Dislocation wall formation during interdiffusion in thin bimetallic films

Abstract

Transmission electron microscopy observations are described of the diffusion-induced behaviour of misfit dislocations originally present in the interface of thin bimetallic films. Experiments were carried out with specimens consisting of a layer of approximately 500 Å Cu vapour deposited onto an electropolished Ni substrate approximately 1000 Å thick. Diffusion anneals were performed in situ in the electron microscope at annealing temperatures in the range 450–600°C. The dislocation behaviour in Cu/Ni bicrystals with originally a (100) interface was photographed and video-recorded. A cross-grid of misfit dislocations parallel to 〈110〉 directions was present in the original interface. The Burgers vectors were of type 1 2 a〈110〉 lying in the interface. During diffusion the misfit dislocations became distributed in the diffusion zone. When 2Dt (where D is the diffusion coefficient and t is the annealing time at a given temperature) exceeded a value of 40–50 Å, the dislocations started to align vertically forming dislocation walls along 〈110〉 directions parallel to the original interface. This resulted in a dislocation cell structure. Lengthwise the dislocation walls grew with shocks. The elastic strain energy of a finite edge dislocation array was estimated. Using this result an energy criterion for the formation of dislocation walls was derived. From this criterion it followed that dislocation wall formation may start to occur when 2Dt ≈ 45 Å, in good correspondence with the experimental results. Some additional observations of recrystallization phenomena during interdiffusion are reported.