Temperature dependence of the strength of Nb-Mo-Ta-W alloys due to screw dislocations

Abstract

The Nb-Mo-Ta-W system has attracted significant attention as a model alloy of the refractory multi-principal element alloy family. Nb-Mo-Ta-W crystallizes as a single phase in the body-centered cubic (bcc) structure and possesses exceptional mechanical properties at high temperature. However, unlike in standard bcc metals, the ground-state configurations of screw dislocations in Nb-Mo-Ta-W are highly twisted, with dislocation lines displaying a high concentration of kinks and cross-kinks at all temperatures. This gives rise to a strengthening of chemical nature associated with overcoming this intrinsic line roughness. In this paper we quantify the contribution to the total strength of this ‘self-pinning’ effect using a kinetic Monte Carlo model of screw dislocation lines that is not subjected to some of the traditional limitations of atomistic simulations. We find that the self-pinning stress remains even at high temperatures due to the balance of two competing effects: strengthening due to higher concentrations of kinks on multiple glide planes, and softening associated with the thermal dissolution of cross-kinks.