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nology, ed. G. Lutjering, U. Zwicker and W. Bunk. (Deutsche Gesellschaft fiir Metallkunde, 1985) p. 1759. [11] H. Nakajima and M. Koiwa, Def. Diff. Forum 66-69.
Journal of Non-Crystalline Solids 150 (1992) 456-459 North-Holland

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The influence of atmosphere on the amorphization reaction by mechanical alloying for the Ni-Ti system K.Y. Wang, T.D. Shen, J.T. W a n g a n d M . X . Q u a n State Key Laboratory of Rapidly Soh'dified Non-equilibrium Alloys, Institute of Metal Research, Academia Sinica, Shenyang 110015, People's Republic of China

A study of the amorphous phase reaction by mechanical alloying (MA) in different atmospheres has been carried out. Structural changes during millingwere examined by means of X-ray analysis. The amounts of the gaseous elements absorbed by the powder particles during milling were determined by chemical analysis. It was found that there is almost no influence on the MA process either in an argon or a nitrogen atmosphere. During the milling process in air or an oxygen atmosphere, intermetallic compounds and oxides appeared, accompanying the formation of the amorphous phase. The intermetallic compounds appeared due to the presence of oxygen atoms.

1. Introduction Within the past few years, interest has focussed on the preparation of amorphous alloys by mechanical alloying (MA). Starting from elemental crystalline powders a large number of alloy systems, for example N i - N b [1], N i - T i [2] and several transition m e t a l - Z r systems [3,4], have been investigated. In general, the milling is performed in an inert gas atmosphere in order to avoid the oxidation of the powders. Koch et al. [1] prepared amorphous Ni60Nb40 from the elemental powders by MA in air and helium atmospheres. The crystallization features and the position of the maximum of the principal peak in the X-ray diffraction pattern are different, due to the different oxygen concentrations in the alloys. Mizutani and Lee [5] have studied the effect of excessive mechanical alloying on glass formation, by continuing ball-milling beyond the completion of glass formation for N i - Z r powders. A partial crystallization [5] took place which was attributed to oxygen contamination and other impurities. Correspondence to: Professor J.T. Wang, Institute of Metal

Research, Academia Sinica, Wenhua Road 72, Shenyang 110015, People's Republic of China. Tel: +86-24 383 531. Telefax: + 86-24 391 320.

Titanium is a reactive metal; commercially available titanium contains oxygen at concentration of the order of a few thousand ppm. Therefore, it is important to know how the solid-state amorphization reaction is affected by oxygen. In previous work [6], we have reported the effects of oxygen on the mechanical alloying of Ni60Ti40 composite powders in an air atmosphere. The results show that there are different transformation paths when this material is milled in different atmospheres. In the present work, the elemental crystalline nickel and titanium powders are milled under different atmospheres and the effects of the gas atoms are investigated.

2. Experimental procedures The mechanical alloying was performed in a planetary ball mill using elemental nickel (99.8%, average size 20 Ixm) and titanium (99.8%, average size 10 Ixm) powders which are blended to the composition 60 at.% N i - 4 0 at.% Ti. At a selected milling time, a small part of the mixed powder was removed for analysis. High purity argon (Ar), nitrogen (N2), air and oxygen (02) were used in four mechanical alloying processes

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K.Y. Wang et al. / Influence of atmosphere on the amorphization reaction

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Table 1 Mechanical process and atmospheres used in this study

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Process

0-6

6-8

8-20

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Ar N2 Ar Ar

MAI MAll MAIII MAIV

20-32

32-60

Ar

Ar

(MAI, MAIl, MAIII and MAIV) in different atmospheres, as shown in table 1. The samples were characterized by X-ray diffraction (XRD) using a Rigaku X-ray powder diffractometer with Cu Kot radiation (A = 0.154 nm). The amounts of the gas elements absorbed by the powder particles during milling were determined by chemical analysis.

3. Results

For the MAI process, the XRD patterns for Ni and Ti powders (in the atomic ratio Ni60Ti40) after different milling times are presented in fig. 1. Starting with elemental nickel and titanium, after 6 h milling the typical broad maximum (around 20=43.3 °) of the amorphous phase shows up in the diffraction pattern (see fig. l(b)). Further milling up to 20 h leads to a completely (d)

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amorphous powder with no indication of any intermetallic phase (see fig. l(d)). The oxygen content has increased from 0.05 wt% in the initial powder mixture to about 0.60 wt% in the powders after 20 h milling in an argon atmosphere. For comparison, the powder after 6 h milling in an argon atmosphere is further milled in a high purity nitrogen atmosphere (MAil). The XRD pattern is shown in fig. 2. After milling for a further 2 h (total time = 8 h) in the nitrogen atmosphere, the compounds Ni3N and Ti2N appear, and coexist with Ni and Ti and the amorphous phase a-NixTil_ x (see fig. 2(a)). There are no intermetallic compounds. Further milling in the nitrogen atmosphere up to 14 h (total time = 20 h) leads to a completely amorphous powder with no indication of any intermetallic phase. The diffraction peaks for Ni3N and Ti2N are also annihilated under the broad maximum of the amorphous phase (see fig. 2(c)). In addition, the nitrogen content has increased from 0.14 wt% in the powders after 6 h milling in an argon atmosphere to about 1.07 wt% in the powders after 20 h of milling (MAIl process). Figure 3 shows the XRD patterns for the MAIII process and it is found that different peaks appear for different milling times. When the MA was performed in air during the 6-8 h milling period, the oxide Ni2TiaO and the intermetallic compound Ni3Ti coexisted with residual Ni and the amorphous phase, a-NixTil_ . (see fig. 3(a)).

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K.Y. Wang et aL / Influence of atmosphere on the amorphization reaction

The oxygen content increased from 0.3 wt% in the powders after 6 h milling in an argon atmosphere to about 3.1 wt% in the powders following further milling for 2 h in air. The MA was then performed in argon during the 8-16 h milling period and it is noted that the amount of Ni2Ti40, Ni and Ni3Ti decreases while the intermetallic compounds NiTi and Ti2Ni and the oxide TiO appear after this milling time (see fig. 3(b)). The MA process in argon was continued during the 16-60 h milling period. It is found that there is small amount of the intermetallic compounds NiTi, Ti2Ni and Ni3Ti at a milling time of 32 h (see fig. 3(c)). At a milling time of 60 h, there are only TiO and the amorphous phase in final powders (see fig. 3(d)). The intermetallic compounds have completely transformed into the amorphous phase by mechanical grinding (MG). The Ni2TiaO phase may be metastable and decompose easily. The following reaction may occur during the 8-16 h milling period: Ni2Ti40 ~ NiTi + Ti2Ni + TiO. The powder after 6 h milling in an argon atmosphere was also further milled for 2 h (total time = 8 h) in a high purity oxygen atmosphere (MAIV). The X R D pattern is present in fig. 4. It is noted that the intermetallic compound Ni3Ti appears in addition to the oxides Ti203, TiO2,

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Fig. 4. XRD patterns for the MAIV process. (a) 6-7 h in 02, (b) 7-8 h in O z, (c) 8-16 h in Ar and (d) 16-32 h in Ar. e, Ni; ©, Ti; ®, Ni3Ti; ,TiO2; II, Ti203; @ Ti305; and D, TiO.

T i 3 0 5 and TiO and the amorphous phase aNixTil_ x (see figs. 4(a) and (b)). The powder color changes from grey to black, which may be the color of the oxide phases. The oxygen content has increased from 0.3 wt% in the powders after 6 h milling in an argon atmosphere to about 4.9 wt% in the powders following further milling for 2 h in an oxygen atmosphere. Further milling up to 32 h (total time) under an argon atmosphere leads to the transformation of the intermetallic compound Ni3Ti into the amorphous phase and the oxide also exists in the final powders (see fig. 4(d)). The color of the powder becomes grey again.

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50 60 70 80 2@(~gree) Fig. 3. XRD patterns for the MAIII process. (a) 6-8 h in air, (b) 8-16 h in Ar, (c) 16-32 h in Ar and (d) 32-60 h in Ar. e, Ni; G, Ni2TiaO; ~, TiaNi; ®, NiaTi; @, NiTi; and 121,TiO.

From the above results, we can see that the effects of an argon or nitrogen atmosphere on the mechanical alloying process are similar. Argon is an inert gas and so it is reasonable that there is no effect on the amorphization reaction in this atmosphere. The mechanical alloying of elemental chromium and C u - C r alloy in the nitrogen atmosphere was reported by Ogino et al. [7]. They found that large amounts of nitrogen were absorbed during milling in a r g o n - a i r and a r g o n -

K.Y. Wang et al. / Influence of atmosphere on the amorphization reaction

nitrogen atmosphere, resulting in significant grain refinement and subsequent amorphization of chromium and Cr70Cu30 powders. In the present case, it seems that the amorphization reaction is not affected by the nitrogen atoms. The solid state amorphization reaction by mechanical alloying in the Ni-Ti system may occur by any of the following mechanisms, or combinations thereof: (a) diffusion of Ti through lattice defects in c~-Ni followed by nucleation of amorphous Ni7~Ti28 [8]; (b) nucleation and growth of an amorphous layer at the Ni/Ti interfaces, followed by diffusion of Ni a n d / o r Ti across this layer [2]; and (c) anomalously fast diffusion of Ni through et-Ti into an already nucleated amorphous alloy [9]. Nakajima and Koiwa [10] made a measurement of the diffusion coefficient of the element Ni in single crystal et-Ti. This element exhibits very fast diffusion. They also indicated that the diffusion of nickel [10] is not much affected by oxygen. The oxygen and the diffusing atoms share common sites formed by the interstices of the hcp Ti lattice. Since the diffusivity of oxygen is much smaller than that of the diffusing atoms, oxygen may be regarded as immobile. They further assume that oxygen does not chemically attract the diffusing atoms. The long-range migration of the diffusing atoms is retarded by the presence of oxygen atoms and the sites occupied by oxygen atoms are no longer available for them [11]. The milling performed under the oxygen atmosphere results in the adhesion of oxygen atoms to the amorphous a - N i x T i ~ _ x / N i , aN i x T i l _ x / T i and Ni/Ti interfaces created by cold welding and fracture. The interdiffusion of Ni and Ti atoms and their diffusion in amorphous a-NixTit_ x will be hindered by the oxygen atoms at the interfaces. From the above discussion, mechanism (a) may be the first to operate during the mechanical alloying process in an argon atmosphere. However, milling in oxygen or air, the adhesion of the oxygen atoms to the Ti atoms hinders the diffu-

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sion of Ti through the lattice defects in tx-Ni which is normally followed by the nucleation of amorphous Ni72Ti28. The hindrance of the diffusion of Ti may result in the production of a Ni-enriched intermetallic compound. Probably the formation of Ni3Ti is kinetically more favored than that of other intermetallic compounds.

5. Conclusions

It can be seen that the amorphization reaction is affected by the milling atmosphere. The amorphous Ni60Ti40 phase can be obtained directly through mechanical alloying in an argon atmosphere. Nitrogen has only a small influence on the MA process. However, oxygen (even in only small amounts) has a great effect on the MA process. The formation of the intermetallic compound Ni3Ti is attribute to the presence of oxygen atoms.

References [1] C.C. Koch, O.B. Cavin, C.G. Mckamey and J.O. Scarbrough, Appl. Phys. Lett. 43 (1983) 1017. [2] R.B. Schwarz, C.K. Petrich and C.K. Saw, J. Non-Cryst. Solids 76 (1985) 281. [3] E. Hellenstern and L. Schultz, Appl. Phys. Lett. 48 (1986) 124. [4] L. Schultz, J. Less-Common Met. 145 (1988) 233. [5] U. Mizutani and C.H. Lee, J. Mater. Sci. 25 (1990) 399. [6] K.Y. Wang, T.D. Shen, M.X. Quan and J.T. Wang, J. Mater. Sci. Lett. 11 (1992) 129. [7] Y. Ogino, S. Murayama and T. Yamasaki, J. Less-Common Met. 168 (1991) 221. [8] H. Schroder, K. Samwer and U. Koster, Phys. Rev. Lett. 54 (1985) 197. [9] R.B. Schwarz and W.L. Johnson, Phys. Rev. Lett. 51 (1983) 415. [10] H. Nakajima and M. Koiwa, Titanium Science and Technology, ed. G. Lutjering, U. Zwicker and W. Bunk (Deutsche Gesellschaft fiir Metallkunde, 1985) p. 1759. [11] H. Nakajima and M. Koiwa, Def. Diff. Forum 66-69 (1985) 395.