Recrystallization and interdiffusion in thin bimetallic films

Diffusion-induced recrystallization phenomena in thin bimetallic films are described. For specimens consisting of a copper layer vapour deposited onto an electropolished nickel substrate, diffusion experiments were carried out in situ in the electron microscope in the temperature range 450–600°C. It is concluded from the observations that new grains were formed by self-induced recrystallization. These new grains were curved in order to accomodate the misfit still present. The degrees of interdiffusion of the curved grains and the matrix were determined respectively from the curvature and from the increase in the average moiré spacing. It was found that the curved grains, compared with the matrix, were in a much more advanced stage of homogenization. Assuming that this was caused by grain boundary diffusion occuring in the recrystallization front while it swept through the material, a realistic value for the grain boundary diffusion coefficient was obtained. It is demonstrated that the driving force on the recrystallization front cannot be supplied by the difference in strain energy between the matrix and the curved grains. Instead, it is shown that the driving force can be provided by the chemical Gibbs free energy for the reaction between the matrix and the recrystallized material.