On the behaviour of Kirkendall markers in solid-state interdiffusion

The migration of inert markers as a result of unequal mobilities of the components during solid-state interdiffusion in a binary system can be rationalized using a Kirkendall velocity construction. It is demonstrated that in a diffusion-controlled interaction the Kirkendall plane, identified by inert particles (markers) placed at the original contact surface of a reaction couple, need not be unique. Multiple planes can develop, but, on the other hand, markers at the original interface sometimes can get dispersed into a diffuse zone on both sides of the contact surface and no unique location of the Kirkendall plans can be defined. It is also shown that the location of the Kirkendall plane(s) inside the diffusion zone can be identified by grain morphology changes within the microstructure of the reaction products. A physico-chemical approach is developed which elucidates the role of the Kirkendall effect in the morphologenesis of interdiffusion systems. The occurence of one or more Kirkendall planes, characterized by morphology changes in the reaction layers, turns out to be related to different nucleation sites of the product grains.