High temperature tensile and creep properties of CrMnFeCoNi and CrFeCoNi high-entropy alloys

High temperature tensile and creep properties of face-centered cubic (FCC) high-entropy alloys (HEAs), CrMnFeCoNi and CrFeCoNi, were evaluated at 500–700 and 650–725 °C, respectively, to evaluate high temperature structural integrity of the most well-known FCC HEAs with special attention on (i) sigma phase formation in CrMnFeCoNi and (ii) difference in solid solution strengthening between the two alloys in the temperature range where commercial heat-resistant wrought alloys including austenitic heat-resistant steels and Ni-based superalloys are used. No remarkable difference was detected in tensile behavior measured both at room and high temperatures between the two alloys. However, creep rupture life turned out to be significantly longer for the CrFeCoNi quaternary alloy. On top of the longer creep rupture life, CrFeCoNi alloy is characterized by lower minimum creep rate and larger creep activation barrier energy which could be attributed to higher level of lattice distortion of CrFeCoNi than CrMnFeCoNi resulting in higher solid solution strengthening. Also, grain boundary strength of CrMnFeCoNi was compromised by the formation of sigma phase during creep deformation which led to lower elongation especially for long-term creep conditions. Therefore, the longer creep life of CrFeCoNi is mainly attributed to the combination of enhanced solid solution strengthening and stronger grain boundary which is free from the deleterious sigma phase due to the reduced thermodynamic driving force for formation of intermetallic phase.