Solid solubility, precipitates, and stacking fault energy of micro-alloyed CoCrFeNi high entropy alloys

The face centered cubic (FCC) high entropy alloys (HEAs) showed highly attractive properties because of their simple crystal structures and low stacking fault energies. However, deep understandings of micro-alloying in FCC HEAs, including solid solubility, precipitates formation and stacking fault energy variation, remain unsatisfactory. In present study, we systematically investigated the solubility of Ti, Al, and Mo in the CoCrFeNi HEA and evaluated the effect of every single element on the precipitation behavior. The results showed that at 750 °C the CoCrFeNi matrix can separately dissolve up to 0.6 at.% Ti, 4 at.% Al, and 3 at.% Mo. The roles of both additional and base elements in the precipitation behaviors were identified. Specifically, Co can stabilize the γ′ phase in the Ti-doped HEAs, while Al leads to the formation of B2 phase in the CoCrFeNiAl_x system. The annealing stacking faults and twins in the matrix changed with the addition of different elements, which was mainly due to stacking fault energy variations. These insights will greatly benefit the design of precipitate-hardening HEAs and provide a deeper understanding of metallurgical behaviors of HEAs.