Formation of solute atmospheres around dislocations

Abstract

Solute atoms form long-range atmospheres around dislocations when in thermal equilibrium. These atmospheres scatter phonons and can be studied through their effect on the thermal conductivity. Previous studies indicate that solute atmospheres are formed after plastic deformation at room temperature in Cu-Al but not in Cu-Ge. Ordinary diffusion at room temperature is too slow to permit atmospheres to form, but excess vacancies produced by plastic deformation can enhance diffusion. One can define a net time-integrated diffusion due to excess vacancies until the excess is exhausted in terms of ${R}^{ 2}$, where $R$ is the range over which the atmosphere attains equilibrium. It depends on the excess concentration and vacancy lifetime. The lifetime of the vacancies is calculated, assuming that the dislocations act as vacancy sinks. The resulting $R$ is independent of the vacancy jump rate, and depends only on the initial vacancy concentration and on the dislocation density. The values of $R$ thus calculated are much smaller than observed. However, the tendency for the vacancies to be associated with the solute atoms, and the possibility of dislocations not acting as perfect sinks of vacancies, may enhance the effectiveness of a vacancy in promoting solute diffusion and bring the value of $R$ closer to the observed magnitude. Estimates of $R$ are given for Cu-Al and Cu-Ge. The case of Al-Mg is also discussed.