Formation of solute atmospheres around dislocations by excess vacancies

Abstract

Long range solute atmospheres tend to form around dislocations by diffusion. They scatter phonons and can be studied through their effect on the lattice thermal conductivity. Previous studies indicate that these atmospheres are formed after plastic deformation in Cu-Al but not in Cu-Ge alloys. Ordinary diffusion at room temperature is too slow to permit atmosphere formation, but excess vacancies originally produced during plastic deformation can enhance diffusion. The vacancies diffuse towards the dislocation, where they are annihilated, and their presence causes solute diffusion and hence atmosphere formation. The atmospheres attain equilibrium over a range R, and R 2 can be expressed in terms of a time-integrated diffusion coefficient due to excess vacancies until the excess is exhausted. This range R is independent of the vacancy jump rate, and depends only on the initial vacancy concentration and the dislocation density. The values of R thus calculated are much smaller than those observed. However, R is increased if there is enhancement of the solute diffusion relative to the solvent diffusion and also if dislocations do not act as perfect vacancy sinks. Estimates of R are given for Cu-Al, Cu-Ge and Al-Mg.