Interdiffusion in Bimetallic Au-Fe Nanowhiskers Controlled by Interface Mobility

Nanoscale interdiffusion at the heterophase interfaces is the elementary process that controls solid-solid phase transformations and intermixing in multilayers. Here, we provide the first experimental proof that the nanoscale multiphase interdiffusion is controlled by the interface mobility. We observed the anisotropic diffusion intermixing in Au-Fe bimetallic nanowhiskers, where the diffusion penetration depth of Fe into single crystalline Au nanowhisker across the incoherent Au-Fe interface was much greater than across its coherent counterpart. By applying a simple kinetic model to the results of diffusion measurements, we obtained the absolute values of mobilities of coherent and incoherent Fe-Au interfaces, and the Arrhenius parameters describing their temperature dependence. Atomic resolution transmission electron microscopy confirmed that the lower mobility of the coherent interface is associated with the difficulties of the nucleation of interface disconnections. We proposed that the sluggish movement of the latter represents a kinetic "bottleneck" which determines the rate of intermixing. Our results indicate that the anisotropy of interface mobilities is a leading factor determining the anisotropic shape of precipitates formed in many heterogeneous solid-solid phase transformations.