Microstructure and mechanical properties of a Ti₅₀V_29.5Zr₁₀Nb₁₀Mo_0.5 lightweight refractory medium-entropy alloy after cold rolling and annealing

Lightweight refractory medium-entropy alloys (LRMEAs) are attractive for their low density, high specific strength, and excellent high-temperature stability, yet their limited room-temperature ductility remains a critical challenge. Thermomechanical processing, particularly cold rolling combined with annealing, offers an effective route to tailor microstructure and overcome this limitation. In this study, we investigated the effects of cold-rolling reductions of 20%, 60%, 80%, and 90% followed by annealing at 900 °C for 1 h on the microstructure and mechanical properties of a Ti₅₀V_29.5Zr₁₀Nb₁₀Mo_0.5 LRMEA. The alloy retains a single body-centered cubic (BCC) structure without new phases formed during cold rolling. Recrystallization behavior and microstructural homogeneity proved strongly dependent on rolling reduction. While CR20 exhibited incomplete recrystallization with a bimodal grain distribution spanning fine grains of 25–45 μm and coarse grains exceeding 125 μm, CR60 and CR80 achieved full recrystallization with uniform equiaxed grains of 45.2 and 41.5 μm, respectively. Despite yielding the most refined microstructure, CR90 suffered a marked reduction in dislocation density, substantially weakening back-stress strengthening. The strength-grain size relationship deviates from the classical Hall-Petch law, showing a non-monotonic trend attributable to the interplay among residual dislocation density, microstructural heterogeneity, and recrystallization. This study clarifies microstructural evolution rules, reveals back stress strengthening, provides a new way to overcome the strength-plasticity trade-off, and provides guidance for the design of lightweight high-temperature materials.