Effect of ageing on the microstructure, mechanical properties, and deformation behavior of a recrystallised metastable medium-entropy alloy

This study investigates the effect of ageing on the microstructure and mechanical properties of a recrystallised metastable Fe₄₅Co₃₅Cr₁₀V₁₀ medium-entropy alloy (MEA), produced by 88% cold-rolling, annealing at 800 °C, and subsequent ageing at 625 and 650 °C. Ageing promotes the co-precipitation of an Fe-rich disordered bcc and Cr–V-rich σ phases, which nucleate preferentially at fcc grain boundaries and triple junctions within 6 h of ageing. Consistent with thermodynamic predictions, ageing at 625 °C beyond 96 h promotes the transformation of the disordered bcc phase into ordered B2, whereas at 650 °C the bcc phase remains disordered. The bcc/B2 and σ phases exhibit diffusion-controlled growth with a low Avrami exponent of ∼0.5. Ageing at 625 °C for 96 h yields an optimal strength–ductility balance (yield strength ∼780 MPa, ultimate tensile strength ∼1240 MPa, total elongation ∼23%) with an ∼0.33 bcc/B2 + σ phase area fraction. Microstructural analysis shows that incoherent fcc–σ interfaces act as crack initiation sites, whereas coherent fcc – bcc/B2 interfaces enhance strain partitioning. In the recrystallised alloy, deformation proceeds via dislocation slip, faulting, twinning, and fcc → bcc martensitic transformation, leading to multi-stage strain hardening. B2 + σ phase precipitation during ageing suppresses twinning and delays martensitic transformation, producing a simplified two-stage hardening regime governed by interfacial dislocation pile-up. These findings demonstrate that controlled ageing enables the precise tailoring of precipitation and microstructure evolution to optimise the mechanical performance of metastable MEAs.