Effect of Ternary Alloying Elements Additionon the Order-Disorder Transformation Temperaturesof B2-Type Ordered Fe-Al-X Intermetallics

MEHMET YILDIRIM, M. VEDAT AKDENIZ, and AMDULLA O. MEKHRABOV

The effect of alloying element additions on B2MA2 order-disorder phase transformation tem-peratures of B2-type ordered Fe0.5(Al1�nXn)0.5 intermetallics (X = Cr, Ni, Mo, Ta, Mn, Ti, andW) that readily form single-phase solid solution for X = 1 at. pct were investigated experi-mentally. It was shown that the type of the ternary substitutional alloying elements have aprofound effect on the variation of order-disorder transition temperature of Fe0.5(Al1�nXn)0.5alloys. Based on the magnitude of partial ordering energies of the Al-X and Fe-X atomic pairs,predicted normalized transition temperatures, DT/To, were verified experimentally. Besides thenormalized transition temperature, the relative partial ordering energy (RPOE) parameter, b,was also defined to estimate the extent of variation in B2MA2 order-disorder phase transfor-mation temperatures upon ternary alloying additions. The RPOE parameter, b, takes intoaccount both the effects of magnitude of partial ordering energies of Al-X and Fe-X atomicpairs and also the lattice site occupation preferences of X element atoms over B2-type orderedFe-Al sublattices. The alloying elements, which are preferentially distributed Fe sublattice sites,b > 0, and owing to b >> 1, are more effective in increasing order-disorder transformationtemperature in Fe-Al (B2) intermetallics. On the contrary, alloying elements having b<1 tendto decrease the transition temperature slightly relative to the binary FeAl intermetallic. Theexperimentally determined B2MA2 order-disorder transition temperatures are in good quali-tative or semiquantitative agreement with theoretical predictions for all X ternary alloyingelements. Accordingly, the present experimental results confirm the validity of the theoreticalmodel and calculations proposed in our previous study on the B2MA2 order-disorder transitiontemperatures of single-phase Fe0.5(Al1�nXn)0.5 intermetallics.

DOI: 10.1007/s11661-011-1059-3� The Minerals, Metals & Materials Society and ASM International 2012

I. INTRODUCTION

THE D03-type ordered Fe3Al and B2-type orderedFeAl alloys were investigated widely as suitable candi-dates for structural applications at high temperatures.[1,2]

Their high-temperature corrosion and oxidation resis-tance and their lower density are the advantages of ironaluminides compared to many other commercial Fe-basematerials such as cast iron and stainless steel.[1–8] Inaddition, iron aluminides offer relatively high meltingtemperatures, low material cost, and conservation ofstrategic elements such as chromium.[9–11] However, themajor drawbacks of iron aluminides are their poorductility and toughness at ambient temperatures, limitedfabricability, poor strength, and creep resistance.[4,8–11]

It was shown experimentally that the mechanicalproperties of Fe-Al intermetallics strongly depend ondeviation from alloy stoichiometry and type and contentof ternary alloying additions.[12] However, the specificphysical and mechanical properties of iron aluminidesare also attributed to the D03 or B2 type of long-rangeordered (LRO) superlattices[1,2] and the arrangement ofternary alloying element atoms in the submicro volumesof these ordered superstructures. Furthermore, thearrangement and distribution of ternary alloying addi-tions on the sublattices of these ordered superstructureshave a significant effect on the magnitude of order-orderand order-disorder phase transformation temperaturesof Fe-Al intermetallics.[13]

The effects of ternary alloying additions on order-disorder phase transformation temperature and thecharacteristics of atomic short-range order in varioustypes of ordered aluminides were analyzed by combiningthe statistico-thermodynamical theory of ordering withthe electronic theory of alloys in pseudo-potentialapproximation.[13–24] A good qualitative agreement withavailable experimental data in the literature wasobtained for L12-type, DO3-type, B2-type, and L10-type

ordered intermetallics of Ni3Al,[18] Fe3Al,[19–22]

FeAl,[13,23] and c-TiAl,[24] respectively. Although a greatdeal of experimental work was performed on the effects

MEHMET YILDIRIM, Postdoctoral Student, and M. VEDATAKDENIZ and AMDULLA O. MEKHRABOV, Professors, are withthe Novel Alloys Design and Development Laboratory (NOVALAB),Department of Metallurgical and Materials Engineering, Middle EastTechnicalUniversity, 06531Ankara, Turkey. Contact e-mail: akdeniz@metu.edu.tr

Manuscript submitted June 14, 2010.Article published online February 22, 2012

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JUNE 2012—1809

of ternary alloying additions for Ni3Al and Fe3Alintermetallics,[25–29] little information is provided inthe literature on the ordering characteristics, espe-cially B2MA2 order-disorder transformation tempera-tures, of Fe-Al-X alloy containing nearly 50 at. pctAl.[7,12,13]

Therefore, it is of research interest to investigate theeffect of ternary alloying element additions on theB2MA2 order-disorder transformation temperatures ofFe0.5(Al1�nXn)0.5 intermetallics with B2-type orderedstructure. Moreover, our effort was directed towardproviding experimental data to confirm the validity ofthe theoretical model proposed in our previous study forsingle-phase Fe0.5(Al1�nXn)0.5 ternary intermetallics.[13]

II. EXPERIMENTAL PROCEDURE

In the present study, the effects of ternary alloyingelement additions on the B2MA2 order-disorder trans-formation temperatures of Fe0.5(Al1�nXn)0.5 intermetal-lics were investigated by using various ternary alloyingelements of X = Cr, Ni, Mo, Ta, Mn, Hf, Zr, Nb, Ti,and W up to 5 at. pct. Samples with 10 to 15 g masswere prepared by mixing the appropriate amounts ofhigh-purity constituents (Fe = 99.97 wt pct, Al = 99.9wt pct, and X > 99.5 wt pct) and produced by arcmelting using a nonconsumable tungsten electrode. Inorder to achieve higher composition homogeneity,alloys were melted several times in Zr-gettered Aratmosphere. The weight loss during the arc-meltingprocess was found to be less than 0.5 wt pct. The Fe-Al-X alloy ingots were encapsulated in evacuated quartztubes filled with argon and then subjected to homoge-nization at 1173 K (900 �C) for 24 hours. Homogeni-zation was followed by annealing heat treatments at 673K (400 �C) for 168 hours to attain a highly ordered statein these alloys. The compositions of the samples wereverified through energy-dispersive X-ray analysis (EDS)using a JEOL* JSM-6400 model scanning electron

microscope (SEM) equipped with a Noran System-6X-ray microanalysis system (Noran, Thermo ElectronCorporation, Middleton, WI).

X-ray diffraction (XRD) analyses were conductedwith a Rigaku D/Max-2200 PC diffractometer (RigakuCorporation, Tokyo, Japan) with Cu Ka radiation toidentify the phases present in the samples. The micro-structures were examined by light optical microscopy,and detailed structural investigation at higher magnifi-cations was carried out by SEM. Samples were preparedby standard metallographic techniques and etched witha solution of 68 mL glycerin, 16 mL 70 pct HNO3, and16 mL 40 pct HF. Thermal characteristics and order-disorder phase transformation temperatures were mea-sured with a Setaram Setsys 16/18 high-temperaturethermal analyzer (Setaram Instrumentation, KEP Tech-nologies, Caluire, France) at various heating/cooling

rates under constant argon flow. The instrument wascalibrated for a wide range of temperatures and scan-ning rates by using high-purity standard elements of Al,Zn, Pb, Ag, Au, and Ni. The accuracy of the temper-ature calibration was found to be ±1 K (1 �C).

III. RESULTS AND DISCUSSION

In this present investigation, a wide range of ternaryalloying elements, X, were prepared and examined up to5 at. pct, Fe0.5(Al1�nXn)0.5. However, alloys goingthrough eutectic phase transformation even at verylow concentration of the transition elements of X = Hf,Zr, and Nb were excluded. Therefore, emphasis wasplaced on Fe0.5(Al1�nXn)0.5 intermetallics having ternaryalloying elements, X = Cr, Ni, Mo, Ta, Mn, Ti, and W,that readily form single-phase solid solution for X £1 at. pct to make a dependable comparison with similaralloys studied theoretically.The microstructures of binary stoichiometric FeAl

and ternary Fe0.5Al0.49X0.01 alloys display single-phasesolid solutions having equiaxed coarse grain structuresin as-cast and heat-treated conditions. The examples ofthese almost similar microstructures are given in Fig-ure 1 for binary and Fe0.5(Al1�nXn)0.5 (X = Cr and Mo)alloys. However, small amounts of second-phase parti-cles, which preferentially form along the grain bound-aries, tend to exist in alloys containing X = Mo, W, Ti,and Ta elements (Figure 1(c)). The formation of thesesecond-phase particles could be attributed to their lowor limited solid solubility in FeAl phase[7,12] and couldnot be detected by XRD analyses due to their smallvolume fractions. However, high-magnification SEMstudies coupled with EDS (Figure 2) confirm the exis-tence of the X-rich second-phase particles in as-cast andheat-treated Fe0.5Al0.49X0.01 (X = Mo, W, Ta, Ti)alloys. Moreover, these intrinsically hard and brittlesecond-phase particles eventually develop continuousnetworks along the grain boundaries where intercrys-talline decohesion is observed. This seems to be one ofthe major reasons for poor ductility behavior of thesealloys. The volume fractions of these second-phaseparticles increase with further addition of X content(X>1 at. pct). This increase leads to formation of second-phase particles not only along the grain boundaries butalso within the grain interior. The solid solubilitybehavior of ternary alloying additions in B2-type FeAlalloys may be ascribed to the site occupancy character-istics of the X alloying elements. It is interesting to notethat these alloying elements that readily form second-phase particles in the microstructure were predicted tosubstitute preferentially Fe sublattice sites in B2 typeordered Fe-Al intermetallics.[13]

XRD analyses of as-cast and heat-treatedFe0.5Al0.49X0.01 alloys reveal almost the same diffractionpattern as that of the stoichiometric binary FeAl alloyand prove the existence of single-phase B2 type orderedFeAl phase (Figure 3) in all alloys under investigation.The relative intensities of diffraction lines are compar-atively larger for the heat-treated than as-cast alloys due

*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.

1810—VOLUME 43A, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

to the homogenization and ordering heat treatmentdescribed previously. Success of the ordering heattreatment procedure, consequently the extent of theatomic ordering, may be evaluated by calculating thelong-range order (LRO) parameter, g, for the as-castand heat-treated binary stoichiometric FeAl alloys.LRO parameters, g, were calculated on the basis ofXRD results given in Figure 4 by employing thefollowing set of equations:[30]

ln g2 ¼ �ðln yF � ln ySSÞ ½1�

where

ln yF ¼ lnIF

4mLP cAlfAl þ cFefFeð Þ2þ cAlDfAl þ cFeDfFeð Þ2h i

½2�

and

ln ySS ¼ lnISS

mLP fAl � fFeð Þ2þ DfAl � DfFeð Þ2h i ½3�

Fig. 1—SEM micrographs of as-cast and heat-treated Fe0.50Al0.49X0.01 alloys: (a) binary, (b) X = Cr, and (c) X = Mo.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JUNE 2012—1811

where IF and ISS are the integrated intensities of thefundamental and superstructure diffraction lines, respec-tively; m is the multiplicity factor; LP is the Lorentzpolarization factor; and cAl, fAl, DfAl, and cFe, fFe, DfFeare the atomic fraction, atomic scattering factors, andscattering-factor corrections of the Al and Fe atoms,respectively.

LRO parameters calculated based on Eqs. [1] through[3] were found to be 0.48 and 0.83 for as-cast and heat-treated FeAl alloys, respectively. The presence of thesuperstructure diffraction lines and the experimentallydetermined magnitude of the LRO parameter for theheat-treated binary FeAl alloy suggest the existence ofnot fully but highly ordered B2-type crystal structure. Afully ordered structure having g = 1.0 could not beachieved because of the presence of rather coarse (>200-lm) equiaxed grains, which tends to reduce the inten-sities of the superstructure diffraction lines.[31] It is wellknown that transformation of B2-type ordered FeAlphase to the disordered A2-type phase occurs through asecond-order transition in the composition rangebetween 23 and 45 at. pct Al.[32,33] However, the alloyunder investigation having Al content around 50 at. pctundergoes an L + a-Fe (A2) M FeAl (B2) type reactionat 1583 K (1310 �C). Although this transformation waspreviously considered a peritectic reaction,[34] it has beenrecently proposed and accepted to be a second-orderFeAl (B2) M a-Fe (A2) transition.[32]

Before going through a detailed analysis of the effect ofternary alloying additions on the order-disorder trans-formation temperature of Fe-Al alloys, it is of interest toconfirm the reliability and reproducibility of the thermalbehavior of Fe0.5Al0.49X0.01 alloys studied in this presentinvestigation by thermal analysis. Therefore, experimen-tally determined thermal characteristics of binary heat-treated Fe-Al alloys in the composition range of 46 to 50at. pct Al having B2 structure were verified by phasediagram[33] and other experimental data on the B2MA2order-disorder phase transition temperatures given in theliterature.[32] Figure 5 shows the DSC thermograms ofthe binary B2-type ordered Fe-Al alloys on heatingaround their melting temperatures. Peak temperaturesappearing in an L + a-Fe (A2) two-phase region aftermelting were identified as B2MA2 order-disorder phasetransition temperatures. The characteristics of the DSCthermograms and values of order-disorder transitiontemperatures, measured as 1589 K (1316 �C) for stoichi-ometric FeAl alloy, are consistent with the phasediagram[33] and other literature data.[32]

The effect of ternary alloying additions on theB2MA2 order-disorder phase transformation tempera-tures of the heat-treated Fe0.5Al0.49X0.01 alloys has beenanalyzed by considering their DSC heating curves(Figure 6), and measured transformation temperaturesare given in Table I. It is evident that all ternaryalloying elements except Cr and Ti tend to increase the

Fig. 2—(a) SEM micrographs of Fe0.50Al0.49Mo0.01 alloys at high magnification. (b) EDS spectrum of Fe0.50Al0.49Mo0.01 as-cast alloy.

1812—VOLUME 43A, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

order-disorder transformation temperature with respectto binary Fe-Al alloy. Ti is the only ternary alloyingelement that slightly decreases this temperature. Itappears that the extent of variation in B2MA2 order-disorder phase transformation temperatures depends onthe type of ternary alloying element additions. The effectof type and content of the ternary alloying elementadditions on the order-disorder transformation temper-atures of Fe0.5(Al1�nXn)0.5 intermetallics with B2-typeordered structure was also investigated theoretically inour previous study[13] by considering the energetic andstructural characteristics of atomic ordering processes inthese intermetallics. Hence, the effect of ternary alloyingadditions on the B2MA2 order-disorder transformationtemperature will now be discussed within the frameworkof the theoretical model and calculations proposed byMekhrabov and Akdeniz[13] and a comparison will bemade with experimental results.It was shown that the distribution of ternary alloying

element atoms over sublattices of B2-type Fe-Al inter-metallics, determined by their partial SRO parameters,have a significant effect on the B2MA2 order-disordertransformation temperature, which has been calculatedusing the following equation:[35]

DTTo� 49 1� 49

48

1

cosh2 WFeX�WAlX

4kBTo

� �24

35cX ½4�

where DT = Tox � To and it is the change in order-disorder phase transition temperature. To and Tox

denote the B2MA2 order-disorder phase transitiontemperature of binary stoichiometric FeAl and Fe-Al-X intermetallics, respectively; WFeX and WAlX are the

Fig. 3—XRD patterns of (a) as-cast and (b) heat-treatedFe0.50Al0.49X0.01 alloys.

Fig. 4—XRD patterns of binary as-cast and heat-treated FeAl alloysused for LRO parameter determination.

Fig. 5—DSC heating (at a heating rate of 10 K/min (10 �C/min))curves of binary heat-treated Fe-Al alloys. Inset shows the 1578 K to1598 K (1305 �C to 1325 �C) interval of binary FeAl alloy in detail.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JUNE 2012—1813

partial ordering energies of Fe-X and Al-X pairs inthe ternary alloy, respectively; kB is the Boltzmannconstant; and cX is the concentration of ternary alloyingelement X.

The normalized transition temperature, DT/To, maybe considered as an indication of the degree of variationin the order-disorder transition temperature thatstrongly depends on the type of ternary alloying Xelements. Consequently, absolute values of relativepartial ordering energies of different atomic pairs ofFe, Al, X, elements, Waa¢(R), in Fe-Al-X intermetallicsbecome an important parameter in determining the Tox

temperatures. Employing the partial ordering energiesof Fe-X and Al-X pairs in Table II (as given inReference 13) and To of 1589 K (1316 �C), as deter-mined experimentally by differential scanning calorim-etry (DSC) measurement for the binary FeAl alloy inthis present investigation, the normalized transitiontemperatures are recalculated. The calculated valuesare compared to the experimentally determined normal-ized temperature changes for Fe0.5(Al1�nXn)0.5 alloys inTable III. The qualitative or semiquantitative agreementbetween theoretical predictions[13] and the presentexperimental observation is excellent in terms of theeffect of ternary alloying element addition on thetendency of the variation of the order-disorder phasetransformation temperature of Fe-Al intermetallics.It has been shown in our previous study[13] that the

variation of critical order-disorder phase transformationtemperature DT depends not only on the magnitude ofpartial ordering energies of Al-X and Fe-X atomic pairs,but also on the distribution of X ternary alloyingelement atoms over Fe and Al sublattices in Fe-Alintermetallics determined by their partial SRO param-eters. Hence, the tendency of variation of order-disorderphase transformation temperature may be determinedexclusively by taking into account both the effects oflattice site occupation preferences of X element atomsand the magnitude of partial ordering energies. Theseenergies would be considered as the measure of the bondstrength of Al-X and Fe-X atomic pairs formed whenthe X element atoms preferentially reside over either Feor Al sublattices in Fe-Al intermetallics. Thus, the

Fig. 6—DSC heating (at a heating rate of 10 K/min (10 �C/min))curves of heat-treated Fe0.50Al0.49X0.01 alloys: (a) X = Cr, W, andNi; and (b) X = Mn, Mo, Ta, and Ti.

Table I. Experimentally Measured Order-Disorder Transformation Temperatures of Heat-Treated Fe0.50Al0.49X0.01 Alloys

Alloying Addition None Ti Cr Mn Ni Mo Ta W

B2MA2 order-disordertransformationtemperature [K (�C)]

1589 (1316) 1586 (1313) 1589 (1316) 1595 (1322) 1599 (1326) 1603 (1330) 1609 (1336) 1609 (1336)

Table II. Partial Ordering Energies of Fe0.50Al0.49X0.01

Alloys at First Coordination Sphere

Elements

Ordering Energy (at. u.) b

WAlX�WFeX

WFeAlWAlX(R1) WFeX(R1)

Ni �3.67 9 10�3 7.18 9 10�4 �1.50Mn �4.84 9 10�3 �7.37 9 10�4 �1.41Cr �2.41 9 10�3 4.00 9 10�4 �0.96Ti 3.09 9 10�3 5.89 9 10�4 0.86Ta 2.54 9 10�2 1.06 9 10�2 5.07Mo 4.44 9 10�2 2.81 9 10�2 5.58W 4.18 9 10�2 2.41 9 10�2 6.06

1814—VOLUME 43A, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

relative partial ordering energy parameter (RPOE), b,has been defined by following equation:

b ¼WAlX �WFeX

WFeAl½5�

In the calculation of the RPOE parameter, we haveused a value for the ordering energy of Fe-Al atomicpairs WFe-Al(R1) = 2.92 9 10�3 at.u., as determined byKrivoglaz and Smirnov.[35] Calculated values of RPOEparameters are tabulated in Table II together with thepartial ordering energies of Al-X and Fe-X atomic pairs.It is worth noting that, while the sign of the RPOEparameter implies the distribution of ternary alloyingelement atoms over Fe and Al sublattices in Fe-Alintermetallics, its magnitude provides useful informationon the bond strengths of atomic pairs of Al-X or Fe-Xrelative to Fe-Al in Fe0.5(Al1�nXn)0.5 intermetallics. Theoutcome of the RPOE parameter and the associatedconditions for sublattice occupation and the change inthe order-disorder transformation temperatures aresummarized in Table IV along with experimentallyand theoretically determined DT/To values.

If the RPOE parameter is negative (b < 0), ternaryalloying element atoms (XI = Ni, Mn or Cr) mainly

occupy Al sublattice sites, and they are surrounded byFe atoms, whereas if the RPOE parameter is positive (b>0), the X alloying element atoms (XII = Ti, Ta, Mo,or W) preferentially substitute Fe sublattice sites and aresurrounded primarily by Al atoms. Current predictionsbased on the RPOE parameter are consistent with ourprevious study on Fe-Al intermetallics, in which thelattice site occupancy preference of the impurity atoms isdetermined by their partial SRO parameters.[13] Fur-thermore, the change in critical order-disorder phasetransformation temperature (DT) upon addition of Xternary alloying elements is related to the magnitude ofthe RPOE parameter featuring the relative bondstrength of Al-X and Fe-X atomic pairs with respectto the Fe-Al atomic bond of the binary Fe-Al alloy.When the relative bond strengths of Al-X and Fe-Xatomic pairs are close to the bond strength of the Fe-Alatomic pair, the RPOE parameter nearly equals unity (b� 1). This would tend to suggest that no changein order-disorder transition temperature ofFe0.5(Al1�nXn)0.5 alloys is predicted compared to thebinary Fe-Al intermetallic, as in the case of Cr addition.If the RPOE parameter is less than unity (b < 1), therelative bond strength of Al-X and Fe-X atomic pairsbecomes weaker than that of the Fe-Al atomic pair,leading to a decrease in transformation temperature, asin the case of Ti addition. Similarly, when alloyingelements tend to form stronger Al-X and Fe-X atomicbonds than the Fe-Al atomic bond, the RPOE param-eter becomes greater than unity (b>1), implying a slightincrease in transformation temperature. However, alloy-ing elements with very strong Al-X and Fe-X atomicbonds have an RPOE parameter much greater thanunity (b>>1); thus, they would have a higher potentialto increase the order-disorder transition temperature.The present experimental observations on the effects

of ternary alloying additions on the B2MA2 order-disorder phase transformation temperatures of theFe0.5Al0.49X0.01 alloys are consistent qualitatively withthe current predictions, based on the relative partialordering energy parameter. Moreover, the theoreticalmodel and calculations, based on the statistico-thermo-dynamical theory of ordering combined with the electronictheory of alloys in pseudo-potential approximation,

Table III. Normalized Order-Disorder Transition

Temperatures of Fe0.5(Al12nXn)0.5 Alloys Determined by DSCMeasurements in Comparison with Theoretical Predictions[15]

(To = 1589 K (1316 �C) for the Binary FeAl Alloy)

TernaryAddition

(DT/To) 9 10�3

TheoreticalExperimental

n = 0.0025 n = 0.005 n = 0.01 n = 0.01

Ti �0.61 �1.26 �2.57 �1.88Cr �0.15 �0.31 �0.59 0Mn 2.43 4.86 10.00 3.78Ni 3.03 6.06 12.81 6.29Mo 52.55 106.02 213.88 8.81Ta 46.47 92.94 185.89 12.58W 58.88 121.29 239.07 12.58

Table IV. Order-Disorder Transition Temperatures, Relative Partial Ordering Energy Parameters, Sublattice Occupations,and Normalized Order-Disorder Transition Temperatures of Fe0.50Al0.49X0.01 Alloys

Elements Tox [K (�C)] b

DT = Tox � To Occupation (DT/To) 9 10�3

Condition Site Condition Exp. Theo

None 1589 (1316) — — — — —Cr 1589 (1316) �0.96 Tox � To b � 1 Al b < 0 0 �0.59Mn 1595 (1322) �1.41 Tox >To b > 1 Al b < 0 3.78 10.00Ni 1599 (1326) �1.50 Tox >To b > 1 Al b < 0 6.29 12.81Ti 1586 (1313) 0.86 Tox <To b < 1 Fe b > 0 �1.88 �2.57Mo 1603 (1330) 5.58 Tox >>To b >> 1 Fe b > 0 8.81 213.88Ta 1609 (1336) 5.07 Tox >>To b >> 1 Fe b > 0 12.58 185.89W 1609 (1336) 6.06 Tox >>To b >> 1 Fe b > 0 12.58 239.07

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JUNE 2012—1815

proposed in our previous study[13] were also verified bythe present experimental results for which the agreementis excellent.

IV. CONCLUSIONS

The effects of ternary alloying element additions onthe variation of B2MA2 order-disorder phase transfor-mation temperatures of Fe0.5(Al1�nXn)0.5 intermetallics(n = 0.01) were investigated experimentally by using thevarious ternary alloying elements of X = Cr, Ni, Mo,Ta, Mn, Ti, and W. The extent of variation order-disorder phase transformation temperatures was definedby the normalized transition temperature (DT/To), and itdepends strongly on the type of ternary alloying elementadditions. Comparisons were made with the theoreticalmodel and calculations; consequently, predictions basedon the variation of B2MA2 order-disorder transitiontemperatures of Fe0.5(Al1�nXn)0.5 intermetallics wereverified experimentally. The tendency and measure ofvariation of the order-disorder phase transformationtemperature were determined exclusively by defining theRPOE, which takes into account both the effects oflattice site occupation preferences of X element atomsand the magnitude of the partial ordering energies ofAl-X and Fe-X atomic pairs relative to Fe-Al pairs. Thesign of the RPOE parameter implies the distribution ofternary alloying element atoms over Fe and Al sublat-tices in B2-type ordered Fe-Al intermetallics, and itsmagnitude provides an indication of the variation of theorder-disorder transformation temperature relative tobinary FeAl alloys. Related outcomes of the study canbe summarized as follows.

1. If their RPOE parameter is negative (b < 0), alloy-ing elements of X mainly substitute Al sublatticesites and are surrounded by Fe atoms in theB2-FeAl superlattice (e.g., XI = Ni, Mn, or Cr).

2. If their RPOE parameter is positive (b > 0), alloy-ing elements of X preferentially substitute Fe sub-lattice sites and are surrounded mainly by Al atoms(e.g., XII = Ti, Ta, Mo, or W).

3. Depending on the magnitude of the RPOE parame-ter, the change of the B2MA2 order-disorder trans-formation temperature in the presence of a ternaryalloying element X can be predicted as follows.

a. b � 1 indicates no change; thus, Tox � To (X = Cr).b. b < 1 indicates a small decrease; thus, Tox < To

(X = Ti).c. b > 1 indicates a slight increase; thus, Tox > To

(X = Mn, Ni).d. b >> 1 indicates a significant increase; thus, Tox

>>To (X = Mo, Ta, W).

ACKNOWLEDGMENTS

The authors gratefully acknowledge the OYP Pro-gram at Middle East Technical University and The

Scientific and Technological Research Council of Tur-key, TUBITAK, National Scholarship Programme forpostdoctoral students.

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