Effect of sigma phase formation on short- and long-range ordering in a metastable medium-entropy alloy

This study presents the first detailed analysis of disorder-to-order transformation kinetics in an 80% cold-rolled medium-entropy alloy during isothermal annealing at 625 °C for 0.25–192 h. The cold-rolled microstructure consists of ∼99% disordered bcc martensite. We show that within 6 h, a Cr- and V-rich σ phase nucleates at bcc grain boundaries and triple junctions. Its precipitation, which depletes the matrix of Cr and V while enriching it in Fe and Co, directly triggers the disorder-to-order transformation. After 6 h, chemically short- and medium-range ordered domains emerge, progressively evolving into a fully developed FeCo-rich long-range ordered B2 phase by 96 h. The transition is confirmed by forbidden superlattice reflections in in-situ X-ray diffraction and intensified sublattice spots in selected area electron diffraction. Elemental mapping and thermodynamic modelling further corroborate the role of the σ phase in initiating this transformation pathway, highlighting precipitation-driven chemical partitioning as a key pathway for ordering in complex concentrated alloys.