Crack-like grain-boundary diffusion wedges in thin metal films

Constrained grain-boundary diffusion in polycrystalline thin metal films on substrates is studied as a strongly coupled elasticity and grain-boundary diffusion problem in which no sliding and no diffusion are allowed at the f ilm/substrate interface. Surface diffusion and grain-boundary grooving are neglected in the present analysis. We show that such a diffusion process le ads to the formation of crack-like grain-boundary wedges which cause the no rmal traction along the grain boundary to decay exponentially with time. PL rigorous mathematical analysis is performed to derive and calculate the tr ansient solutions for diffusion along a single grain boundary and along a p eriodic array of grain boundaries. An approximate closed-form solution is a lso given as a simple description of constrained grain-boundary diffusion. A most remarkable feature of the solution is that the diffusion wedges indu ce crack-like singular stress concentrations which could also enhance dislo cation plasticity processes in a metal film. (C) 1999 Acta Metallurgica Inc . Published by Elsevier Science Ltd. AN rights reserved.