coarsening process of coherent β ′ precipitates in fe–10...

Coarsening process of coherent b9 precipitatesin Fe–10 wt-%Ni–15 wt-%Al and Fe–10wt-%Ni–15 wt-%Al–1 wt-%Cu alloys

H. J. Rosales-Dorantes1, N. Cayetano-Castro2, V. M. Lopez-Hirata*1,M. L. Saucedo-Munoz1, D. Villegas-Cardenas1 and F. Hernandez-Santiago1

The coarsening process of the NiAl ordered b9 precipitates was studied in the isothermally aged

Fe–10 wt-%Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys. The aging treatments

at 750, 850 and 950uC caused the precipitation of the b9phase in a ferritic matrix. In general, the

coarsening process of Cu containing alloy was slower with an activation energy of 248 kJ mol21

compared to 194 kJ mol21 for the other alloy. However, the coarsening kinetics for the former

alloy was faster at 950uC. The precipitation hardening response was also better in the Cu

containing alloy, with a slower overaging stage. The coarsening kinetics followed the behaviour

predicted by the modified Lifshitz–Slyozov–Wagner theories for coarsening controlled by volume

diffusion.

Keywords: Fe–Ni–Al based alloys, Aging, b9 precipitates, Coarsening kinetics, Hardness

IntroductionThe precipitation of coherent b9 phase with an NiAlordered crystalline structure type confers good mechan-ical properties at high temperatures for the ferritic Fe–Ni–Al alloys.1–3 This makes this type of alloys to be apotential structural material at high temperatures as analternative to substitute austenitic steels or wroughtnickel base superalloys.1 The b9 precipitates are presentin different industrial alloys such as maraging steels4 andadvanced martensitic hot work steels.5 The coarseningprocess of these precipitates is an important issue tokeep the mechanical properties at high temperatures.6,7

Thus, the control of coarsening kinetics of precipitates iscrucial for the above purpose. The addition of alloyingelements is a good alternative to control the coarseningkinetics because of its effect on the interfacial energy,solubility of precipitates or atomic diffusion.8 Forinstance, the addition of Cu is expected to have, atleast, effect on the solubility since Cu has a lowsolubility in body centred cubic (bcc) Fe,9 and it has agood solubility in Ni and .4 at-% in Al. Therefore, asmall addition of Cu to the Fe–Ni–Al alloys is expectedto modify the coarsening kinetics of b9 precipitates. Noreports of the Cu addition effect on coarsening kineticsof b9 precipitates in Fe–Ni–Al alloys were found in theliterature.

Thus, the purpose of this work was to comparethe aging behaviour of Fe–10 wt-%Ni–15 wt-%Al and

Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys to analysethe effect of Cu addition on the coarsening kinetics of b9

precipitates.

ExperimentalFe–10 wt-%Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys were prepared in an electric arcfurnace under an argon atmosphere using high purity Fe(99?9%), Ni (99?99%), Al (99?7%) and Cu (99?999%).The chemical composition of alloys was checked byatomic absorption chemical analysis, and it was veryclose to the nominal one. The alloy ingots werehomogenised at 1100uC for 1 week. The alloy specimensof 30630610 mm were cut and encapsulated in aquartz tube under argon atmosphere. The solutiontreatment was conducted at 1100uC for 1 h, and theaging treatments were carried out at 750, 850 and 950uCfor times from 0?5 to 1500 h. The heat treated specimenswere prepared metallographically and etched with asolution of 2 vol.-% nitric acid in methanol and thenobserved with a scanning electron microscope (SEM)equipped with an energy dispersive spectrometer (EDS)at 25 kV. The X-ray diffraction analysis with Mo Ka

radiation of both the aged alloys was carried out inorder to confirm the presence of the ordered b9 phase.The equivalent circus radius of precipitates r wasevaluated by measuring the precipitate area measuredon SEM images using an image analysing system. Inorder to obtain reliable data, .200 measurements werecarried out in different areas of the specimens. Thesemeasurements mainly corresponded to precipitates witha cuboid shape. The volume fraction of precipitationwas determined using the area method in both the alloyspecimens aged at 750, 850 and 950uC for 25, 50, 100

1Instituto Politecnico Nacional, ESIQIE-ESIME-AZC, Mexico, D.F., Mexico2Instituto Potosino de Investigacion Cientıfica y Tecnologica A.C., SanLuis Potosı, Mexico

*Corresponding author, email [email protected]

1492

� 2013 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 14 January 2013; accepted 25 May 2013DOI 10.1179/1743284713Y.0000000315 Materials Science and Technology 2013 VOL 29 NO 12

and 200 h. The Vickers hardness, HV 0?1/12 s, wasdetermined in the aged specimens. The high angleannular dark field Scanning transmission electronmicroscopy (HAADF-STEM) image was obtained inthe aged Cu containing alloy with an FEI Tecnai F30microscope equipped with a field emission gun operatedat 300 kV. The elemental composition of precipitateswas determined by EDS with an EDAX spectrometerattached to the STEM.

Results

Microstructural evolutionThe SEM images of the microstructural evolution of theFe–10 wt-%Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys are shown in Figs. 1 and 2respectively for aging at 750, 850 and 950uC. The b9

phase is precipitated in the ferrite matrix. The presenceof b9 phase with an NiAl ordered crystalline structuretype was confirmed with the detection of superreflectionsfor this phase in the diffractograms of alloy specimensaged at 950uC for 25 h, shown in Fig. 3. In addition,

only the X-ray diffraction peaks of bcc ferrite are seen inboth the solution treated alloys. The morphology of theb9 precipitates seems to be rounded cuboids since theearly stages of aging (Figs. 1 and 2a and b). Precipitatesare aligned in relation to the matrix phase (Figs. 1 and2c). The alignment of precipitates in the ferritic matrix ismore evident as the aging time increases. The size ofprecipitates increases with the increase in aging time andtemperature, while the volume fraction of precipitatesdecreases with the increase in aging temperature. Thevolume fraction of precipitates was determined to beabout 0?28–0?29 at 750uC, 0?27–0?28 at 850uC and 0?26–0?27 at 950uC for the Fe–10 wt-%Ni–15 wt-%Al alloyaged up to 200 h, and 0?32–0?34 at 750uC, 0?30–0?32 at850uC and 0?28–0?30 at 950uC for the Cu containingalloy aged up to 200 h. The faces of rounded cuboidsremain flat for the aging of both alloys at 750 and 850uC(Figs. 1 and 2a–f ). Nevertheless, the morphology ofprecipitates is quite different at the aging temperature of950uC in both the aged alloys. That is, elongated platescan be seen in both the aged alloys (Figs. 1 and 2g–i).However, the elongated plates are formed by several

1 Images (SEM) of Fe–10 wt-%Ni–15 wt-%Al alloy aged at 750uC for a 25 h, b 200 h and c 500 h; at 850uC for d 25 h, e

200 h and f 500 h; and at 950uC for g 25 h, h 200 h and i 500 h

Rosales-Dorantes et al. Coarsening process of coherent b9 precipitates in alloys

Materials Science and Technology 2013 VOL 29 NO 12 1493

http://www.maneyonline.com/action/showImage?doi=10.1179/1743284713Y.0000000315&iName=master.img-000.jpg&w=472&h=427

cuboids in the Fe–10 mass-%Ni–15 mass-%Al alloy, butthis characteristic was not observed in the Cu containingalloy. These plates seem to be the product of acoalescence reaction between smaller cuboid particles

after prolonged aging. These plates are also aligned withrespect to the phase matrix. It is interesting to noticethat the end of plates is like a rounded tip for the agedFe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloy (Fig. 2g–i),while it is more or less flat for the other aged alloycomposition (Fig. 1g–i). This fact seems to suggest thatthe coherency between matrix and precipitates is lost inthe former alloy, and thus, the plate grows more easily inthat direction, causing a larger length of plates in thiscase. The alignment of precipitates on elastically softestdirections occurred faster in both the aged alloys, whichcan be attributed to the high energy caused by thecoherency strain between precipitates and matrix. Nosplitting of precipitates was observed to occur during theaging process in both the aged alloys. An abundantfraction of small precipitates was observed to occurduring the aging at 950uC in the Fe–10 wt-%Ni–15 wt-%Al alloy (Fig. 1i), which were formed after aging,during the cooling of specimen to room temperature.

Coarsening processThe time evolution of the average precipitate radiusduring the coarsening process is described by a t1/3

2 Images (SEM) of Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloy aged at 750uC for a 50 h, b 100 h and c 500 h; at 850uC for

d 25 h, e 100 h and f 200 h; and at 950uC for g 25 h, h 75 h and i 200 h

3 X-ray diffraction patterns of Fe–10 wt-%Ni–15 wt-%Al

and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys solution

treated and aged at 950uC for 25 h


1494 Materials Science and Technology 2013 VOL 29 NO 12



power law according to the Lifshitz–Slyozov–Wagner(LSW) and modified LSW theories10–12 for diffusioncontrolled coarsening. That is, the same power law isexpected to be followed for the coarsening of precipi-tates in alloys with either low or high volume fractionfor precipitates, low or high elastic strain energy,precipitate morphology different from spheres andternary alloys. Nevertheless, the precipitate size distri-bution is expected to be different from that predicted bythe LSW theory.

Thus, the t1/3 power law for precipitate coarsening wasverified using a linear regression analysis of a doublelogarithmic plot of the average equivalent circus radiusversus time. It is important to mention that theequivalent circus radius was measured for the cuboidprecipitates in almost all the aging conditions for bothalloys. In the case of Cu containing alloy aged at 950uC,the morphology of precipitates was elongated plates.The average time exponent n was determined to be closeto one-third for the two alloy compositions (seeTable 1). The exponent values in Table 1 are close tothe ideal time exponent n51/3 predicted for most of themodified coarsening theories.8 Thus, the variation of theaverage equivalent circus radius of coherent precipitateswith time was fitted by the method of the least squares tothe following equation8

r3zro3zkrt (1)

where ro and r are the average radius of precipitates atthe onset of coarsening and time t respectively, and kr isa rate constant. The ro and kr values were determinedfrom the linear regression analysis. The variation of the

b9 precipitates size expressed as r3{r3o with aging time is

shown in Fig. 4a and b for both the Fe–10 wt-%

Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cualloys respectively, aged at 750, 850 and 950uC. It can benoticed that the experimental data fit to a straight line foreach temperature.

The size distributions of precipitates are shown inFig. 5a–c for the Fe–10 wt-%Ni–15 wt-%Al alloy agedat 750uC for 50, 100 and 200 h respectively and inFig. 6a–c for the Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cualloy aged at 750uC for 25, 100 and 200 h respectively.For comparison, the shape invariant distribution func-tion of the LSW theory is also included in these figures.The probability density r2h(r) was determined with thefollowing equation6,7

r2h(r)~Ni(r,rzDr)P

Ni(r,rzDr)

r�

Dr(2)

where r* is the average radius of the particle, andNi(rzDr) represents the number of particles in a givenclass interval Dr. The normalised radius is defined as theratio of r/r*.

The size distributions of precipitates shown in Figs. 5and 6 are very close to those predicted by the LSWtheory; however, the size distributions became wider andmore symmetrical for aging treatments longer than200 h.

Aging curvesFigure 7 shows the aging curves for the Fe–10 wt-%Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cualloys respectively at 750, 850 and 950uC. The solutiontreated and subsequently quenched specimen showed thelowest hardness for both alloys. However, the initialhardness was higher in the Cu containing alloy becauseof the higher content of solutes (Fig. 7b). Additionally,the hardness peak is, in general, higher in the aged Cu

Table 1 Values of time exponent n, rate constant kr and energy activation Q for Fe–10 wt-%Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys

Alloy

n kr/nm3 h21

Activation energy Q/kJ mol21750uC 850uC 950uC 750uC 850uC 950uC

Fe–10 wt-%Ni–15 wt-%Al 0.32 0.27 0.28 1.16104 1.26105 4.46105 194Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu 0.29 0.27 0.28 5.56103 9.56104 1.76106 248

4 Plot of r3{r3o versus aging time of a Fe–10 wt-%Ni–15 wt-%Al and b Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys aged at

750, 850 and 950uC




containing alloy. This behaviour seems to be also relatedto the presence of copper, as will be shown in the nextsection.

Discussion

Coarsening growth kineticsAccording to Fig. 4, the growth kinetics of coarseningfollowed the behaviour predicted by the LSW theoryand modified LSW theories10 for coarsening controlledby volume diffusion. This fact also shows a goodagreement with the modified theory11 for the diffusion

controlled coarsening in ternary alloys, which predictsthat growth kinetics is similar to that of LSW theory.

The elastic stresses can come, for instance, from theinternal stress due to misfit between the precipitates andthe matrix. In the absence of this elastic stress, thecoarsening process is driven by the reduction ininterfacial energy, and the coarsening kinetics followedthe equation predicted by the LSW theory (equa-tion (1)). In the case of coherency between matrix andprecipitates, the coarsening process is driven by thedecrease in the sum of both the elastic and the interfacialenergies. High elastic strain energy may cause changes in

5 Size distribution of precipitates for Fe–10 wt-%Ni–15 wt-%Al alloy aged at 750uC for a 50, b 100 and c 200 h

6 Size distribution of precipitates for Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloy aged 750uC for a 25, b 100 and c 200 h

7 Aging curves for a Fe–10 wt-%Ni–15 wt-%Al and b Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys aged at 750, 850 and 950uC






shape and spatial correlation. Thornton et al.12 analysedthe effect of elastic stresses on the microstructuralevolution during coarsening. They found that if theelastic stress is absent, the morphology of coarsenedprecipitates is, in general, circular or spherical in thealloy system. In contrast, if the elastic stress is present,the precipitate shape is more like square or rectangular,cuboids or plates. Nevertheless, the coarsening kineticsfor the average particle size is described by a t1/3 powerlaw in a similar manner as stated in LSW and modifiedLSW theories.

The values of the rate constant kr were determinedfrom the slope of the straight lines in Fig. 4, and theseare also shown in Table 1 for both alloys. Theseconstants increase with aging temperature because ofthe higher atomic diffusion. The rate constant at 750 and850uC for the Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cualloy is lower than that observed in the Fe–10 wt-%Ni–15 wt-%Al alloy at the same temperatures, whichsuggests a higher coarsening resistance for the formeralloy; however, this coarsening behaviour changes at theaging temperature of 950uC. This behaviour may berelated to the loss of coherency between precipitates andmatrix, as explained above. The temperature depen-dence of the rate constant kr is expected to follow theArrhenius relationship. Thus, the activation energy forthe coarsening process was determined, from theArrhenius plot of kr, to be ,194 and 248 kJ mol21 forthe Fe–10 wt-%Ni–15 wt-%Al and Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys respectively. The activationenergy for the interdiffusion process reported in bcc Ferich Fe–Al alloys is between 188 and 234 kJ mol21,13

which is in the value magnitude found for the coarseningprocess in these alloys. Therefore, the coarsening kineticscan be confirmed to be controlled by volume diffusion.Additionally, the activation energy for the Fe–10 wt-%Ni–15 wt-%Al–1 mass-%Cu alloy is higher than thatdetermined in the Fe–10 wt-%Ni–15 wt-%Al alloy. Thissuggests that the coarsening is more difficult in the formeralloy, which confirms the growth kinetics shown in Fig. 4.

It was observed that the size distribution is broaderand lower than that predicted by the LSW theory in the

alloys aged for longer times, while it is more similar tothat predicted by the LSW theory for alloys aged forshorter times. This fact is in agreement with the expectedbehaviour of the modified LSW theories.10 The formerbehaviour is attributed to the high volume fraction ofprecipitates, which has been reported in the coarseningprocess of several alloy systems.14–16 The growth orshrinkage rate of an individual particle has beenobserved17 to depend not only on its normalised radiusbut also on its local environment. That is, a particlesurrounded by several larger particles will grow slower,or shrink faster, than a particle of the same size whoseneighbours are smaller. Thus, as the volume fractionincreases, the particle size distribution widens.

Hardening behaviourThe higher hardness peak was observed to occur in theCu containing alloy, and it seems to be associated withthe higher volume fraction of precipitates, 0?28–0?34, inthe Cu containing alloy than that of the Fe–10 wt-%Ni–15 wt-%Al alloy, 0?26–0?29. Additionally, the aging timefor causing overaging is also longer in this alloy. Thiscan also be explained by the presence of copper, whichretards the volume diffusion, and thus, the coarseningresistance is improved. Figure 8 shows a HAADF-STEM image of the 10Ni–15Al–1Cu alloys aged at750uC for 100 h. The corresponding HAADF-STEMEDS linescan profile is shown in Fig. 8b for thisspecimen. The Fe rich content of the ferritic matrix isevident. In contrast, the b9 precipitates are composed ofFe, Ni, Al and Cu. Furthermore, most of the Cu contentis located within the b9 precipitates. This fact verifies thehardening effect and coarsening delay described above.

ConclusionsThe small addition of copper to the Fe–10 wt-%Ni–15 wt-%Al alloy improves its coarsening resistance of b9

precipitates, as well as its response to precipitation hard-ening. The lower coarsening rate at 750 and 850uC and moreretarded overaging behaviour are related to the lowerinterdiffusion process in the Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloy as shown by a higher activation energy

8 a image (HAADF-STEM) of Fe–10 wt-%Ni–15 wt-%Al–1 wt-%Cu alloys aged at 750uC for 100 h and b its corresponding

HAADF-STEM EDS linescan profile




of 248 kJ mol21. The higher hardening behaviour is alsoattributed to the copper addition, which caused a slightlyhigher volume fraction of precipitates.

Acknowledgement

The authors wish to thank the financial support fromSIP-COFAA-IPN.

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coarsening process of coherent β ′ precipitates in fe–10...

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