Effects of Annealing on Phase Transformation and Corrosion Mechanism of Severely Deformed Al 0.3 CoCrFeNi High-Entropy Alloy

The effects of transitional phase states on corrosion behavior in the equiatomic-derived high-entropy alloy Al₀.₃CoCrFeNi were investigated following 90% cold-rolling and subsequent isothermal annealing at 620°C for durations ranging from 1 to 50 hours. Electron and X-ray diffraction analysis reveal a progressive transformation from single-phase FCC to an FCC + B2 + σ trinity. Nano-scale B2 and σ nuclei emerge within partially recrystallized grains after 1–2 h, lowering the cold rolling micro-strain from 0.26 to 0.13. This heterogeneous state has the maximum hardness (552 VHN) while enlarging the chloride-passive window by > 200 mV relative to the solutionized material. Prolonged annealing (> 10 h) coarsens the intermetallics and exhausts Al from the matrix, thereby reducing pitting resistance due to the loss of protective Al₂O₃ film. Characterization by electron microscopy and atom probe tomography reveals that Al-rich B2 nuclei are the primary Al sink that compromises Al ₂ O ₃ formation at long times. The results demonstrate that a narrowly defined heat-treatment window balances strengthening with passivation in Al _0.3 CoCrFeNi, providing a mechanistic blueprint for designing corrosion-tolerant high-entropy alloys (HEAs).