Interdiffusion and reaction of metals: The influence and relaxation of mismatch-induced stress

The early interdiffusion stages in epitaxially grown Ag/Au and Cu/Au reaction couples are investigated by high-resolution and Z-contrast electron microscopy. While the interdiffusion in the lattice-matched system Ag/Au follows Fick's diffusion laws from the very beginning. a complex two-stage reaction is observed in the lattice-mismatched Cu/Au samples. A fast diffusion at the beginning of the heat treatment produces a planar zone of lattice defects along the interface. which release most of the induced stress. This first reaction stage stops after reaching a diffusion length of about 15 am. Subsequent interdiffusion takes place by a recrystallization mechanism comprising heterogeneous nucleation of new grains and diffusion-induced grain boundary migration. During this second reaction stage, discontinuous composition profiles are determined, evidencing stress release at high-angle grain boundaries. The recrystallization mechanism also dominates the formation Of ordered intermetallics at lower reaction temperatures.