Solid-state amorphization, interdiffusion, and ion-beam mixing in Au/Zr and Ni/Zr
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. INTRODUCTION Schwartz and Johnson1 first demonstrated that binary amorphous alloys can be formed by solid-state interdiffusion in elemental, thin-film multilayers. These authors proposed two criteria, one kinetic and one thermodynamic, that must be satisfied for amorphization to occur: (1) one of the elements must be an anomalously fast diffuser in the other and (2) the elements must have a large negative heat of mixing in the solid state. Ni/Zr fulfills both criteria. The heat of mixing2 is approximately —0.5 eV per atom and the diffusion coefficient3 of Ni in hexagonal Zr is ~10~ 13 cm 2 /sat 525 K. Indeed, a continuous film of amorphous NiZr has been shown4'5 to form during solid state reaction of Ni/Zr multilayers at temperatures =£ 550 K. Consistent with the kinetic criterion, Ni was found to be the dominant diffusing species during the reaction. 6 Au and Zr have an even larger negative heat of mixing, ~ 0.75 eV per atom,7 but there is no indication of anomalously fast diffusion of one element in the other. In fact, the tracer diffusion coefficient of Au in crystalline Zr is approximately five orders of magnitude smaller than that of Ni. 8 Colgan and Mayer9 found several Au-rich intermetallic phases after thermal reaction of Au/Zr bilayers at temperatures between 625 and 975 K; no evidence of an amorphous phase was reported. However, ion-beam mixing of multilayer Au/Zr films has been shown to produce an amorphous product up to temperatures as high as 540 K.7 Hence it is of interest to examine the reaction products at temperatures below 600 K in Au/Zr for evidence of amorphous phase formation.
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with the Department of Technical Physics, Peking University, Beijing, People's Republic of China.
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Recent reviews of ion-beam mixing are available.10'11 Measurements of ion-beam mixing of Au/Zr7 and Ni/Zr12 as a function of temperature have been reported previously. There is considerable interest in both the product phases of ion-beam mixing, as well as in understanding the increased mixing efficiency that has been reported in many alloy systems (including these) at intermediate temperatures. In particular, many authors have attributed this increased efficiency to thermally activated mass transport arising from the free migration of irradiation-induced point defects.1314 However, as has been argued in detail elsewhere,11 the observed diffusion behavior differs in several respects from the known contributions of freely-migrating irradiation defects in large-grained and single-crystal specimens. A comparison between measured interdiffusion and ion-beam mixing data in the same alloy systems can be expected to help resolve this issue. II. EXPERIMENTAL Markers, including metals and inert gases, are widely used to measure diffusion in bilayer specimens. In general, gases such as Ne, Ar, Kr, and Xe are effective markers because they are chemically inert. Au/Zr and Ni/Zr bilayer specimens with Xe markers embedded at the interface were prepared in an oil-free, ultra-high vacuum chamber equipped with two rate-control
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