Revisiting the phase transformation pathways in the Al0.7CoCrFeMnNi high-entropy alloy

The phase transformation pathways and microstructural evolution of the Al 0.7 CoCrFeMnNi high-entropy alloy were systematically investigated to clarify the origin of its characteristic FCC + BCC/B2 microstructure. Arc melting, directional solidification, heat treatments, differential scanning calorimetry, electron microscopy, and CALPHAD calculations were combined to reassess its solidification and solid-state transformations. The results indicate that solidification proceeds through the formation of a disordered BCC phase as the sole product, rather than via a eutectic reaction. The Widmanstätten-type FCC plates develop through a subsequent solid-state transformation within the BCC matrix. Upon further cooling, the remaining BCC decomposes into BCC + B2, followed (at lower cooling rates) by the formation of a secondary Cr-rich BCC phase within B2. The σ phase can also form at intermediate temperatures. Cooling rate significantly influences phase fractions and coarsening behavior but does not alter the fundamental transformation sequence. Microstructural refinement achieved at higher cooling rates enhances hardness and compressive yield strength while maintaining comparable plasticity. These findings clarify the non-eutectic origin of the microstructure and demonstrate the strong processing-microstructure-property relationships in this alloy. • Non-eutectic solidification confirmed in Al 0.7 CoCrFeMnNi HEA. • Widmanstätten FCC forms via solid-state transformation. • BCC decomposes into BCC + B2 followed by Cr-rich BCC formation. • Cooling rate refines microstructure and increases strength. • Experimental pathway deviates from CALPHAD predictions.