Ti induced phase evolution and precipitation strengthening in CoCrNiV-based high-entropy alloys with improved mechanical properties and oxidation resistance

Non-equiatomic CoCr 0.6 NiV 0.6 Tix high entropy alloys containing 0, 2, 4, and 6 wt % Ti (denoted Ti-0, Ti-2, Ti-4, and Ti-6, respectively) were prepared by vacuum arc melting. The effects of Ti addition on the microstructure, deformation behaviour, mechanical properties, and cyclic oxidation resistance at 700 °C were investigated. Ti promoted the co-precipitation of σ and η-Ni 3 Ti phases, including the formation of a core-shell σ@η-Ni 3 Ti structure in Ti-4. In Ti-6 alloy, the area fractions of σ and η-Ni 3 Ti reached 49.5% and 11.0%, respectively. The Ti-4 alloy exhibited a compressive yield strength of 1062.4 MPa, approximately 240% higher than that of Ti-0, while retaining a compressive fracture strain of 40.1%. Its hardness reached 48.4 HRC, compared with 4.2 HRC for Ti-0. This favorable compressive strength-deformation balance arose from dislocation slip in the FCC matrix, precipitation strengthening, and the deflection and elongation of crack paths by the core-shell precipitates. Excessive σ precipitation in Ti-6 promoted stress concentration and premature cracking, reducing the fracture strain to 18.3%. Under tensile loading, the Ti-4 alloy exhibited a yield strength of 581 MPa, while its fracture strain decreased to approximately 7.0%. During cyclic oxidation, Ti-4 showed the lowest mass gain and oxidation rate. Ti addition promoted the formation of continuous Cr 2 O 3 and (Co,Ni)Cr 2 O 4 spinel, thereby improving scale integrity. However, Ti-6 underwent non-uniform oxidation and developed more pores and cracks. However, Ti-6 underwent non-uniform oxidation and developed more pores and cracks.