Thermal super-jogs control the high-temperature strength plateau in Nb-Mo-Ta-W alloys

Abstract

Refractory multi-element alloys (RMEA) with body-centered cubic (bcc) structure have been the subject of much research over the last decade due to their high potential as candidate materials for high-temperature applications. Most of these alloys display a remarkable strength at temperatures above 1000 ∘C, which cannot be explained by the standard model of bcc plasticity dominated by thermally-activated screw dislocation motion. Recent research on Nb-Mo-Ta-W alloys points to a heightened role of edge dislocations during mechanical deformation, which is generally attributed to atomic-level chemical fluctuations in the material and their interactions with dislocation cores during slip. However, while this effect accounts for levels of strength that are larger than what might be found in a pure metal, it is not sufficient to explain the high yield stress found at elevated temperatures, particularly in the so-called strength ‘plateau’ region. In this work, we propose a strengthening mechanism based on the existence of thermal super-jogs in edge dislocations that act as strong obstacles to dislocation motion. The basis for the formation of these super-jogs is found in the unique properties of RMEA, which display vacancy formation energy distributions with tails that extend to almost zero energy. This facilitates vacancy formation at dislocation cores, which subsequently relax into atomic-sized super-jogs. While these super-jogs result in alloy strengthening in a wide temperature range, they can also displace diffusively along the glide direction, relieving with their motion some of this extra stress and eventually leading to strength softening above 2000 K. We implement these mechanisms into a specially-designed hybrid kinetic Monte Carlo/Discrete Dislocation Dynamics approach (kMC/DD) parameterized with vacancy formation and migration energy distributions obtained with machine-learning potentials designed specifically for the Nb-Mo-Ta-W system. The kMC module sets the timescale dictated by thermally-activated events, while the DD module relaxes the dislocation line configuration in between events in accordance with elastic forces and the applied stress. We find that the balance between super-jog pinning and super-jog diffusion confers an extra strength to edge dislocations at intermediate-to-high temperatures which is consistent with the experimental observation of a strength plateau in equiatomic Nb-Mo-Ta-W. We derive an analytical model based on the computational results that captures this improved understanding of plastic processes in these alloys and could help to partially explain the strength measurements at high temperatures.