Sigma phase embrittlement in a tempered Fe₄₅Cr₂₅Ni₁₀Mn₁₅Al₄C₁ medium-entropy alloy

Precipitation of intermetallic compounds is an effective strategy for strengthening high- and medium-entropy alloys (HEAs/MEAs). Here, we developed a strong, ductile and cost-effective duplex MEA with a chemical composition of Fe45Cr25Ni10Mn15Al4C1 (at.%) in hot-rolled state and explored the potential of strengthening this MEA by forming hard σ phase via tempering within the temperature range of 400–900°C. Severe embrittlement is observed when the MEA is tempered at between 700°C and 800°C. Microstructural characterisation reveals that σ phase with a lamellar morphology is formed via a diffusion-controlled process. The severe embrittlement results from the intrinsic brittleness of σ phase and the lamellar morphology that provides continuous paths for crack propagation.