A study on the mechanism of amorphous phase formation by interdiffusion in Ni/Zr multilayers
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Geun-Hong Kim and Chang-Hwan Chun Advanced Technology Research Center, Agency for Defense Development, P.O. Box 35, Taejon, Korea (Received 7 February 1994; accepted 11 February 1995)
The mechanism of the solid-state amorphization has been investigated by means of the microstructural studies on the evolution of Ni/Zr diffusion couples, prepared at different sputtering pressures, during short heating times at high temperature. In the sample deposited at 8 mTorr compared to that at 3 mTorr, the possibility of the supersaturation sequence prior to amorphization is observed, and the amorphous phase grows extremely fast with diffuse interface. A high-resolution TEM image shows that the amorphous phase preferentially penetrates along the Zr grain boundary into the Zr layer and selectively grows from the grain boundary into the region of Zr grain with many defects. From the results, the importance of interstitial diffusion has been discussed, and a modified mechanism has been suggested.
I. INTRODUCTION In the first report of amorphization by hydriding, Yeh et al. suggested that the fast diffusion of hydrogen at low temperatures was essential to producing and retaining the amorphous-reacted phase.1 This report promoted Schwarz and Johnson2 to investigate whether other systems with abnormally fast diffusion at low temperature could produce a similar metastable phase. They found the formation and growth of an amorphous interlayer between two elemental crystalline layers when Au/La binary diffusion couples were heated to an appropriate temperature in 1983.2 In this appropriate temperature range, the formation of equilibrium compounds with a still lower free energy than the amorphous phase is prohibited by a kinetic constraint. This constraint has been considered to be correlated with the "diffusional asymmetry" that one constituent diffuses anomalously faster than the other.2~A If the annealing is performed at high temperatures that permit sufficient atomic mobilities of both constituent atoms, it has been believed that interdiffusion had to produce only a crystalline phase. On the other hand, when the annealing temperature is too low, the system is kinetically frozen. This intuitive concept about the diffusional asymmetry thus presents a "temperature window" for amorphization by interdiffusion in multilayer thin films. When polycrystalline zirconium is replaced by single-crystal zirconium in Ni/Zr diffusion couples, no reaction and/or interdiffusion takes place in the temperature window, for example, 300-360 °C.5'6 It has thus been believed that the grain boundary in the Zr layer does act as a heterogeneous nucleation site for J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
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amorphous formation in Ni/Zr diffusion couples. By contrast, the free energy curves calculated by Saunders and Miodownik7 show quite clearly that the free energy of the Ni-Zr system would be substantially lowered by first forming a supersaturated crystalline solid solution, which could then transform in a
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