Stress and Temperature Dependence of Screw Dislocation Motion in Refractory Multi-Principal Element Alloys

Abstract The motion of dislocations controls the strength of alloys with a body-centered cubic structure. Recently, new alloys with body-centered cubic structure, called refractory multi-principal element alloys, have demonstrated outstanding high-temperature strength, with some compositions exhibiting exceptional stability in strength with respect to temperature—a so-called athermal regime. Despite recent advances, the understanding of the impact and glide mechanisms of dislocations at high temperatures, particularly corresponding to the athermal regime, in these unusual yet superior alloys are lacking. Here, a phase field dislocation dynamics model is employed to predict the energetically favorable pathways taken by initially screw-character, long dislocations as a function of temperature and driving stress. The simulations resolve the critical stress at which glide becomes jerky to smooth, the changes in the glide mechanisms as temperature increases, and the local impact of variations in composition-dependent energetic barriers. It is shown that the jerky-to-smooth stress exhibits a two-stage response, where it decays with temperature at low temperatures and transitions to an athermal regime at high temperatures, like that measured for these alloys. The analysis elucidates the changes in the glide processes responsible for the onset of the athermal regime in critical stress and shows a close connection to experimentally measured athermal temperatures, suggesting that screw dislocations might impact the high-temperature strength of these alloys.