Effect of V content on microstructure and mechanical properties of the CoCrFeMnNiV_x high entropy alloys

Highlights: • Structure and mechanical properties of CoCrFeMnNiV_x alloys reported. • Formation of intermetallic sigma phase due to V addition. • Volume fraction of sigma phase is proportional to cumulative Cr and V concentration. • Strengthening and loss of ductility due to increase in sigma phase volume fraction. - Abstract: Crystal structure, microstructure, microhardness and compression properties of CoCrFeMnNiV_x (x = 0, 0.25, 0.5, 0.75, 1) high entropy alloys were examined. The alloys were produced by vacuum arc melting and studied in as-solidified and homogenized (annealing at 1000 °C for 24 h) conditions. The CoCrFeMnNi alloy was a single-phase fcc solid solution in both conditions. The CoCrFeMnNiV_0.25 alloy had a single-phase fcc structure in as-solidified condition, but ∼2 vol.% fine particles of a sigma phase precipitated after annealing. The alloys with x = 0.5, 0.75 and 1.0 contained the sigma phase already in as-solidified condition. The sigma-phase volume fraction increased with an increase in the V content, and in CoCrFeMnNiV the sigma phase became the matrix phase. After homogenization treatment, the volume fraction of the sigma phase increased in all three alloys by ∼8% due to additional precipitation of fine particles inside the fcc phase. Phase composition and microstructure of the alloys was analyzed employing criteria for solid solution/intermetallic phase formation. The effect of alloys' chemical composition on the volume fraction of constitutive phases was discussed. A modified valence electron concentration (VEC) criterion, which takes into account localized lattice distortions around V atoms, was suggested to correctly predict sigma phase formation in the CoCrFeNiMnV_x alloys. It was demonstrated that the volume fraction of sigma phase was proportional to the cumulative Cr and V concentration. Mechanical properties of the alloys were greatly affected by the sigma phase. The CoCrFeMnNi and CoCrFeMnNiV_0.25 alloys were soft and ductile, but an increase in the sigma-phase volume fraction resulted in continuous strengthening and loss of ductility