In-situ observation of ‘chemical’ strengthening induced by compositional fluctuations in Nb-Mo-Ta-W

Abstract

The mechanical response of refractory high-entropy alloys (RHEA) can be greatly influenced by local chemical fluctuations. Here, we perform in-situ transmission electron microscopy (TEM) nanocompression tests at room temperature of body-centered cubic Nb-Mo-Ta-W RHEA single-crystal nanopillars to understand the reasons behind this at atomic scale. We find that (i) chemical composition is not uniform in space, following certain oscillatory patterns and (ii) that specimen near-surface regions with local ‘soft’ spots favor dislocation nucleation over ‘hard’ ones. Under stress these dislocations glide inwardly, inevitably running into hard local regions adding hardening stresses of up to 400 MPa to the nominal yield stress. These hard local regions act as ‘chemical’ obstacles as revealed by the occurrence of double cross-slip and the formation of prismatic loops. Using electron dispersive spectroscopy along with atomic resolution TEM, we demonstrate that the obstacles correspond to atomic environments presumably rich in W surrounded by misfit edge dislocations.