Abstract: Medium-entropy alloys (MEAs) have garnered significant interest due to their unique mechanical properties, but phase instabilities such as the formation of brittle sigma (σ) phase during annealing pose challenges to their practical application. This study investigates the microstructural evolution and mechanical behavior of an 80 % cold-rolled Fe₄₅Co₃₅Cr₁₀V₁₀ MEA that was isochronally annealed between 100 °C and 900 °C for 300 s and characterized using hardness indentations, in-situ X-ray diffraction, and thermodynamic calculations, with high-resolution electron microscopy detailing microstructural evolution at 625 °C, 675 °C, and 725 °C. The results show increases in Vickers hardness between 500 °C and 625 °C, attributed to the nucleation of a Cr- and V-rich sigma (σ) phase, primarily at the bcc grain boundaries. Beyond 625 °C, the hardness decreased due to σ-phase dissolution, recovery of bcc and fcc phases, bcc→fcc phase reversion, and recrystallization of the reverted fcc phase. Scanning-transmission electron microscopy and transmission Kikuchi diffraction revealed a Kurdjumov-Sachs orientation relationship (OR) at 675 °C and a near Nishiyama-Wassermann OR at 725 °C for bcc-fcc interfaces, whereas bcc-σ and fcc-σ interfaces showed no dominant OR. In addition to σ phase, two types of bcc phase were identified at 625 °C. Type 1 bcc initially retained a near-nominal composition and a disordered crystal structure from deformation-induced bcc martensite but gradually became Fe-enriched and Cr- and V-depleted up to 725 °C. In contrast, Type 2 bcc phase was Fe-depleted and Co-enriched at 625 °C but disappeared at 675 °C, coinciding with the onset of bcc→fcc phase reversion. This phase also exhibited B2-like chemical short-range ordering, with alternating FeCo-rich and CrV-rich domains. This study provides insights into the complex phase transformation occurring between 500 °C and 725 °C in a Fe₄₅Co₃₅Cr₁₀V₁₀ MEA, which can be leveraged to design alloys with optimized mechanical properties for practical applications.

Key words: Medium entropy alloy, Sigma phase, Scanning-transmission electron microscopy, Transmission Kikuchi diffraction