Grain Boundary Migration in Iron During Zincification

Abstract

Diffusion of a solute into a polycrystalline sample at temperatures where only grain boundary diffusion is active has been observed to induce otherwise stable grain boundaries to migrate. This phenomenon has been observed in many binary metal systems^1–5 under a variety of experimental conditions. The region swept by the boundary has a much higher solute concentration than can be accounted for by volume diffusion. The solute is left behind as the boundary migrates, while the region ahead of the migrating boundary remains solute free. Since no large scale size change in the samples is observed (above that due to a change in the molar volume), solvent atoms must leave the interior of the sample and diffuse to the surface via grain boundaries, or cause dislocation motion or void formation. Besides this net flux of solvent atoms from the grain boundary area, a net flux of the solvent atoms across the boundary is necessary for boundary motion. While it is clear that the driving force for this boundary motion must be the reduction in the total energy of the system that accompanied mixing, how the driving force couples with the atomic transport necessary for boundary motion is not known⁴.