Triple hierarchical precipitation and serrated grain boundaries strengthened co-free medium-entropy alloys†

Microstructure design effectively achieves strength-ductility synergy in high/medium entropy alloys (H/MEAs). A new type of non-equiatomic FeCrNiAl_0.3V_0.1Ti_0.2 (Al3V1Ti2) MEA was designed and prepared in an as-cast state; the unique microstructures consist of high-density hierarchical L2₁ nanoparticles and serrated grain boundaries (SGBs). Using Ti to replace part of V in the Al3V3 (FeCrNiAl_0.3V_0.3) alloy, the Al3V1Ti2 alloy obtained two additional L2₁ phases in nanospherical and lamellar shapes in addition to the near-spherical sub-micron L2₁ phase of the Al3V3 alloy. The triple hierarchical precipitation (THP) and SGBs in Al3V1Ti2 MEA contribute to its super-strength and plastic properties: the compressive yield strength reaches 1.15 GPa, the maximum strength is 3.02 GPa, and elongation is maintained at 24%. The mechanical properties are superior to most of the FeCrAlNiCo and FeCrNiAl-based HEAs. These results indicate that the Co-free low-cost H/MEAs designed using multiple microstructure optimisation strategies are promising new systems for engineering applications.