Effects of Ta addition on microstructure, compression and wear properties of CoCrNiTa_x medium entropy alloys

Effect of Ta content on microstructure and mechanical properties of CoCrNiTa_x medium entropy alloys (x = 0.1–0.5) was discussed. With increasing Ta content, CoCrNiTa_x alloys changed from hypoeutectic (x = 0.1, 0.3) to eutectic (x = 0.35) and to hypereutectic (x = 0.4, 0.5) alloys, and CoCrNiTa_x alloys were all composed by γ phase and C14-type Laves phase. Meanwhile, microhardness of CoCrNiTa_x alloys increased with increasing Ta content. With increasing Ta content, compressive strengths of CoCrNiTa_x alloys firstly increased and then decreased, and CoCrNiTa_0.35 alloy had the highest compression strength (2852 MPa). Nanohardness of primary γ phase was much lower than that of primary Laves phase, and nanohardness of eutectic phases firstly increased and then decreased with increasing Ta content. In addition, nanohardness of γ phase in CoCrNiTa_x alloy was higher than that of γ phase in CoCrFeNiTa_x alloy, which provided a notable contribution to compression strengths of CoCrNiTa_x alloys. Wear mechanism gradually changed from adhesive wear to abrasive wear with increasing Ta content, and CoCrNiTa_0.5 alloy exhibited the highest wear resistance.