Age Heat Treatment of the CoCrFeNiTi0.3 High-Entropy Alloy

The hardness and microstructure of an as-cast CoCrFeNiTi0.3 high-entropy alloy after age heat treatment 24–144 h at 500–1000°C are investigated. The as-cast alloy has a dendritic structure in which the dendrite is a Ti-lean face-centered cubic solid solution phase (FCC1) and the interdendrite consists of a Ti-rich face-centered cubic solid solution phase (FCC2), a mixture of (Cr, Fe)-rich σ phase and (Ni, Ti)-rich η phase. After a 144 h of age heat treatment, age hardening is observed at temperatures of 600–800°C because the amount of σ phase increases in the interdendrite. The optimum hardness is obtained at an aging temperature of 700°C, which makes the hardness increase from HV366 to HV508. However, age softening occurs at 900–1000°C. The vanishing σ phase at 1000°C brings the alloy hardness to the minimum, HV223.