Effect of Al/Co ratio on phase stability and mechanical behavior of mechanically alloyed AlxCoyCrFeNi HEAs

This study investigates the effect of the Al/Co ratio on phase evolution and mechanical behavior in Al x Co y CrFeNi high-entropy alloys synthesized via mechanical alloying followed by high-temperature sintering. The variation in Al/Co ratio (0.14 to 1.00) was employed to tailor phase formation and optimize mechanical performance. XRD, EBSD/EDS, and TEM analyses revealed a compositional-dependent transition from a predominantly FCC matrix to the formation of BCC (AlNi-rich B2 and Cr-rich A2) phases and eventually to a brittle (Cr,Fe)-rich sigma phase at high Al/Co ratios. Mechanical testing demonstrated that the low-Al/Co alloy exhibited exceptional compressive strength and ductility due to its refined FCC grains and deformation twins. In contrast, alloys with high Al/Co ratios showed increased hardness but reduced toughness, attributed to sigma phase formation and oxide dispersion. The results highlight the critical role of Al/Co ratio in controlling solid-solution strengthening, lattice distortion, and phase stability. This work provides design insights for optimizing microstructure and performance in HEAs via Al/Co tuning and non-equilibrium processing, contributing to the rational development of next-generation structural materials.