Dislocation behavior and slip plane preferences in refractory multi-principal element alloys: Insights into strength-ductility trade-offs

The intricate balance between strength and ductility in refractory multi-principal element alloys (MPEAs) represents a pivotal research challenge. This study delves into the room-temperature tensile deformation mechanisms in TiZrNbMo_x MPEAs, focusing on dislocation behavior and slip plane preference. Our findings reveal that an augmentation in Mo content significantly enhances strength while concurrently diminishing plasticity. Through an integrative approach, encompassing experimental observations, theoretical analyses, and computational modeling, we discern a pronounced transition in deformation mechanisms prompted by Mo incorporation. TiZrNb primarily exhibits stochastic activation of slip systems across diverse crystallographic planes, with mixed dislocations assuming a pivotal role. However, in the TiZrNbMo_0.3 alloy, the deformation mechanism transitions towards a predominant {110} slip plane, governed by the lower generalized stacking fault energy relative to other slip planes. Concurrently, the dislocation nature undergoes a transition from mixed to screw dislocations, which significantly impedes glide due to their compact core structures, thereby ultimately diminishing ductility. We ascribe the observed alterations in dislocation characteristics and slip plane preferences to variations in valence electron concentration (VEC), thus offering a mechanistic explanation for the VEC-based ductility criterion in refractory MPEAs. These findings underscore the critical role of high-VEC Mo in modulating the delicate balance between strength and ductility in these alloys. Our results furnish pivotal insights into the strategic design of MPEAs with bespoke mechanical properties, achieved through the deliberate manipulation of dislocation behavior and slip system activation.