Effects of Al addition on microstructure and mechanical properties of a novel FeNi2VAlx high-entropy alloy system

Multi-phase high-entropy alloys (HEAs) exhibiting an exceptional strength-ductility synergy represent a novel paradigm in advanced alloy design. In this study, a series of FeNi₂VAl_x (x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1) HEAs were designed and fabricated, and the effects of Al addition on the phase formation and mechanical properties were systematically investigated. With Al content increasing, the phase constitution in this alloy system evolves from a single face-centered cubic (FCC) phase to a mixture of FCC and L2₁ phase – an ordered body-centered cubic (BCC) derived phase, and finally to a single L2₁ phase. There is a localized short-range ordered L1₂ structure in the FCC phase in the alloys with high Al concentration, and the L2₁ phase in this series of alloys exhibits antiphase domains (APDs) structure, confirming its long-range periodicity. The increasing Al addition leads to the improvement of the strength of the alloys, yield strength from 303 to 1115 MPa, and ultimate tensile strength from 565 to 1407 MPa. The Al0.6 alloy achieves an excellent strength-ductility balance (761 MPa yield strength, 1194 MPa ultimate tensile strength, and 21.2 % elongation) due to the following combined strategy: synergistic FCC/L2₁ phase deformation, Taylor lattices, high-density dislocation walls, and FCC stacking faults, which collectively optimize strain hardening and dislocation restriction. This study provides essential insights into the strategic design of multi-phase HEA structures, enabling the achievement of an extraordinary strength-ductility synergy.