The influence of Al elements on the structure and the creep behavior of Al x CoCrFeNi high entropy alloys

Abstract The microstructure, crystal structure, creep behavior and mechanism of AlxCoCrFeNi (x is in molar ratio) high entropy alloys were examined. The low Al content of Al0.15CoCrFeNi alloy had single FCC structure with columnar cell microstructure, while the high Al content of Al0.60CoCrFeNi alloy contained FCC+BCC duplex crystal structure with columnar dendrite microstructure. The creep property, evaluated by stress relaxation test, showed that the Al0.15CoCrFeNi alloy had a higher creep resistant property than that of the Al0.60CoCrFeNi alloy. The creep constitutive equation for the Al0.15CoCrFeNi alloy was established with an average stress exponent of 5.56 and an average activation energy of 385 kJ mol−1, while it was with a larger average stress exponent of 8.82 and a smaller average activation energy of 334 kJ mol−1 for the Al0.60CoCrFeNi alloy. The analysis of apparent activation volume reveals that both alloys were in a rate limiting mechanism of cross-slip. Then, the higher stacking fault energy of the Al0.60CoCrFeNi alloy was considered to result in its higher creep rate. Therefore, it was concluded that the increased Al element tended to reduce the creep resistant property of the AlxCoCrFeNi alloys by increasing the stacking fault energy.