Effect of Ni on Microstructure and Mechanical Property of a Co-Ti-V-Based Superalloy

The effect of Ni on microstructure, elemental partition behavior, γ' phase solvus temperature, lattice misfit between γ and γ' phases, and mechanical properties of the Co-8Ti-11V-xNi alloys was investigated. The result shows that the lattice misfit in the alloys decreases from 0.74% to 0.61% as the Ni content increases from 0 to 10%, and the average sizes of the cuboidal γ' phase were measured to be 312.10 nm, 112.86 nm, and 141.84 nm for the Co-8Ti-11V, Co-8Ti-11V-5Ni, and Co-8Ti-11V-10Ni, respectively. Ti, V, and Ni exhibit a slight tendency to partition into the γ' phase, while Co shows a slight tendency to partition into the γ phase. The solvus temperatures of the γ' phase were measured to be 1167°C, 1114°C, and 1108°C for the Co-8Ti-11V, Co-8Ti-11V-5Ni, and Co-8Ti-11V-10Ni alloys, respectively, by using differential scanning calorimetry (DSC). Moreover, the yield strength and ultimate strength of the Co-8Ti-11V, Co-8Ti-11V-5Ni, and Co-8Ti-11V-10Ni alloys were investigated, and the yield strength and ultimate strength of the 10Ni alloy were highest, at 219 MPa and 240 MPa. After compression at 1000°C, the dislocations cannot effectively shear the γ' phase in the 0Ni and 10Ni alloys, resulting in a relatively high compressive strength of the 0Ni and 10Ni alloys. However, the γ' phase of the 5Ni alloy is no longer visible, and its strength is the lowest.