In situ observations of chemically induced grain-boundary migration and discontinuous precipitation in the aluminum-zinc system

In situ observations are reported for chemically induced grain-boundary migration (CIGM) and discontinuous precipitation (DP) in the Al-Zn system. In each case, it is found that the migration of the grain boundary is a steady continuous process, at least on the scale of the resolution of the electron optical techniques employed. However, there are significant differences; the grain-boundary velocities are such that a diffusion field must exist in the parent crystal for the case of CIGM but little or no volume diffusion penetration for steady DP. For CIGM, it is argued that the coherency strain energy in the diffusion field is sufficient for the development of a small residual chemical force; for steady DP, a large fraction of the total chemical force is available to act directly across the grain boundary. In the latter case, agreement is obtained between observed and calculated dynamic boundary shapes, using a method in which the chemical force (corrected for losses due to continuous precipitation in the parent phase) is balanced against capillary forces. Formerly with McMaster University