A Study of Diffusion of Zinc Along the Grain Boundaries and Boundary Migration in Polycrystalline Aluminum

A detailed study has been carried out to find out if diffusion of zinc along grain boundaries in pure aluminum can cause simultaneous boundary migration similar to that observed during diffusion induced grain boundary migration (DIGM). Light and electron microscopy techniques have been used to study the morphological features of boundary migration, migration rate and composition profiles across the boundaries. The migration rates of migrating boundaries were found to be relatively slow and the activation energy for boundary migration was determined to be 5 kcal/mol, considerably lower than that for boundary diffusion of Zn in Al. The symmetric nature of the zinc profiles across grain boundaries as determined by EDS in an AEM indicate that boundaries or boundary segments remained stationary. The volume diffusion coefficient of zinc in pure aluminum in the temperature range 180 to 235°C was determined from the solute profiles using the same analysis as that used for the collector plate mechanism of precipitate growth from grain boundaries. The (D0 and Q) were found to be 5.6 ⨯ 10-6 m2/sec and 111 kJ/mol, in good agreement with the values of (D0 and Q) obtained by tracer measurements in the temperature range 450-600°C. Coherency strain energy resulting from diffusional penetration of zinc into the grains ahead of migrating boundaries were found to be too small to cause DIGM. A calculation of the total free energy available for boundary migration shows that it is great enough to cause DIGM. The fact that the coherency strain energy is not great enough to cause DIGM suggests that the boundary migration observed resulted from thermally activated boundary migration and not DIGM