Atomistic simulation of thermally activated glide of dislocations in FeNiCrN austenite

Thermally activated glide of edge and screw dislocations with the Burgers vector b = a/2〈110〉 in FeNiCr austenite without and with nitrogen has been analyzed using the conjugate gradient method to minimize the potential energy of the crystal (subject to appropriate constraints) and the embedded-atom method to quantify the atomic interactions. The results of the analyses were compared with their experimental counterparts. In austenite without nitrogen, the variation of the crystal energy with the dislocation position gives rise to a Peierls frictional stress. However, the magnitude of this stress is relatively low and both edge and screw dislocations can readily overcome it by the assistance of thermal activation. The presence of nitrogen, on the other hand, causes the core of a screw dislocation to dissociate onto two {111} planes containing the dislocation line. This renders the screw dislocation sessile. Such changes in the core structure of an edge dislocation were not observed. Instead, the interaction of nitrogen with edge dislocations was found to give rise to both an athermal (long-range) and a thermal (short-range) component of the frictional stress. A detailed analysis of the interactions between the nitrogen atoms and the edge dislocations suggests that the athermal component of the frictional stress observed in the present work is most likely responsible for the experimentally observed athermal flow stress in FeNiCr polycrystalline austenite. On the other hand, with a possible exception of very low temperature, the contribution of the thermal frictional stress associated with the interaction of edge dislocations with nitrogen to the strength of FeNiCr polycrystalline austenite is small.