Achieving dual-phase structure and improved mechanical properties in AlCoCrFeTi0.5 high-entropy alloys by addition of Ni

High-entropy alloys (HEAs) have attracted increasing attention as structural materials due to their excellent mechanical properties; however, most single-phase HEAs still encounter the conflict between strength and plasticity, which greatly limits their engineering applications. Here we reported the use of composition design, that is the addition of Ni element into a base alloy of AlCoCrFeTi0.5 HEA, to obtain the body-centered cubic (BCC)/face-centered cubic (FCC) dual-phase structure, for avoiding the strength-plasticity trade-off. With the Ni addition increasing from 0 to 3 mol ratio, the initial ordered BCC (B2 phase) structure first transformed to a duplex BCC (A2 + B2) structure, and then to an FCC (L12)/BCC (A2 + B2) dual-phase structure. Meanwhile, the Ni-containing HEAs displayed greatly improved mechanical properties. More specifically, dual-phase AlCoCrFeTi0.5Ni2.5 HEA showed the most promising combination of strength and plasticity with a yield strength of 1.41 GPa and compressive strain of 31.8%, as well as a fracture toughness of 64.3 MPa m1/2. Post-deformation structural analyses of the AlCoCrFeTi0.5Ni2.5 HEA revealed that the activation of dislocations slip and stacking fault in the FCC (L12) phase, precipitation strengthening in BCC phase (B2), together with the synergic deformation between FCC and BCC phases, contribute to the desirable capability of strain hardening. Our work shows that the use of dual-phase architecture can be an effective microstructure-design strategy for developing HEAs with a favorable strength-ductility synergy.