Mo in the form of solid solution atom or compound phase is distributed in CoCrFeMnNi high entropy alloy, which has the effect of solution strengthening or second phase strengthening. The method of annealing was used to heat treated (CoCrFeMnNi)97.02Mo2.98 high entropy alloy to investigate effects of σ phase on mechanical properties of (CoCrFeMnNi)97.02Mo2.98 high entropy alloy. SEM, EDS and XRD were used to analyze effects of annealing temperature on precipitation σ phase (CrMo phase) in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy. The mechanical properties were tested by microhardness and tensile test, and the influencing mechanism of σ phase on the mechanical properties was investigated. The results show that with increase of the annealing temperature, the quantity of precipitation σ phase increases in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy, and the σ phase is first precipitated at the grain boundary, and is after precipitated in intracrystalline. The morphologies of σ phase at the grain boundary are changed gradually from tiny strips of discontinuous distribution to thick strip of continuous distribution. With the annealing temperature increases further, the morphologies of σ phase are changed from strip of continuous distribution to granular of continuous distribution. The precipitation σ phases in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy by annealing have the effect of second phase reinforcement, with the annealing temperature increase, the numbers of precipitation σ phase increase, and the hardness and strength both increase, which is obviously at temperature higher than 900 ℃. The σ phase precipitation in intracrystalline, and its refinement, can improve the strength and plasticity of (CoCrFeMnNi)97.02Mo2.98 high entropy alloy synchronously.

Keywords： CoCrFeMnNi high entropy alloy ; Mo ; annealing ; σ phase (CrMo phase) ; mechanical property