On the superior hot hardness and softening resistance of AlCoCrxFeMo0.5Ni high-entropy alloys

Effects of Cr content on microstructures and hot hardness of AlCoCrxFeMo0.5Ni high-entropy alloys (x = 0–2.0) were investigated. The cast microstructure of AlCoCrxFeMo0.5Ni consists of B2 and σ phases, both being multi-element solid solutions. Increasing Cr content increases the volume fraction of σ phase and causes the matrix phase of the dendrite to change from B2 phase to σ phase. The alloy hardness increases from Hv 601 at x = 0 to Hv 867 at x = 2.0 as a result of increasing amount of hard σ phase. A phase diagram for the AlCoCrxFeMo0.5Ni alloy system is constructed based on SEM, HTXRD and DTA analyses, providing useful information for understanding and designing high-entropy alloys. All the AlCoCrxFeMo0.5Ni alloys possess higher hot-hardness level than that of Ni-based superalloys, In 718 and In 718 H, from room temperature to 1273 K. Cr-1.5 and Cr-2.0 alloys exhibit a transition temperature higher than that of Co-based alloy T-800 by about 200 K and have respective hardness values, Hv 374 and Hv 450 at 1273 K, being much higher than those, around Hv 127, of In 718 and In 718 H. The mechanism of larger strengthening and softening resistance is related with B2 and σ phases, both having multi-principal-element effect. The AlCoCrxFeMo0.5Ni alloy system has a potential in high-temperature applications.