Corrosion–wear synergy in AlxCoCrNiMo high-entropy alloys: Role of B2/σ phase balance

High-entropy alloys (HEAs) show great potential for extreme environments due to their multi-element synergy and properties. We studied the CoCrNiMo system, incorporating varying Al amounts to develop HEAs. Using multi-scale computations and experiments, we examined how Al content affects microstructure, mechanical properties, tribological behavior, and corrosion resistance. Our results indicate that increasing Al content promotes the formation of the B2 phase while suppressing the growth of the σ phase. The phase composition evolved from FCC + σ (at x = 0) to B2 + σ (at x ≥ 0.5), ultimately forming a B2-dominated multiphase structure (at x = 2.0). Consequently, the alloy’s hardness first increased and then decreased. The Al0.5 alloy achieves the highest hardness both at room temperature and high temperature, with a hardness value of 936.9 ± 17.56 HV at room temperature and 819 ± 0.5 HV at 650 ℃. However, corrosion resistance steadily decreased with increased Al. This decline occurred because the higher Al content raised the proportion of the Cr/Mo-depleted B2 phase, destabilizing the protective passive film. The Al0.5 alloy exhibited the best synergistic resistance to corrosion-wear due to its optimal balance of good corrosion resistance and high hardness. In contrast, higher Al content diminished the alloy’s overall resistance under complex service conditions by increasing the B2 phase proportion. These findings highlight that an optimal B2/σ phase balance—achieved at intermediate Al content—can significantly enhance corrosion–wear synergy. This work provides practical guidance for the compositional design of HEAs used in corrosive and mechanically demanding environments such as marine and chemical industries.