Age-hardening of the CoCrFeNiMo0.85 high-entropy alloy

• Age-hardening is observed over the temperature range of 600–1000 °C.
• The optimal age-hardening occurs at 700 °C.
• Hardening is attributed to the precipitation of the σ or μ phase in the matrix.
• σ Phase is completely converted to the μ phase at aging temperatures above 900 °C.

In this study, the effect of aging treatment for 24–144 h at 600–1000 °C on the hardness and microstructure of an as-cast CoCrFeNiMo0.85 high-entropy alloy was investigated. The microstructure of the as-cast alloy consists of three different phases that include an face-centered cubic solid solution matrix, a (Mo,Cr)-rich σ phase, and a minor (Mo,Cr)-rich μ phase. The σ phase can be completely transformed into the μ phase via aging at 900–1000 °C. The optimum hardness is obtained upon precipitation of fine needles of σ phase in the face-centered cubic matrix, which occurs at an aging temperature of 700 °C. The hardening effect is observed to decrease with any further increase in temperature beyond 700 °C. Comparison of the hardness of aged alloys and the as-cast alloy reveals that no obvious softening occurs after aging at temperatures of up to 1000 °C.