Materials Science and Engineering A 367 (2004) 218–224

Synthesis and stability of L12–Al3Ti by mechanical alloying

S.S. Nayak, B.S. Murty∗

Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India

Received 30 May 2003; received in revised form 30 September 2003

Abstract

The present paper reports the synthesis of nanocrystalline L12–Al3Ti by mechanical alloying (MA) of elemental blends of compositionAl67M8Ti25 (M = Cr, Mn, Fe, Co, Ni and Cu). Among all the ternary elements studied, Cu appears to be the best in stabilizing thenanocrystalline L12–Al3Ti phase in the as milled condition. However, the results of annealing in the temperature range of 823–1273 K indicatethat the L12–Al3Ti phase is stable up to higher temperature in case of Cr containing alloy in comparison to that containing Cu. The as milledL12–Al3Ti phase does not show the superlattice peaks on even annealing up to 1273 K, which has been attributed to the nanocrystalline natureof the phase and the low relative intensity of superlattice peaks of L12–Al3Ti phase even in the fully ordered structure.© 2003 Elsevier B.V. All rights reserved.

Keywords: Mechanical alloying; L12–Al3Ti; Nanocrystalline materials

1. Introduction

Mechanical alloying (MA) is a high energy ball millingprocess to synthesize materials with homogeneous mi-crostructure and novel properties from the elemental blends[1–3]. MA is known to lead to the formation of a variety ofstable and metastable phases depending on the alloy systemand MA conditions studied. MA has been shown in the lit-erature as a viable route for the synthesis of intermetallicswith high melting points, which are difficult to form by aconventional ingot metallurgy route[2–4]. In addition, theseintermetallics can be synthesized in nanocrystalline formby MA, which potentially leads to improvement in theirproperties[4–6]. Among all the intermetallics, aluminidesare quite popular and a significant amount of research ef-forts in MA were concentrated on the synthesis of variousaluminides[2,3,7].

Among the aluminides in Al–Ti system, namely, AlTi,AlTi 3 and Al3Ti, Al 3Ti is outstanding due to the lowestdensity, 3.3 g cm−3, and the highest oxidation resistance[8]resulting in the formation of an impervious Al2O3 layerduring its exposition to air. Experimental results have shownthat the Young’s modulus of polycrystalline Al3Ti at roomtemperature is 216 GPa, which clearly surpasses other tita-

∗ Corresponding author. Tel.:+91-3222-83270; fax:+91-3222-55303.E-mail address: bsm@metal.iitkgp.ernet.in (B.S. Murty).

nium aluminides, and is of the order of superalloys[9] andthus Al3Ti is regarded as a candidate for lightweight struc-tural applications in aerospace and automobile industries.However, its brittleness at low temperature limits Al3Tifrom being widely used in industrial applications[10].Al3Ti crystallizes into a D022 (bct) structure under equi-librium conditions. This phase does not show any ductilityat temperatures below 620◦C and cracks nucleate readilyat structural heterogeneities[11]. The lack of ductility inthis alloy is often associated with its insufficient number ofslip systems to satisfy the von Mises’ criterion for the slipdeformation in polycrystalline materials[7].

In order to improve the ductility of Al3Ti, two types ofapproaches have been attempted[11]. The first one is themicroalloying of D022 structure with little success[12].The second, which is more popular, is to transform theD022 structure to the L12 (ordered cubic) structure by theaddition of a number of ternary elements[11–13]. TheL12 structure has a higher symmetry and hence a largernumber of slip systems and thus can have better ductility.The various elements that have been tried for this are Cr[14], Cu, Fe, Ni[15], Mn [16], etc. However, single phaseL12–Al3Ti prepared by conventional casting route did notyield high ductility [13]. Grain refinement is suggested asa viable method for making intermetallics ductile[12]. Mi-crostructural refinement to a nanometer level is expected toincrease both the strength and ductility of intermetallics.

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2003.09.097

S.S. Nayak, B.S. Murty / Materials Science and Engineering A 367 (2004) 218–224 219

There are very few reports on the synthesis of L12–Al3Tiby MA [17–21]. The present paper reports the attemptsto synthesize nanocrystalline L12–Al3Ti by MA with theternary additions of Cr, Mn, Fe, Co, Ni and Cu. The po-tency of these elements in stabilizing the L12 phase has beenstudied. This paper also reports the thermal stability of theL12–Al3Ti obtained by MA.

2. Experimental details

Different elemental blends with the nominal composi-tion Al67M8Ti25 (M = Cr, Mn, Fe, Co, Ni and Cu) werechosen for the synthesis of L12–Al3Ti compounds. Allblends of elemental powders were mechanically alloyedfor 40 h using a high energy planetary mill (Fritsch Pul-verisette 5) in toluene at 300 rpm, with tungsten carbide(WC) balls and vial, and the ball to powder ratio was10:1. For all the compositions, the elemental powders of−325 mesh (<45�m) were chosen except for titaniumwhich was taken in the sponge form. All the MA sam-ples were characterized by X-ray diffraction technique(XRD) with Co K� radiation by PW 1729 X-ray diffrac-tometer. After the MA of 40 h the powders were subjectedto the thermal treatment in an argon atmosphere at dif-ferent temperatures for different holding times to studythe stability of the phases formed during MA. The crys-tallite size of the milled samples and the lattice straindeveloped due to MA were calculated by separating theirpeak broadening contributions in terms of Cauchy andGaussian components of peak profile, respectively, us-ing Voigt analysis[22]. The nanocrystalline nature of themilled powder has been confirmed by transmission elec-tron microscopy (TEM) using PHILIPS CM30 electronmicroscope.

Fig. 1. XRD patterns of Al67Cu8Ti25 after different durations of MA.

3. Results and discussion

Fig. 1 shows the XRD pattern of the Al67Cu8Ti25 afterdifferent durations of MA. Within 10 h of MA, the Cu peaksmore or less completely vanish suggesting the formation ofa solid solution in Al. This is associated with a decreasein the lattice parameter of Al from 0.4051 to 0.4046 nm(Fig. 2). This indicates that the dissolution of Ti and Cuinto the Al lattice caused a decrease in the lattice parame-ter. Cu has an atomic radius of 0.128 nm, which is smallerthan that of Al (0.143 nm) and hence dissolution of Cu intoAl is expected to decrease the lattice parameter of Al. Atthis stage the crystallite size of Al is 22 nm and that of Tiis 35 nm. Twenty hours of MA resulted in the disappear-ance of both Al and Ti peaks and the appearance of a newbroad peak at 2θ of 45.95◦, which has been identified asthe most intense peak of L12–Al3Ti. At this stage the lat-tice parameter was calculated as 0.3980 nm, which matchesexactly with the previously reported value by Hong et al.[23]. With increase in MA time, a marginal decrease in thelattice parameter to 0.3967 nm after 30 h has been observed,which remained unchanged after 40 h of MA (Fig. 2). Evenafter 40 h of MA, the lattice parameter remained close tothat of L12 phase as reported earlier[23,24]. The absenceof the superlattice reflections makes it difficult to call theAl3Ti phase as ordered L12 phase. However, the relative in-tensity of the superlattice reflections is very low in the stan-dard JCPDS file and hence it is difficult to find them in thenanocrystalline state, as the intensity of peaks broaden andbecome less intense and merge into the background noise inthe XRD diffractograms. In addition, the fundamental peaksin the present XRD pattern showed a very close match to thatof L12 phase. Hence, it can be concluded that the L12 phasehas formed within 20 h of MA. Interestingly, even the earlierworks [17–21] on the synthesis of the L12–Al3Ti by MA

220 S.S. Nayak, B.S. Murty / Materials Science and Engineering A 367 (2004) 218–224

0 10 20 30 400.3950

0.3975

0.4000

0.4025

0.4050

0.4075

L12 Phase

Lat

tice

Par

amet

er (

nm

)

MA (hours)

Al67

Cu8Ti

25 MA

Fig. 2. Variation of lattice parameter with MA time for Al67Cu8Ti25.

do not show the presence of superlattice reflections in theXRD pattern. However, it has been reported[21] that Braggpeaks for the superlattice of the L12–Al3Ti compound arerather weak due to small difference in the scattering factorsof Al and Ti. It is also possible that the L12–Al3Ti formedis partially disordered in the MA conditioned.

The XRD pattern of Al67M8Ti25 (where M = Cr, Mn,Fe, Co, Ni and Cu) after 20 h of MA is shown inFig. 3.All elemental peaks have disappeared after 20 h of MA, ex-cepting the case of Mn containing sample, wherein Mn peakremained. This suggests Mn does not completely dissolvein Al within 20 h of MA. However, the fundamental peaksof L12–Al3Ti have been observed in all the cases. No peaksother than L12–Al3Ti peaks have been observed after 20 h

Fig. 3. XRD patterns of Al67M8Ti25 (M = Cr, Mn, Fe, Co, Ni and Cu)after 20 h of MA.

of MA suggesting the formation of single phase L12–Al3Tiin all the ternary cases excepting the Mn containing alloy.These results indicate that the addition of ternary elements(excepting Mn) result in the early formation of single phaseL12–Al3Ti. The superlattice peaks of the L12–Al3Ti phasewere not observed in all the cases, which could be due tothe reasons explain earlier.

Fig. 4 compares the XRD plots of all the compositionsstudied after 40 h of MA. In the Mn containing alloy theformation of L12–Al3Ti phase has been observed even after40 h of MA and some peaks of Mn retained (not shown inFig. 4). However, all the ternary elements studied showedthe retention of L12 phase. In addition, the formation ofsome amorphous phase along with the L12 phase is evident

Fig. 4. XRD patterns of Al67M8Ti25 (M = Cr, Mn, Fe, Co, Ni and Cu)after 40 h of MA.

S.S. Nayak, B.S. Murty / Materials Science and Engineering A 367 (2004) 218–224 221

0 10 20 30 400.3950

0.3975

0.4000

0.4025

0.4050

0.4075

Al67

M8Ti

25 MA

Lat

tice

Par

amet

er (

nm

)

MA (hours)

Cr Mn Fe Co Ni Cu

Fig. 5. Variations of lattice parameter of different alloys with MA time.

in all the compositions studied. The extent of amorphisa-tion is a function of ternary elements added. In case of Crand Ni almost complete amorphisation is visible from theXRD patterns, and the Cu containing alloy showed the leasttendency for amorphisation among all ternary elementsstudied. Amorphisation in Al–Ti system over a wide com-position range of 10–75%Ti has been reported earlier byone of the present authors under different milling conditions[25]. From the present results it can be concluded that Cuhas a greater tendency to stabilize L12–Al3Ti phase duringMA, among all the elements studied.

Fig. 5 shows the lattice parameter of different composi-tions studied as a function of MA time. In all the cases, thelattice parameter decreases with increase in MA time andreaches almost the same value after 40 h of MA, which isvery close to that of L12–Al3Ti [17,23,24]. The variation inlattice parameters (after 40 h of MA) among different com-

10 15 20 25 30 35 400

5

10

15

20

25

30

35

40

MA (hours)

Cry

stal

lite

Siz

e (n

m)

Al67

M8Ti

25 MA Cr

Mn Fe Co Ni Cu

Fig. 6. Variations of crystallite size of different alloys with MA time.

positions studied is in the range of 0.4%. Even after 30 h ofMA the lattice parameter is more or less the same in all thecompositions, except Mn containing alloy, suggesting thatsingle phase L12–Al3Ti phase forms by 30 h in all compo-sitions, except in Mn case where formation of L12–Al3Tiphase was not complete.

The variation of the crystallite size with MA time for dif-ferent compositions is shown inFig. 6. This figure showsthe effects of the ternary elements on the crystallite size ofthe mechanically alloyed L12–Al3Ti. After 40 h of MA, theCr containing alloy showed the smallest crystallite size andthe Ni containing alloy had the largest. The crystallite sizecalculated from the X-ray peak broadening is below 15 nm

Fig. 7. TEM bright field images: (a) Al67Cu8Ti25 and (b) Al67Co8Ti25

after 40 h MA.

222 S.S. Nayak, B.S. Murty / Materials Science and Engineering A 367 (2004) 218–224

10 20 30 400.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Lat

tice

Str

ain

(%

)

Al67

M8Ti

25 MA Mn

Fe Co Ni Cu

MA (hours)

Fig. 8. Variation of lattice strain of different alloys with MA time.

in all the cases after 40 h of MA. A close look at the crystal-lite size and lattice parameter variation with MA time showsthat L12–Al3Ti phase formed after the crystallite size of Alreached about 15 nm, which occurred after 20 h of MA. TheTEM studies have confirmed the crystallite size calculatedby the X-ray peak broadening.Fig. 7(a) shows the brightfield image of Cu containing alloy after 40 h of MA, whichreveals nanocrystalline Al3Ti particles with size of about10 nm. The rings in the selected area diffraction pattern (in-set of Fig. 7(a)) indicate the nanocrystalline nature of theL12–Al3Ti. Fig. 7(b)shows the TEM bright field image ofCo containing alloy after 40 h of MA, which also revealssimilar sized particles of about 10 nm. These observed crys-tallite sizes are slightly larger (by about 5 nm) than thosecalculated from the X-ray peak broadening (Fig. 6). The lat-tice strain by Voigt analysis is shown inFig. 8 for a number

Fig. 9. XRD pattern of the Al67Cu8Ti25 after annealing for different timesat 823 K.

of alloys as a function of MA time. A continuous increase inthe lattice strain with increase in MA time is observed in allthe alloys except Mn containing alloy. The extent of latticestrain developed is almost the same in all the alloys, with ex-ception of Mn containing alloy. The Mn containing alloy didnot show any significant increase in the lattice strain evenup to 40 h of MA. This could be attributed to the fact thatMn did not completely dissolve in Al even after 40 h of MAunder the milling conditions employed in the present study.

The stability of the nanocrystalline L12–Al3Ti phaseobtained by MA has been studied by annealing in the tem-perature of 823–1273 K for times up to 12 h.Fig. 9 showsthe XRD patterns for the Al67Cu8Ti25 after annealing at823 K for different durations. It is clear from the figurethat the L12 phase formed after MA is stable up to 12 hat 823 K. However, a new peak appeared near 50◦, whichcould not be identified as any known phase in the system.

Fig. 10. XRD pattern of (a) Al67Cu8Ti25 and (b) Al67Cr8Ti25 after 40 hof MA and on subsequent annealing at different temperature for 8 h.

S.S. Nayak, B.S. Murty / Materials Science and Engineering A 367 (2004) 218–224 223

The lattice parameter of the Al67Cu8Ti25 alloy after an-nealing was found to be 0.3989 A, which is same as that ofL12–Al3Ti phase reported earlier[23]. Interestingly, no su-perlattice peaks were visible, even after annealing at 823 Kfor 12 h, suggesting that nanocrystalline Al3Ti has strongresistance to ordering. The earlier reports also indicatedthat the superlattice reflections did not appear even after an-nealing[18–21]. Fan et al.[18] studied MA of Al67M8Ti25(M = Cr, Zr and Cu) and showed no superlattice reflectionseven after annealing up to 973 K in all the compositionsstudied. Similarly, Zhang et al.[21] reported the thermalstability of the binary Al–25%Ti after MA with the absenceof superlattice reflections.

Fig. 10(a and b)show the XRD patterns of Al67Cu8Ti25and Al67Cr8Ti25, respectively, after annealing for 8 h at dif-ferent temperatures in the range of 823–1273 K. The L12

0 2 4 6 8 10 12 140

5

10

15

20

25

30

35

40

45

0.5

1.0

1.5

2.0

2.5

Cry

stal

lite

Siz

e (n

m)

Annealing Time (hours)

Al67

Cu8Ti

25 HT at823K

StrainCrystallite Size

Str

ain

(%

)

(a)

200 400 600 800 1000 12000

5

10

15

20

25

30

35

40

45

Al67

M8Ti

25 HT for 8 hours

Cry

stal

lite

Siz

e (n

m)

Temperature (K)

8% Cu 8% Cr

(b)

Fig. 11. (a) Variation of crystallite size and lattice strain of Al67Cu8Ti25 with annealing time at 823 K and (b) variation of crystallite size of Al67Cu8Ti25

and Al67Cr8Ti25 with annealing temperature (for 8 h annealing time).

phase peaks are evident even at 1273 K, suggesting that it isstable even at high temperatures. However, some new peaksappeared from 823 K onwards, which could not be identi-fied as peaks belonging to D022–Al3Ti phase. Thus, the newphase appears to be emerging out of L12–Al3Ti. Interest-ingly, the position of L12–Al3Ti XRD peaks did not changeeven after annealing at 1273 K, suggesting that the lattice pa-rameter and hence the composition of the L12–Al3Ti phaseremains same up to the highest temperature studied. Thus,the new phase that forms is not expected to be of differentcompositions and hence could be of a new crystallographicform of Al3Ti. In case of Cu containing alloy, the formationof D022 phase is observed after annealing at 1273 K, whichis not observed in Cr containing alloy, suggesting that Cr canretain the L12–Al3Ti phase up to high temperatures. Stud-ies on other alloying elements show that D022 phase forms

224 S.S. Nayak, B.S. Murty / Materials Science and Engineering A 367 (2004) 218–224

in all the compositions excepting Cr, after annealing for 8 hat 1273 K. Thus among all the compositions studied, Cr ap-pears to be the strongest stabilizer of the L12–Al3Ti phaseformed after MA, which is consistent with the earlier reportof Fan et al.[18].

Sharpening of the peaks in annealed alloys inFigs. 9 and10suggest an increase in crystallite size or grain growth anddecrease in lattice strain.Fig. 11(a)shows the variation ofcrystallite size and lattice strain of Cu containing alloy as afunction of annealing time at 823 K. Though grain growth isevident on annealing, the Al3Ti phase remained nanocrys-talline with a crystallite size of about 40 nm even after 12 hof annealing at 823 K. There exists inverse relationship be-tween crystallite size and the lattice strain both after MAand also after annealing, which is expected (Figs. 6, 8 and11(a)). Fig. 11(b)shows the variation of crystallite size ofCu and Cr containing alloys as a function of annealing tem-perature (annealing time 8 h). The figure suggests that Crcontaining alloy has a lower crystallite size (38 nm) than Cucontaining alloy (45 nm) even at 1073 K. This could be dueto the higher diffusivity of Cu than that of Cr in Al.

4. Conclusions

1. Nanocrystalline L12–Al3Ti intermetallic compound withthe crystallite size less than 15 nm has been successfullysynthesized directly by MA of Al67M8Ti25 (M = Cr,Mn, Fe, Co, Ni and Cu). Among all the elements studiedCu appears to stabilize L12–Al3Ti phase most in the MAcondition.

2. The L12–Al3Ti phase forms after the crystallite size ofAl reaches about 15 nm in all the compositions studied.

3. The L12–Al3Ti phase is stable up to 1273 K in all thecases. However, a new phase forms after 823 K. Forma-tion of D022 structure has been observed at 1273 K inall the cases except in Cr containing alloy, suggestingthat latter is the best in retaining the L12–Al3Ti phaseup to high temperatures among all the ternary elementsstudied.

4. Superlattice peaks of L12–Al3Ti phase have not beenobserved in any of the compositions studied, both in theas milled and annealed conditions.

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