Microstructural observations and mechanisms of lamellar coarsening in iron meteorites

In many iron meteorites, a lamellar structure is found which consists of ferrite (α) and another phase which is probably austenite (γ). In some of these regions, this lamellar structure has decomposed to a coarser structure of α and another phase which may be austenite. Microstructural evidence is presented that the coarsening occurs by movement of a high-angle boundary separating the two lamellar structures. Examination of diffusion coefficients in the Fe-Ni system shows that the coarse structure could not have formed by volume diffusion, but the lamellar spacing is consistent with formation by diffusion along the interface. Diffusion calculations are made which support the argument that high-angle interface diffusion allows the coarsening to occur at temperatures considerably below those where volume diffusion is too low.