Effect of Stress, Temperature and Langevin Friction on the Rate of Reaction in Dislocation Dipoles

Thermal activation of dislocations is critical to predict the mechanical response of materials under common experimental conditions. According to transition state theory (TST), the rate for the system to overcome free energy barriers is a function of an attempt frequency, activation free energy, and temperature. Using Molecular Dynamics (MD), we have computed the power spectrum of a dislocation dipole at different temperatures to relate it to the attempt frequency. For small Langevin frictions, the power spectrum remains the same, but if the friction is strong, there is a change in the power spectrum. We have also computed the rates from MD, both dynamically and statically. Dynamically, we observed a strong dependence on the Langevin friction. As the friction becomes weaker, the rate increases, and we observe an increase in the number of correlated events. Statically, we computed the minimum energy path and activation free energy relying on Schoeck’s formalism to compute the activation entropy and observed a significant entropic effect. We have also extended the equations of motion in Dislocation Dynamics (DD) to add stochastic forces, and we have computed the rates for similar configurations and compared them with MD results. We have analyzed the effect of discretization of the dislocation in the results for different temperatures and applied stresses. We discuss here the agreements and differences between the two methods, MD and DD.