Mechanochemical Behavior of NiO-Al-Fe Powder Mixturesto Produce (Ni, Fe)3Al-Al2O3 Nanocomposite Powder

Z. ADABAVAZEH, F. KARIMZADEH, and M.H. ENAYATI

(Ni, Fe)3Al-30 vol pct Al2O3 nanocomposite powder was synthesized by mechanochemicalreaction of Fe-NiO-Al powder mixtures. Structural evolution during mechanical alloying wasstudied by employing X-ray diffractometry (XRD), differential thermal analysis (DTA), andtransmission electron microscopy (TEM). After 78 minutes of milling, the (Ni, Fe)3Al-30 volpct Al2O3 nanocomposite can be synthesized by reaction 3Fe + 7Al + 6NiO with a combustionmode. DTA results revealed that milling for 60 minutes decreases the temperature of reactionfrom 1040 K to 898 K (767 �C to 625 �C). TEM images corroborate a homogenous dispersionof reinforcements in the matrix of the nanocomposite proving that the reduction in the crys-tallite size of both reinforcements and matrix is within the nanometer range.

DOI: 10.1007/s11661-012-1138-0� The Minerals, Metals & Materials Society and ASM International 2012

I. INTRODUCTION

THE Ni3Al intermetallic compound exhibits advan-tageous properties including high melting temperature,high tensile strength and yield point, low density, highhardness, good thermal stability, high creep resistance,good corrosion, and oxidation resistance at elevatedtemperatures.[1,2] This is why the intermetallic compoundis particularly ideal for many special applications such ascutting tools, coating blades in gas turbines, jet engines,corrosion resistant materials, and heat treatment fix-tures. The major barriers for the use of Ni3Al asstructural materials are their low toughness and ductilityat ambient temperature.[3,4] One practical processingroute for direct synthesis of compounds and nanocom-posites is mechanical alloying (MA).[5–7] In recent years,there has been increased activity in research on theaddition of a third alloying element to different inter-metallic compounds. It leads to the improvement ofductility and toughness at ambient temperature.[8–10]

Mechanochemical synthesis, in which chemical reactionsare induced by mechanical treatment, can raise themechanical properties.[11–14] Several mechanochemicallysynthesized nanocomposites such as NiAl-Al2O3,

[15]

NiTi-Al2O3,[16] Fe3Al-Al2O3,

[17] and (Fe, Ti)3Al-Al2O3

[18] were previously reported. Rafiei et al.[18] stud-ied the formation of (Fe, Ti)3Al-Al2O3 nanocompositesby mechanical alloying of Fe, Al, and TiO2 powdermixture. Consolidation of the ball-milled powders intobulk, full density compacts while retaining nanoscalegrain size is a major challenge. Many sintering tech-niques, e.g., hot pressing, hot extrusion, sintering forg-ing, and HIPing, were employed to consolidate the

mechanically alloyed powders. Tavoosi et al.[19] investi-gated the fabrication of bulk Al-Zn/Al2O3 metal matrixnanocomposites prepared by mechanical alloying andhot pressing. In this work, the aluminum and zinc oxidepowder mixture was milled by a planetary ball mill andthen hot pressed in a uniaxial die. However, there are fewreports in the literature on the MA of ternary Ni-Fe-Alpowders. Liu et al. studied the effects of Fe substitutionfor Al in NiAl alloy. Investigations were performed onthe mechanical alloying of Ni50Al50–xFex (x = 5, 10, 15,20, 25, 30) powders mixture.[20] In our previous work,[21]

the formation of (Ni, Fe)3Al intermetallic compoundwas investigated. In this study, the formation mechanismof the (Ni, Fe)3Al-30 vol pct Al2O3 nanocompositesynthesized by mechanochemical reaction betweenAl, Fe, and NiO is reported. Finally, the formationmechanism and thermal stability in the milled powder atelevated temperatures was studied by employing X-raydiffractometry (XRD), differential thermal analysis(DTA), and transmission electron microscopy (TEM).

II. MATERIALS AND METHODS

The powders of Fe (99.8 pct purity, particle size of~100 lm, Merck, Darmstadt, Germany), Al (99.5 pctpurity, particle size of 50 to 100 lm, Iran PowderMetallurgy Co., Mashhad, Iran), and NiO (99.9 pctpurity, particle size of <40 lm, Merck, Darmstadt,Germany) were mixed in the appropriate proportion togive the (Ni, Fe)3Al-Al2O3 nanocomposite. Mechanicalalloying was carried out in a planetary ball mill,nominally at room temperature and under Ar atmo-sphere. Al, Fe, and NiO with starting composition NiO-23 wt pct Al-21 wt pct Fe were mixed according toReaction [1] to produce a (Ni, Fe)3Al based nanocom-posite containing 30 vol pct Al2O3:

3Feþ 7Alþ 6NiO! 3 Ni; Feð Þ3Al ð70 vol pctÞþ 2Al2O3ð30 vol pctÞ ½1�

Z.ADABAVAZEH,MScGraduate Student, andF.KARIMZADEHand M.H. ENAYATI, Associate Professors and Supervisors, are with theDepartment of Materials Engineering, Isfahan University of Technology,Isfahan 84156-83111, Iran. Contact e-mail: z.adabavazeh@ma.iut.ac.ir

Manuscript submitted August 3, 2011.Article published online June 9, 2012

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, SEPTEMBER 2012—3359

The phase evolution during mechanochemical reac-tion was investigated by XRD in a PHILIPS* X’Pert

MPD diffractometer using filtered Cu Ka radiation.Crystallite size and internal strain in the milled sampleswere calculated from the XRD line broadening using theWilliamson–Hall equation:[22]

b cos h ¼ 2e sin hþ Kk=D ½2�

where b is the peak breadth in midheight, k representsthe X-ray wavelength of the incident copper X-rayradiation (k = 1.5404 A), h is the Bragg diffractionangle, D is the average crystallite size, K is a constantwith a value of 0.89, and e is the mean value of internalstrain. Morphology and microstructure of the mechan-ically alloyed powder particles were observed by TEM ina PHILIPS CM120FEG.

DTA was also conducted to study phase transforma-tions during heating of (Ni, Fe)3Al-Al2O3 nanocompos-ite. In this case, the sample was placed in Al2O3 pansand heated in dynamic argon atmosphere up to 1473 K(1200 �C) at a rate of 10 �C/min. In order to study thethermal behavior of milled powders, the samples wereheat treated in a tube furnace under Ar atmosphere.

III. RESULTS AND DISCUSSION

A. Thermodynamic Aspects

As it is observed, Reaction [1] consists of two otherreactions: reduction of NiO by Al to form Ni (Reaction[3]) followed by the reaction of Ni with extra Fe and Alto form (Ni, Fe)3Al (Reaction [4]) according to thefollowing reactions:

4Alþ 6NiO! 6Niþ 2Al2O3 ½3�

3Feþ 3Alþ 6Ni! 3 Ni; Feð Þ3Al ½4�

In order to produce the matrix of the nanocomposite,which is (Ni, Fe)3Al according to Reaction [4], Ni, Al,and Fe elemental powders with stoichiometric compo-sition of Ni50Fe25Al25 were mechanically alloyed. It isconsidered in calculations too. All of details are com-pletely explained in our previous work.[21]

Because of negative DG298� and DH298

� values ofReaction [3], this reaction can take place at roomtemperature thermodynamically. The reactions withnegative free energy change do not necessarily takeplace at room temperature because of their slowkinetics. The most important thermodynamic parameterin combustion reaction is the adiabatic temperature ofreaction (Tad) because of its controlling effect on the rateof chemical reaction. It also plays an important role inthe determination of combustion efficiency. In fact, themaximum temperature achieved under adiabatic condi-tions as a result of evolution of heat from the reaction isknown as the adiabatic temperature (Tad).

[23]

Mechanical alloying can be done in one of two modes:gradual or combustion. Combustion reactions occurduring the mechanical activation process with a highlynegative reaction enthalpy; however, gradual reactionsoccur during subsequent thermal treatment with amoderate reaction enthalpy. An adiabatic temperatureabove 1800 K (1527 �C) indicates a self-propagatingcombustion reaction for a thermally ignited system.[24–26] By calculating the value of Tad, the existence ofgradual or combustion reaction in the milling processcan be recognized. In this case, Tad was calculated usingthe following relation:

�DH�

298 ¼X

np

ZTPM

298

CPSPdTþ nd

ZTdM

298

CPSddT

2

64

3

75

þX

nPDHPM þ ndDH

dM

� �

þX

np

ZTad

TPM

CPPLdTþ nd

ZTad

TdM

CdPLdT

264

375 ½5�

In the preceding equation, DH298� is the enthalpy

change of reaction at room temperature; np and nd arethe molar fractions of products and diluents, respec-tively; TM

P and TMd are the melting points; CPL

P andCPLd are the heat capacities in the liquid phase; CPS

P andCPSd are the heat capacities in the solid phase;

and DHMP and DHM

d are the enthalpy of fusion ofproducts and diluents, respectively.[27,28]

The value of Tad for Reaction [4] was calculated withconsideration of dilution effects according to Eq. [5].The value of Tad for 4Al + 6NiO is about 3010 K(2737 �C).[29–31] In this work, dilution of reactants (Feand Al in order) caused Tad to decrease. The Tad in thepresence of extra Fe and Al was calculated using theheat of Reaction [3] and thermodynamic data,[32] andwas found to be 2837 K (2564 �C), which is higher thanthe critical value of 1800 K (1527 �C). Consequently, acombustion reaction takes place during MA of Al, Fe,and NiO powder mixture.

B. Phase Evolution and Reaction Mechanism

Figure 1 shows XRD patterns of a stoichiometricmixture of Al, Fe, and NiO after different milling times.As can be seen, the XRD peaks of the raw material (Al,Fe, and NiO) still exist after 1 hour of MA. The (Ni,Fe)3Al and Al2O3 peaks initially appear on the XRDpattern after 2 hours milling. The presence of superlat-tice diffraction peaks for (Ni, Fe)3Al and Al2O3 phaseformed during MA suggests their ordered structure.Because the XRD pattern of 60 hour milled sample isvery similar to that of the 80 hour milled sample, onlyone is shown in Figure 1. By continuing milling, theintensity of the sharp crystalline diffraction peaksdecreases and their width increases progressively withincreasing processing time.According to the result in the last section, the

nanocomposite was synthesized by a combustion reaction

*PHILIPS is a trademark of FEI Company, Hillsboro, OR.

3360—VOLUME 43A, SEPTEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

and the XRD results confirmed it completely. In orderto determine the modality of reaction experimentally,the vial temperature was measured during ball milling.Figure 2 shows variation of vial temperature duringMA. At room temperature, Reaction [3] can thermody-namically occur because of its negative DG298

� and DH298�

values. As can be seen, the formation of nanocompositeleads to a sudden increase from room temperature to108.3 �C in the vial temperature, which confirms thatthe reduction reaction occurs in self-propagating com-bustion mode between reactants. The rate of tempera-ture rise was fast, and then it slowed. The temperaturethen slowly decayed to the previous steady-state value.

Figure 3 shows XRD patterns of the (Ni, Fe)3Al-Al2O3 composite for 80 hours of MA and annealing at1173 K (900 �C) for 1 hour. Comparing XRD patternsbefore and after annealing shows the appearance of newsuperlattice diffraction peaks for (Ni, Fe)3Al and Al2O3

phases, which suggests an increase in L12 order degreeduring annealing. Meanwhile, the peaks become sharperthan the previous ones due to an increase in the size ofaverage crystallite and the decrease in lattice strain.Because of the stress releasing effects, the width of

(Ni, Fe)3Al peaks decreased; however, their intensityincreased after annealing. With a decrease in internalstrain, grain growth occurred.Table I displays the average crystallite size and

internal strain of 60 and 80 hour milled samples beforeand after annealing. During milling, the average crys-tallite sizes of the 80 hour milled sample decreased toabout 9 and 11 nm, respectively, because of formationof brittle (Ni, Fe)3Al phase (according to Table I). Thesevalues for the 60 hour milled sample decreased to about35 and 20 nm, respectively. After heat treatment, theaverage crystallite size is increased and internal strain isdecreased, as indicated in Table I. The width of (Ni,Fe)3Al and Al2O3 peaks in Figure 3 decreased and theirintensity increased after annealing due to stress releaseas well as grain growth.

C. Thermal Analysis

The microstructure changes during the MA process,and the formation mechanism of reaction between thestarting materials was investigated by differential ther-mal analysis (DTA).Figure 4 shows the DTA curve of the as-blended and

1 hour milled sample for NiO-Fe-Al powder. Fourpeaks appeared in the DTA curve of the as-blended (Ni,Fe)3Al-30 vol pct Al2O3 powder at around 938 K,1040 K, 1251 K, and 1303 K (665 �C, 767 �C, 978 �C,and 1030 �C). The DTA curve shows one endothermicand three exothermic peaks.The first endothermic peak (about 938 K (665 �C)) is

due to the melting of aluminum. To analyze theexothermic peaks, the as-blended powder was heatedto 1123 K (850 �C) and 1473 K (1200 �C) in Ar atmo-sphere at a heating rate of 10 �C min�1 similar to theDTA condition and was analyzed by XRD. Figure 5(a)shows the XRD patterns of the as-blended powderbefore and after annealing at 1123 K (850 �C). Beforeannealing, powder mixtures of NiO, Al, and Fe wereobserved. Considering the XRD pattern, the annealed

Fig. 1—XRD patterns of (Ni, Fe)3Al-30 vol pct Al2O3 at different milling times.

Fig. 2—Variation of vial temperature during milling of (Ni, Fe)3Al-30 vol pct Al2O3.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, SEPTEMBER 2012—3361

sample consists of Ni, Al, Fe, and Al2O3 phases. Theformation of these phases is due to the reduction of NiOby Al. Therefore, the first exothermic peak at 1040 K(767 �C) corresponds to the reduction of NiO by Al.

Figure 5(b) shows the XRD patterns of theas-blended powder before and after annealing at 1473 K(1200 �C). According to the XRD results, (Ni, Fe)3Aland Al2O3 phases were observed in Figure 5(b). Thisindicates that the exothermic peaks at 1251 K (978 �C)and 1303 K (1030 �C) correspond to the occurrence ofReaction [4]. According to the XRD results, c-Al2O3

forms, and all of the NiO is reduced completely duringcombustive reaction. The crystalline c-Al2O3 wasformed instead of amorphous Al2O3 because of thehigh value of the local heat generated during the millingprocess. Thus, after a short time of milling (about

2 hours), Ni, Al2O3, and the remaining Al and Fe, whichcoexist in the milling media, lead to the formation of(Ni, Fe)3Al and Al2O3 phase in the XRD patterns, asseen in Figure 5(b).Figure 4(b) shows the DTA curve of the NiO-Fe-Al

sample after 1 hour MA. According to the XRD results,the reduction temperature of NiO-Fe-Al decreases withmilling time. A comparison of Figures 4(a) and (b)indicates that milling NiO-Fe-Al decreases the reductiontemperature from 1040 to 898 K (767 to 625 �C). Infact, the formation of (Ni, Fe)3Al-30 vol pct Al2O3

nanocomposite powder with a nanometer crystallite sizeincludes an extensive interface area, which providesshort circuit diffusion paths (because of the existence ofhigh defect densities such as dislocations and grainboundaries) and, therefore, can improve the reactionkinetics at ambient temperature.[33]

Figure 6 shows a TEM image and the related selected-area diffraction pattern (SADP) of the powder milledfor 80 hours. Figures 6(a) and (b) show the bright-field(BF) and the dark-field (DF) micrographs, respectively.There is a homogenous dispersion of reinforcementswith size of about 10 nm in the matrix. Furthermore, theTEM images also confirm the nanocrystalline structureof the matrix, which was obtained by the Williamson–Hall method. As can be seen, the TEM image shows thegrains of the nanocomposite and a wide region betweengrains. Because the crystalline peaks of the SADP arethe broadened peaks, they are not observed in the TEMimages, and the amorphous phase is observed in the DF.On the other hand, because the diffraction intensity ofthe crystalline phase in the SADP is high and both thediffraction intensity of the amorphous phase and itsquantity are low, this phase is not observed in theSADP. This wide region between grains consists of the

Table I. Average Crystallite Size and Internal Strain of (Ni, Fe)3Al-30 Vol Pct Al2O3 after 80 and 60 Hour Millingand After Annealing

60 h MA 80 h MA 80 h MA + Annealing

(Ni, Fe)3Al Al2O3 (Ni, Fe)3Al Al2O3 (Ni, Fe)3Al Al2O3

Crystallite size (nm) 35 20 9 11 45 98Internal strain (pct) 2.1 1.3 0.95 1.05 0.4 0.6

Fig. 3—XRD patterns of (Ni, Fe)3Al-30 vol pct Al2O3 powder before and after subsequent heat treatment at 1173 K (900 �C) for 1 h.

Fig. 4—DTA analysis results (a) of as-blended for NiO-Fe-Al pow-der and (b) of NiO-Fe-Al powder after milling for 1 h.

3362—VOLUME 43A, SEPTEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

percentage of amorphous phases. The SADP (Figure 6(c))is called the ring pattern, because it has rings thatindicate that a fine crystalline size of reinforcements andmatrix are located in all directions.

IV. CONCLUSIONS

(Ni, Fe)3Al-30 vol pct Al2O3 nanocomposite powderwas successfully synthesized by mechanochemical reac-tion of Fe, Al, and NiO powder mixtures. Occurrence ofhighly exothermic 3Fe + 7Al + 6NiO reaction takes

place in a combustion mode after 78 minutes of millingtime. From DTA results, it was revealed that millingfor 1 hour decreases the temperature of reaction bet-ween the starting materials. DTA results revealed thatmilling for 60 minutes decreases the temperature ofreaction from 1040 K to 898 K (767 to 625 �C). TEMimages corroborate a homogenous dispersion of rein-forcements in the matrix of the nanocomposite, provingthat reducing the crystallite size of both the reinforce-ments and matrix is within the nanometer range. TEMimages also confirm the nanocrystalline nature of a

Fig. 5—XRD patterns of as-blended for NiO-Fe-Al powder before and after (a) annealing up to 1123 K (850 �C) and (b) after annealing up to1473 K (1200 �C).

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, SEPTEMBER 2012—3363

nanostructured material. TEM results also approved theXRD results thoroughly.

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