Study of dislocation - vacancy clusters interaction by numerical simulations

Vacancy clusters have been observed and characterized experimentally in highly pure metals after plastic deformation or after a particular sequence of heat treatments. These clusters hinder the dislocation propagation and can therefore harden the metal. Using numerical simulations we have explored different mechanisms of dislocation propagation through a vacancy-cluster distribution, for several applied shear stress and temperature. At high stresses, the force applied on the dislocation becomes greater than the pinning forces acting on the line. The dislocation gets through the cluster distribution by gliding and shearing the clusters. The dependence of the pinning force with the cluster size is adjusted on our molecular static simulations. In this stress range, the pinning configurations are rare and the thermal activation is sufficient to unpin the line. The probability for the line to pass the pinning configuration depends on the activation enthalpy, a parameter that we have also estimated using an analytical model adjusted on our atomistic results. At lower stresses, when the applied force is below the pinning forces induced by the cluster, the probability that the dislocation unpins by pure glide becomes negligible. The diffusion of vacancies, emitted preferentially from the vacancy clusters, intervenes and promotes the formation of jogs that contributes to the unpinning of the line. Such a mechanism is the glide assisted by climb. The emission, the absorption and the vacancy migration barriers have been determined by molecular static and are highly dependent on the elastic field and the atomic network distortion induced by the dislocation. This promotes a strong diffusion anisotropy in the vicinity of the dislocations which leads in particular to the pipe diffusion mechanism. The evolution with time of all these mechanisms has been studied using an elastic line model coupled to a kinetic Monte Carlo algorithm in which the parameters come from our atomistic simulations. According to the model assumptions, we obtained an estimation of dislocation velocity as a function of the applied shear stress and the temperature. We used the Orowan's law to estimate the strain rate related to such mechanisms. (author)