Diffusion driven grain boundary migration in iron during zincification

Abstract

At temperatures where only grain boundary diffusion is active in a diffusion couple, solute diffusion along the boundary can cause the boundary to migrate, mixing solute into the lattice swept by the moving boundary. This phenomenon—diffusion driven boundary migration (DDBM) — was investigated in the iron-zinc system. The (∼40 μm) iron foils and zinc vapor sources were sealed together in evacuated Vycor tubes and annealed at 550 °C to 630 °C for different times. The migration rate of grain boundaries was nearly constant throughout the transformation of the foil to alloy. Migration rates were about the same in pure iron and decarburized steel sheet. The apparent activation energy of DDBM (269 kJ/mol) was well above that expected for boundary diffusion of zinc in iron and is felt to reflect the role of creep in accommodating the local expansion that accompanies the addition of zinc. This expansion is in the plane of the sheet and indicates that D′_Zn in the boundary greatly exceeds D′_Fe. Inbulk samples annealed under the same conditions boundaries did not move (in 16 hours). This is consistent with a marked difference in the boundary mobility of Zn and Fe.