Precipitation induced by diffusivity anisotropy in Sn grains under electron current stressing

Nickel diffuses five orders of magnitude faster along the c-axis of white Sn than along the a-axis. Experimental evidence is presented to show that, when Ni atoms diffuse from a Sn grain with its c-axis aligned with the Ni flux direction, across the grain boundary, and then into a neighboring Sn grain with its a-axis aligned with the Ni flux direction, Ni3Sn4 will preferentially precipitates at the Sn grain boundary. This is a direct proof that precipitation can be caused by the diffusivity anisotropy of a diffusion medium.