Microstructural evolution, phase formation and mechanical properties of multi-component AlCoCrFeNix alloys

Microstructure evolution and mechanical property of the AlCoCrFeNix (1.0 <= x <= 3.0) high-entropy alloys were investigated in this paper. Nickel element facilitates the formation of the FCC phase from the BCC matrix. Correspondingly, the microstructure changes from dendrite morphology (1.0 <= x <= 1.5) to eutectic structure (x = 2.1) and finally to oriented cellular structure (x >= 2.4). The onset of the structure transition was observed in the AlCoCrFeNi1.5 alloy with the formation of the ordered B2 single-phase dendrite, resulting in the observed high strength and hardness in comparison to the AlCoCrFeNi alloy. The compressive yield strength and hardness present a general decreasing trend, dropping from 1360 MPa (x = 1.5) to 450 MPa (x = 3.0) and from 524 HV (x = 1.5) to 241 HV (x = 3.0), respectively. The equiaxed grain with dendritic morphology in the AlCoCrFeNi and AlCoCrFeNi1.5 alloys could be attributed to multiple-nucleation and subsequent growth during the solidification process. The thermodynamic properties of the AlCoCrFeNix (1.0 <= x <= 1.8) were also studied by DTA measurement, indicating the decreasing fraction of the metastable phase in the as-cast alloys as the Ni content is increased.