The effect of non-linear diffusion on solid state amorphization in Ni-Hf: experiment and simulation*

Solid state amorphization i the Ni-Hf system has been studied by measuring the reaction rate of terminal solutions with as-prepared amorphous alloys of various compositions. This methodology allows direct determination of the common-tangent compositions corresponding to metastable quilibrium. While good agreement with previously reported values was achieved for amorphous alloys in contact with Ni-rich terminal solution, the metastable quilibrium composition of amorphous alloys in contact with the Hf-rich terminal solution is in significant disagreement with previous reports. Analysis of the reaction rate data shows that the apparent diffusion coefficient increases dramatically with increasing initial Ni content in the amorphous layer. Numerical simulation of non-linear diffusion, including the effect of a moving phase boundary, is used to calculate composition profiles for a strongly composition dependent interdiffusion coefficient/)(C). These profiles provide a consistent explanation for the disagreement with previous interfacial composition measurements estimated in reacted elemental diffusion couples. By assuming a specific functional form of the interdiffusion coefficient /)(C), apparent values of/5 corresponding to the experimental samples are obtained and provide semiquantitative agreement. 1