Diffusional Asymmetry in Amorphous Alloys:Implications for Interfacial Reactions

Abstract

AbstractEarlyllate transition metal systems such as Ni-Zr and Co-Zr exhibit solid state amorphization (SSA) in which the amorphous phase is formed by reaction between the crystalline elements. The rate of the amorphization is governed by the diffusion of the faster species, Ni or Co. Here results are presented on the homogenization of compositionally modulated thin films which show that the Zr diffusion is up to 106 times slower. The difference in diffusivities is correlated with atomic size. The consequences of the marked diffusional asymmetry are considered, particularly for the interpretation of results on the indiffusion of Co into amorphous Co-Zr. It is proposed that for amorphous alloys such as Ni-Zr and Co-Zr, changes in composition by rapid diffusion of Ni or Co can yield structures which are not in internal equilibrium. This would affect, for example, the validity of the common tangent construction as applied to predict the limiting compositions of the amorphous phase in contact with the elemental layers during SSA.