Effect of molybdenum on phases, microstructure and mechanical properties of Al0.5CoCrFeMoxNi high entropy alloys

Effect of molybdenum on phases, microstructure and mechanical properties of Al0.5CoCrFeMoxNi (x = 0-0.5 mol) high entropy alloys has been investigated in this paper. The microstructure, phase constituents and mechanical properties of the alloys have been studied using scanning electron microscopy, X-ray diffraction, transmission electron microscopy, compressive and hardness tests. The possible equilibrium phases existing in the alloys have also been evaluated using the Thermo-Calc program. The as-cast Al0.5CoCrFeMoxNi alloys have typical dendrite microstructure. The alloys with x = 0 and 0.1 consist of FCC and (Ni,Al)-rich ordered BCC phase, while the other four alloys are composed of FCC, (Ni,Al)-rich ordered BCC phase and (Cr,Mo)-rich σ phase. The thermodynamic calculation shows that Mo changes the phase formation order, mole fraction and composition of the equilibrium phases in the Al0.5CoCrFeMoxNi alloys. The addition of Mo enhances formation of σ phase in the Al0.5CoCrFeMoxNi alloys, improves the hardness and compressive strength of as-cast alloys, and reduces the ductility of the alloys. The Al0.5CoCrFeMo0.3Ni and Al0.5CoCrFeMo0.4Ni alloys have balanced properties of compressive strength and ductility. Al0.5CoCrFeMo0.3Ni alloy has a σ0.2 of 814 MPa, an ultimate strength of 2101 MPa, and a fracture strain of 31%, while Al0.5CoCrFeMo0.4Ni alloy has a σ0.2 of 1091 MPa, an ultimate strength of 2117 MPa, and a fracture strain of 18%.