Abstract:
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.
