degrees of freedom

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292 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

relatively small differences in the alloy compositions signifi-cantly alter the quantity of G phase present in the micro-structure The degradation in mechanical properties is aconsequence of the spinodal decomposition of the ferritetogether with G phase precipitation that occurs duringaging

Acknowledgments

This research was sponsored by the Division of MaterialsSciences US Department of Energy under contractDE-AC05-840R21400 with Martin Marietta EnergySystems Inc The authors would like to thankDr H M Chung and Dr O K Chopra of ArgonneNational Laboratory for supplying the aged CF8 steelsDr J A Spitznagel of Westinghouse RampD CenterPittsburgh PA for supplying the CF8M alloyDr M G Hetherington of the University of Oxfordfor providing the sample distribution softwareDr G D W Smith of the University of Oxford for useof the atom probe for some of the analyses andMs K F Russell for her technical assistance

References

1 o K CHOPRAand H M CHUNGin Proc 13th Water ReactorSafety Research Information Meeting 1985 GaithersburgMD US National Bureau of Standards

2 H M CHUNGand o K CHOPRAin Proc 2nd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (ed J T Roberts et al) 287-2921985 Monterey CA American Nuclear Society

3 H M CHUNGand o K CHOPRAProperties of stainless steels inelevated temperature service (ed M Prager) PVP-ASMEVol 132 MPC Vol 26 17-34 1987 New YorkASMEjMaterials Properties Council

4 M K MILLERJ BENTLEYS s BRENNERand J A SPITZNAGELJPhys 1984 45-C9 385-390

5 M K MILLERand 1 BENTLEYJ Phys 1986 47-C7 239-244

6 M K MILLER1 BENTLEYS s BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 326-327 1985 SanFrancisco San Francisco Press

7 1 BENTLEYM K MILLERS S BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 328-329 1985 SanFrancisco San Francisco Press

8 1 BENTLEYand M K MILLERin Analytical electron micros-copy (ed D C Joy) 73-75 1987 San Francisco San Fran-cisco Press

9 M K MILLERand 1 BENTLEYin Proc 3rd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (eds G J Theus and J R Weeks)341-349 1988 Pittsburgh PA TMS

10 1 BENTLEYand M K MILLERin MRS Symp Characterizationof defects in materials Vol 82 (eds R W Siegeletal) 163-168 1987 Pittsburgh PA Materials Research Society

11 T J GODFREYand G D W SMITHJ Phys 1986 47-C7 217-222

12 T1 GODFREYM G HETHERINGTON1 M SASSENand G D WSMITHJ Phys 1988 49-C6 421-426

13 1 M SASSENM G HETHERINGTONT 1 GODFREYG D W SMITHP H PUMPHREYand K N AKHURSTin Properties of stainlesssteels in elevated temperature service (ed M Prager) PVP-ASME Vol 132 MPC Vol 26 65-78 1987 New YorkASMEjMaterials Properties Council

14 G SLAMA P PETREQUINS H MASSONand T MAGERinStructural Mechanics in Reactor Technology (SMIRT) Post-conference Seminar 6 Assuring structural integrity ofsteel reactor pressure boundary components MontereyCA August 1983

15 M K MILLERJ Phys 1986 47-C2 493-498 and 499-50416 M K MILLERInt Mater Rev 198732221-24017 M K MILLERand G D W SMITHAtom probe microanalysis

principles and applications to materials problems 1989Pittsburgh PA Materials Research Society

18 c A JOHNSONand 1 H KLOTZTeehnometries 1974 16 483-493

19 M G HETHERINGTONand M K MILLERJ Phys 1987 48-C6559-561

20 F X SPIEGELD BARDOSand P A BECKTrans AIME 1963227 575

21 E H LEEP 1 MAZIASZandA F ROWCLIFFEin Phase stabilityduring irradiation (ed J R Holland et al) 191-218 1981Warrendale PA The Metallurgical Society of AIME

22 1 M VITEKMetall Trans 1987 18A 154-15623 s S BRENNERM K MILLERand w A SOFFASer Metall 1982

16 831-836

IRONMAKING AND STEELMAKING(alternate-monthly)

Provides international coverage of all aspects of iron and steelmaking including therolling and application of ferrous products1990 Subscription Rates pound9500 US$22500 Members pound5400 US$10800

Orders with remittance to The Institute of Metals Sales amp Marketing Dept 1 CarltonHouse Terrace London SW1Y 5DB Tel 071-976 1338 Fax 071-839 2078

Materials Science and Technology March 1990 Vol 6

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290 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

abca 333 precipitate-reflection dark field TEM b weak beam dark field TEM c superposition of a and b

9 Coarser G phase precipitates associated with dislocations of CF8 steel aged at 400degC

ca b TEM c FIM showing brightly imaging a and darklyimaging a phases

10 Spinodal decomposition into a and a phases of CF8 steelaged at a 300degC and b C 400degC

Materials Science and Technology March 1990 Vol 6

where the darkly imaging a and the brightly imaging aphases are evident The periodicity of the modulations ofthe two phases was measured from FIM micrographs to be7 nm Field evaporation sequences revealed that themodulated microstructure was interconnected indicativeof phase separation by isotropic spinodal decomposition

Atom probe composition profiles through the ferrite alsoindicated that the ferrite had decomposed into a chromiumenriched a phase and an iron rich a phase A short sectionof an atom probe composition profile through the ferrite ofthe CF8 steel aged for 70 000 h at 400degC is shown in Fig 11The large amplitude fluctuations that are evident is indica-tive of phase separation on a fine scale Extended compos-ition profiles for both austenite and ferrite phases in theCF8 material were subjected to statistical analyses Theresults of these analyses are summarised in Table 4 Thesample distribution analysis autocorrelation function rbthe Johnson and Klotz (JK) Markov chain ordering para-meter () and the Hetherington and Miller mean separationmethod all indicated that the ferrite had phase separatedinto iron rich and chromium enriched regions and the chi-squared tests of the frequency distributions indicate that thesolute was not randomly distributed With the exception ofthe Pa sample distribution analysis the statistical analysis ofthe austenite indicated a random distribution of chromiumThe significance of Pa was much smaller for the austenite(3middot1) than for the ferrite (22)

Atom probe composition profiles were obtained from theferrite phase in the CF8M steel avoiding the silicide pre-cipitates and also revealed decomposition into the a and aphases as shown in Fig 12 The absolute compositions ofthe two phases are probably more extreme than those indi-cated from the composition profile since the probe aper-ture was larger than the extent of the chromium enrichedregions and therefore some averaging of the composition ofthe two phases occurred The phase separation was notresolved in the field ion micrographs partly because of theextremely fine scale (2 nm) and partly because the silicideprecipitates altered the local imaging conditions

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Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 291

2 3 4 5 6 7DISTANCE (nm)

12 Atom probe composition profile in ferrite phase of agedCF8M steel showing phase separation into iron rich a andchromium enriched a phases

70 60

60 AGED 70 OOOh AT 400degC AGED 7500h AT 4000C50

50fl ~

40

~40 E~ 2 30~30

E0

I 00 r

20u20

10 10

0 o 100 200 300DISTANCE - 50 ION BLOCKS

11 Section of atom probe composition profile throughferrite phase of CF8 steel aged for 70 000 h at 400degCshowing phase separation into iron rich a andchromium enriched a phases

Discussion

The microstructures of the CF8 and CF8M steels weresimilar In both types of steel the ferrite spino dally decom-posed into an isotropic network of a and a phases and Gphase precipitates The major difference between the twotypes of steel was the size and volume fraction of the Gphase precipitates In the CF8M material aged for 7500 hat400degC the G phase precipitates exhibited a much largervolume fraction compared with the CF8 steel that was alsoaged at 400degC but for almost 10 times longer This largervolume of G phase is related to the differences in initialcomposition between the two alloys The G phase silicide isrich in nickel and molybdenum which were present athigher levels in the CF8M steel than in the CF8 steelAlthough a small fraction of the G phase precipitates wasobserved pinning dislocations it should be noted that theseresidual dislocations will not be of the same type or behavein the similar manner as those generated during furtherdeformation G phase precipitates on dislocations in ferritehave also been observed by Vitek22 in similar steels Areduction in the levels of silicon nickel and perhapsmolybdenum in the ferrite would reduce or even suppressthe amount of G phase that is precipitated

The composition of the ferrite phase is in the range wherea miscibility gap exists at low temperatures Phase separa-tion of the ferrite into a chromium enriched phase and aniron rich phase is similar to that observed in Fe-Cr andmany Fe-Cr-X systems which undergo isotropic spinodaldecomposition within a miscibility gap under certain condi-tions Relatively small changes in the chromium and molyb-denum levels in the ferrite will alter the position in themiscibility gap and therefore the volume fraction of the ironrich and chromium enriched phases This could affect themorphology of the transformation products and hence alterthe mechanical properties and aging behaviour However

changing the chromium content of the alloy does not neces-sarily change the composition of the ferrite it may merelyalter the quantity of ferrite

The fine scale spinodal decomposition and the G phaseprecipitation in the ferrite bo~h contribute to the changes inmechanical properties that occur during aging Howeversince the volume of G phase was much lower in the CF8than inmiddot the CF8M steel and the behaviour of the steelsis similar the degradation in mechanical properties isprimarily due to the spinodal decomposition of the ferriteduring aging In addition the observation that the increasein hardness in the CF8 and CF8M steels is similar to thatpreviously observed in a spino dally decomposed Fe-30Cralloy which did not contain G phase23 also suggests thatspinodal decomposition is the primary factor influencingmechanical properties The results presented here arespecific to the material and heat treatment and may varyconsiderably in each individual casting depending on theprecise alloy composition and casting conditions

Conclusions

The results of this study have pointed out the complexityand the very fine scale of the decomposition processes thatoccur at low temperatures in CF8 and CF8M type stainlesssteels This study has also shown that the near atomicresolution of the atom probe was able to detect and quantifythe extremely fine scale phase separation that occurredBoth APFIM and AEM results indicate that the chromiumenriched ferrite had decomposed into a very fine network ofchromium enriched a and iron rich a phases as a result ofisotropic spinodal decomposition Coarse M23C6 precipi-tates were observed at the ferriteaustenite interface in theCF8 steel aged at 400degC Very fine G phase silicide precipi-tates were observed in the ferrite A comparison betweenthe results from the CF8 and CF8M steels indicates that

Table 4 Summary of statistical analysis of atom probe data for CF8 stainless steel aged for 70 000 h at400degC

Sample J and K Markov Frequencydistribution Autocorrelation chain Mean distributionanalysis function separation

Phase Pa r () Sig Sig X2 DF

Ferrite 0middot066 plusmn 0middot003 0middot467 plusmn 0middot06 1middot129 4middot26 6middot35 43middot0 22Austenite 0middot028 plusmn 0middot009 0middot24 plusmn 0middot09 1middot011 0middot34 1middot65 19middot1 13

Sig significance DF degrees of freedom

Materials Science and Technology March 1990 Vol 6

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292 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

relatively small differences in the alloy compositions signifi-cantly alter the quantity of G phase present in the micro-structure The degradation in mechanical properties is aconsequence of the spinodal decomposition of the ferritetogether with G phase precipitation that occurs duringaging

Acknowledgments

This research was sponsored by the Division of MaterialsSciences US Department of Energy under contractDE-AC05-840R21400 with Martin Marietta EnergySystems Inc The authors would like to thankDr H M Chung and Dr O K Chopra of ArgonneNational Laboratory for supplying the aged CF8 steelsDr J A Spitznagel of Westinghouse RampD CenterPittsburgh PA for supplying the CF8M alloyDr M G Hetherington of the University of Oxfordfor providing the sample distribution softwareDr G D W Smith of the University of Oxford for useof the atom probe for some of the analyses andMs K F Russell for her technical assistance

References

1 o K CHOPRAand H M CHUNGin Proc 13th Water ReactorSafety Research Information Meeting 1985 GaithersburgMD US National Bureau of Standards

2 H M CHUNGand o K CHOPRAin Proc 2nd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (ed J T Roberts et al) 287-2921985 Monterey CA American Nuclear Society

3 H M CHUNGand o K CHOPRAProperties of stainless steels inelevated temperature service (ed M Prager) PVP-ASMEVol 132 MPC Vol 26 17-34 1987 New YorkASMEjMaterials Properties Council

4 M K MILLERJ BENTLEYS s BRENNERand J A SPITZNAGELJPhys 1984 45-C9 385-390

5 M K MILLERand 1 BENTLEYJ Phys 1986 47-C7 239-244

6 M K MILLER1 BENTLEYS s BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 326-327 1985 SanFrancisco San Francisco Press

7 1 BENTLEYM K MILLERS S BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 328-329 1985 SanFrancisco San Francisco Press

8 1 BENTLEYand M K MILLERin Analytical electron micros-copy (ed D C Joy) 73-75 1987 San Francisco San Fran-cisco Press

9 M K MILLERand 1 BENTLEYin Proc 3rd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (eds G J Theus and J R Weeks)341-349 1988 Pittsburgh PA TMS

10 1 BENTLEYand M K MILLERin MRS Symp Characterizationof defects in materials Vol 82 (eds R W Siegeletal) 163-168 1987 Pittsburgh PA Materials Research Society

11 T J GODFREYand G D W SMITHJ Phys 1986 47-C7 217-222

12 T1 GODFREYM G HETHERINGTON1 M SASSENand G D WSMITHJ Phys 1988 49-C6 421-426

13 1 M SASSENM G HETHERINGTONT 1 GODFREYG D W SMITHP H PUMPHREYand K N AKHURSTin Properties of stainlesssteels in elevated temperature service (ed M Prager) PVP-ASME Vol 132 MPC Vol 26 65-78 1987 New YorkASMEjMaterials Properties Council

14 G SLAMA P PETREQUINS H MASSONand T MAGERinStructural Mechanics in Reactor Technology (SMIRT) Post-conference Seminar 6 Assuring structural integrity ofsteel reactor pressure boundary components MontereyCA August 1983

15 M K MILLERJ Phys 1986 47-C2 493-498 and 499-50416 M K MILLERInt Mater Rev 198732221-24017 M K MILLERand G D W SMITHAtom probe microanalysis

principles and applications to materials problems 1989Pittsburgh PA Materials Research Society

18 c A JOHNSONand 1 H KLOTZTeehnometries 1974 16 483-493

19 M G HETHERINGTONand M K MILLERJ Phys 1987 48-C6559-561

20 F X SPIEGELD BARDOSand P A BECKTrans AIME 1963227 575

21 E H LEEP 1 MAZIASZandA F ROWCLIFFEin Phase stabilityduring irradiation (ed J R Holland et al) 191-218 1981Warrendale PA The Metallurgical Society of AIME

22 1 M VITEKMetall Trans 1987 18A 154-15623 s S BRENNERM K MILLERand w A SOFFASer Metall 1982

16 831-836

IRONMAKING AND STEELMAKING(alternate-monthly)

Provides international coverage of all aspects of iron and steelmaking including therolling and application of ferrous products1990 Subscription Rates pound9500 US$22500 Members pound5400 US$10800

Orders with remittance to The Institute of Metals Sales amp Marketing Dept 1 CarltonHouse Terrace London SW1Y 5DB Tel 071-976 1338 Fax 071-839 2078

Materials Science and Technology March 1990 Vol 6

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels 291

2 3 4 5 6 7DISTANCE (nm)

12 Atom probe composition profile in ferrite phase of agedCF8M steel showing phase separation into iron rich a andchromium enriched a phases

70 60

60 AGED 70 OOOh AT 400degC AGED 7500h AT 4000C50

50fl ~

40

~40 E~ 2 30~30

E0

I 00 r

20u20

10 10

0 o 100 200 300DISTANCE - 50 ION BLOCKS

11 Section of atom probe composition profile throughferrite phase of CF8 steel aged for 70 000 h at 400degCshowing phase separation into iron rich a andchromium enriched a phases

Discussion

The microstructures of the CF8 and CF8M steels weresimilar In both types of steel the ferrite spino dally decom-posed into an isotropic network of a and a phases and Gphase precipitates The major difference between the twotypes of steel was the size and volume fraction of the Gphase precipitates In the CF8M material aged for 7500 hat400degC the G phase precipitates exhibited a much largervolume fraction compared with the CF8 steel that was alsoaged at 400degC but for almost 10 times longer This largervolume of G phase is related to the differences in initialcomposition between the two alloys The G phase silicide isrich in nickel and molybdenum which were present athigher levels in the CF8M steel than in the CF8 steelAlthough a small fraction of the G phase precipitates wasobserved pinning dislocations it should be noted that theseresidual dislocations will not be of the same type or behavein the similar manner as those generated during furtherdeformation G phase precipitates on dislocations in ferritehave also been observed by Vitek22 in similar steels Areduction in the levels of silicon nickel and perhapsmolybdenum in the ferrite would reduce or even suppressthe amount of G phase that is precipitated

The composition of the ferrite phase is in the range wherea miscibility gap exists at low temperatures Phase separa-tion of the ferrite into a chromium enriched phase and aniron rich phase is similar to that observed in Fe-Cr andmany Fe-Cr-X systems which undergo isotropic spinodaldecomposition within a miscibility gap under certain condi-tions Relatively small changes in the chromium and molyb-denum levels in the ferrite will alter the position in themiscibility gap and therefore the volume fraction of the ironrich and chromium enriched phases This could affect themorphology of the transformation products and hence alterthe mechanical properties and aging behaviour However

changing the chromium content of the alloy does not neces-sarily change the composition of the ferrite it may merelyalter the quantity of ferrite

The fine scale spinodal decomposition and the G phaseprecipitation in the ferrite bo~h contribute to the changes inmechanical properties that occur during aging Howeversince the volume of G phase was much lower in the CF8than inmiddot the CF8M steel and the behaviour of the steelsis similar the degradation in mechanical properties isprimarily due to the spinodal decomposition of the ferriteduring aging In addition the observation that the increasein hardness in the CF8 and CF8M steels is similar to thatpreviously observed in a spino dally decomposed Fe-30Cralloy which did not contain G phase23 also suggests thatspinodal decomposition is the primary factor influencingmechanical properties The results presented here arespecific to the material and heat treatment and may varyconsiderably in each individual casting depending on theprecise alloy composition and casting conditions

Conclusions

The results of this study have pointed out the complexityand the very fine scale of the decomposition processes thatoccur at low temperatures in CF8 and CF8M type stainlesssteels This study has also shown that the near atomicresolution of the atom probe was able to detect and quantifythe extremely fine scale phase separation that occurredBoth APFIM and AEM results indicate that the chromiumenriched ferrite had decomposed into a very fine network ofchromium enriched a and iron rich a phases as a result ofisotropic spinodal decomposition Coarse M23C6 precipi-tates were observed at the ferriteaustenite interface in theCF8 steel aged at 400degC Very fine G phase silicide precipi-tates were observed in the ferrite A comparison betweenthe results from the CF8 and CF8M steels indicates that

Table 4 Summary of statistical analysis of atom probe data for CF8 stainless steel aged for 70 000 h at400degC

Sample J and K Markov Frequencydistribution Autocorrelation chain Mean distributionanalysis function separation

Phase Pa r () Sig Sig X2 DF

Ferrite 0middot066 plusmn 0middot003 0middot467 plusmn 0middot06 1middot129 4middot26 6middot35 43middot0 22Austenite 0middot028 plusmn 0middot009 0middot24 plusmn 0middot09 1middot011 0middot34 1middot65 19middot1 13

Sig significance DF degrees of freedom

Materials Science and Technology March 1990 Vol 6

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ey P

ublis

hing

(c)

IOM

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mun

icat

ions

Ltd

292 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

relatively small differences in the alloy compositions signifi-cantly alter the quantity of G phase present in the micro-structure The degradation in mechanical properties is aconsequence of the spinodal decomposition of the ferritetogether with G phase precipitation that occurs duringaging

Acknowledgments

This research was sponsored by the Division of MaterialsSciences US Department of Energy under contractDE-AC05-840R21400 with Martin Marietta EnergySystems Inc The authors would like to thankDr H M Chung and Dr O K Chopra of ArgonneNational Laboratory for supplying the aged CF8 steelsDr J A Spitznagel of Westinghouse RampD CenterPittsburgh PA for supplying the CF8M alloyDr M G Hetherington of the University of Oxfordfor providing the sample distribution softwareDr G D W Smith of the University of Oxford for useof the atom probe for some of the analyses andMs K F Russell for her technical assistance

References

1 o K CHOPRAand H M CHUNGin Proc 13th Water ReactorSafety Research Information Meeting 1985 GaithersburgMD US National Bureau of Standards

2 H M CHUNGand o K CHOPRAin Proc 2nd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (ed J T Roberts et al) 287-2921985 Monterey CA American Nuclear Society

3 H M CHUNGand o K CHOPRAProperties of stainless steels inelevated temperature service (ed M Prager) PVP-ASMEVol 132 MPC Vol 26 17-34 1987 New YorkASMEjMaterials Properties Council

4 M K MILLERJ BENTLEYS s BRENNERand J A SPITZNAGELJPhys 1984 45-C9 385-390

5 M K MILLERand 1 BENTLEYJ Phys 1986 47-C7 239-244

6 M K MILLER1 BENTLEYS s BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 326-327 1985 SanFrancisco San Francisco Press

7 1 BENTLEYM K MILLERS S BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 328-329 1985 SanFrancisco San Francisco Press

8 1 BENTLEYand M K MILLERin Analytical electron micros-copy (ed D C Joy) 73-75 1987 San Francisco San Fran-cisco Press

9 M K MILLERand 1 BENTLEYin Proc 3rd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (eds G J Theus and J R Weeks)341-349 1988 Pittsburgh PA TMS

10 1 BENTLEYand M K MILLERin MRS Symp Characterizationof defects in materials Vol 82 (eds R W Siegeletal) 163-168 1987 Pittsburgh PA Materials Research Society

11 T J GODFREYand G D W SMITHJ Phys 1986 47-C7 217-222

12 T1 GODFREYM G HETHERINGTON1 M SASSENand G D WSMITHJ Phys 1988 49-C6 421-426

13 1 M SASSENM G HETHERINGTONT 1 GODFREYG D W SMITHP H PUMPHREYand K N AKHURSTin Properties of stainlesssteels in elevated temperature service (ed M Prager) PVP-ASME Vol 132 MPC Vol 26 65-78 1987 New YorkASMEjMaterials Properties Council

14 G SLAMA P PETREQUINS H MASSONand T MAGERinStructural Mechanics in Reactor Technology (SMIRT) Post-conference Seminar 6 Assuring structural integrity ofsteel reactor pressure boundary components MontereyCA August 1983

15 M K MILLERJ Phys 1986 47-C2 493-498 and 499-50416 M K MILLERInt Mater Rev 198732221-24017 M K MILLERand G D W SMITHAtom probe microanalysis

principles and applications to materials problems 1989Pittsburgh PA Materials Research Society

18 c A JOHNSONand 1 H KLOTZTeehnometries 1974 16 483-493

19 M G HETHERINGTONand M K MILLERJ Phys 1987 48-C6559-561

20 F X SPIEGELD BARDOSand P A BECKTrans AIME 1963227 575

21 E H LEEP 1 MAZIASZandA F ROWCLIFFEin Phase stabilityduring irradiation (ed J R Holland et al) 191-218 1981Warrendale PA The Metallurgical Society of AIME

22 1 M VITEKMetall Trans 1987 18A 154-15623 s S BRENNERM K MILLERand w A SOFFASer Metall 1982

16 831-836

IRONMAKING AND STEELMAKING(alternate-monthly)

Provides international coverage of all aspects of iron and steelmaking including therolling and application of ferrous products1990 Subscription Rates pound9500 US$22500 Members pound5400 US$10800

Orders with remittance to The Institute of Metals Sales amp Marketing Dept 1 CarltonHouse Terrace London SW1Y 5DB Tel 071-976 1338 Fax 071-839 2078

Materials Science and Technology March 1990 Vol 6

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

292 Miller and Bentley APFIM and AEM of CF8 and CF8M primary coolant pipe steels

relatively small differences in the alloy compositions signifi-cantly alter the quantity of G phase present in the micro-structure The degradation in mechanical properties is aconsequence of the spinodal decomposition of the ferritetogether with G phase precipitation that occurs duringaging

Acknowledgments

This research was sponsored by the Division of MaterialsSciences US Department of Energy under contractDE-AC05-840R21400 with Martin Marietta EnergySystems Inc The authors would like to thankDr H M Chung and Dr O K Chopra of ArgonneNational Laboratory for supplying the aged CF8 steelsDr J A Spitznagel of Westinghouse RampD CenterPittsburgh PA for supplying the CF8M alloyDr M G Hetherington of the University of Oxfordfor providing the sample distribution softwareDr G D W Smith of the University of Oxford for useof the atom probe for some of the analyses andMs K F Russell for her technical assistance

References

1 o K CHOPRAand H M CHUNGin Proc 13th Water ReactorSafety Research Information Meeting 1985 GaithersburgMD US National Bureau of Standards

2 H M CHUNGand o K CHOPRAin Proc 2nd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (ed J T Roberts et al) 287-2921985 Monterey CA American Nuclear Society

3 H M CHUNGand o K CHOPRAProperties of stainless steels inelevated temperature service (ed M Prager) PVP-ASMEVol 132 MPC Vol 26 17-34 1987 New YorkASMEjMaterials Properties Council

4 M K MILLERJ BENTLEYS s BRENNERand J A SPITZNAGELJPhys 1984 45-C9 385-390

5 M K MILLERand 1 BENTLEYJ Phys 1986 47-C7 239-244

6 M K MILLER1 BENTLEYS s BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 326-327 1985 SanFrancisco San Francisco Press

7 1 BENTLEYM K MILLERS S BRENNERand 1 A SPITZNAGELinProc 43rd Annual Meeting of the Electron MicroscopySociety of America (ed G W Bailey) 328-329 1985 SanFrancisco San Francisco Press

8 1 BENTLEYand M K MILLERin Analytical electron micros-copy (ed D C Joy) 73-75 1987 San Francisco San Fran-cisco Press

9 M K MILLERand 1 BENTLEYin Proc 3rd Int Symp onEnvironmental degradation of materials in nuclear powersystems - water reactors (eds G J Theus and J R Weeks)341-349 1988 Pittsburgh PA TMS

10 1 BENTLEYand M K MILLERin MRS Symp Characterizationof defects in materials Vol 82 (eds R W Siegeletal) 163-168 1987 Pittsburgh PA Materials Research Society

11 T J GODFREYand G D W SMITHJ Phys 1986 47-C7 217-222

12 T1 GODFREYM G HETHERINGTON1 M SASSENand G D WSMITHJ Phys 1988 49-C6 421-426

13 1 M SASSENM G HETHERINGTONT 1 GODFREYG D W SMITHP H PUMPHREYand K N AKHURSTin Properties of stainlesssteels in elevated temperature service (ed M Prager) PVP-ASME Vol 132 MPC Vol 26 65-78 1987 New YorkASMEjMaterials Properties Council

14 G SLAMA P PETREQUINS H MASSONand T MAGERinStructural Mechanics in Reactor Technology (SMIRT) Post-conference Seminar 6 Assuring structural integrity ofsteel reactor pressure boundary components MontereyCA August 1983

15 M K MILLERJ Phys 1986 47-C2 493-498 and 499-50416 M K MILLERInt Mater Rev 198732221-24017 M K MILLERand G D W SMITHAtom probe microanalysis

principles and applications to materials problems 1989Pittsburgh PA Materials Research Society

18 c A JOHNSONand 1 H KLOTZTeehnometries 1974 16 483-493

19 M G HETHERINGTONand M K MILLERJ Phys 1987 48-C6559-561

20 F X SPIEGELD BARDOSand P A BECKTrans AIME 1963227 575

21 E H LEEP 1 MAZIASZandA F ROWCLIFFEin Phase stabilityduring irradiation (ed J R Holland et al) 191-218 1981Warrendale PA The Metallurgical Society of AIME

22 1 M VITEKMetall Trans 1987 18A 154-15623 s S BRENNERM K MILLERand w A SOFFASer Metall 1982

16 831-836

IRONMAKING AND STEELMAKING(alternate-monthly)

Provides international coverage of all aspects of iron and steelmaking including therolling and application of ferrous products1990 Subscription Rates pound9500 US$22500 Members pound5400 US$10800

Orders with remittance to The Institute of Metals Sales amp Marketing Dept 1 CarltonHouse Terrace London SW1Y 5DB Tel 071-976 1338 Fax 071-839 2078

Materials Science and Technology March 1990 Vol 6