Simple ordered L12 phase and complex ordered µ phase were found in the (FeCoNiMo)90Al10 alloy, while disordered FCC and µ phases were detected in the (FeCoNiMo)90Cr10 alloy. After annealing at 900°C, nano-size precipitates were observed in the L12 phase, and splitting and spheroidisation were occurred in some regions of the eutectic structures in the (FeCoNiMo)90Cr10 alloy. After holding for 100 h at 900°C, (FeCoNiMo)90Al10 alloy showed an excellent hardness resistance. The oxidation kinetics of both (FeCoNiMo)90Al10 and (FeCoNiMo)90Cr10 alloys followed a parabolic rate law at 900°C. An external layer of Al2O3 and an underlying MoO2 subscale were found in the (FeCoNiMo)90Al10 alloy. A semi-continuous oxide layer of MoO2 and Cr2O3 was observed in the (FeCoNiMo)90Cr10 alloy.
Liquid-phase separation occurred in CoCrCu x FeMoNi (x ≥ 0.5) alloys when the mixing entropies are positive in our previous work. So in this work, CoCr 0.5 Cu x Fe y MoNi alloys are designed to investigate the microstructure, component phases and properties. FCC and BCC were detected in CoCr 0.5 Cu x Fe y MoNi alloys, accompanied with a topologically close-packed μ phase. A parameter, V R , was defined, where V R is the ratio of the volume fraction of BCC plus μ phases and that of FCC phase. The maximum strength and the ductility were obtained at the minimum V R of the alloys, whereas the hardness increased with the increasing V R . It can be presumed that the strange balance between strength, ductility and hardness is an indirect result of the degree of brittle failure.
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