Effect of Ni/Al Ratio on Solidification Structure and Properties of NiAl-Based Multi-principal Element Alloy

A series of (NixAl)65Cr10Mo5V20 (x = 0.6,0.8,1.0,1.2,1.4,1.6) multi-principal element alloys (MPEAs) (or eutectic high-entropy alloys) were fabricated using a non-self-consumable vacuum melting method, and their solidification microstructures and mechanical properties were examined. As the Ni/Al ratio increases, the microstructure of the alloy changes from hypereutectic (primary BCC + eutectic, x = 0.6, 0.8) to eutectic (x = 1.0) to hypoeutectic (primary B2 + eutectic, x = 1.2, 1.4, 1.6). Within the range of 0.6-1.4, increasing the Ni/Al ratio promotes the production of the B2 phase, and the alloy's fracture strength increases initially and then decreases. The strengthening mechanisms are solid solution strengthening, B2 phase strengthening, and fine-grain strengthening. Among them, (Ni1.2Al)65Cr10Mo5V20 exhibits exceptional mechanical properties, with a yield strength of 1600 MPa, a fracture strength of 3106 MPa, a total compression plasticity of 25%, and a hardness of 697 HV. (Ni-1.2, Ni-1.4, and Ni-1.6 show the same mechanical properties within the confidence interval.) Furthermore, the phase prediction criteria for multi-principal element alloys containing Ni and Al were proposed: when ΔHmix ≤ 12.31(kJ/mol), 5.76(%) ≤ δr ≤ 7.44(%), as well as 0.75 ≤ Ω ≤ 1.53, the structure of MPEAs is BCC phase + B2 phase.