Study on the γ + γ′ microstructure characterization of the Co–V–Zr system based on CALPHAD method

The γ + γ′ microstructure in novel Co-based superalloys is often obtained by means of alloying method. Therefore, this study focuses on exploring the evolution of the γ + γ′ microstructure through the addition of Zr using CALculation of PHAse Diagram (CALPHAD) method. The heat capacity of τ was experimentally determined by the sapphire method, and the enthalpy of formation of τ at 0 K was calculated using Density Functional Theory (DFT). The thermodynamic parameters were derived based on experimental results from phase equilibrium data and first-principles calculations using CALPHAD method. According to the thermodynamic analyses, the alloy Co80.0V18.5Zr1.5 (at. %) was homogenized at 1473 K for 10 h and aged at 1173 K for different time, the ordered L1₂-γ′ precipitates coarsened and dissolved after 2 h, and transformed into needle-like D0₁₉-Co₃V after 67 h of aging, which indicated that the γ′ phase was not in a thermodynamically stable state in the Co–V–Zr system. If the stable γ′ phase is obtained, additional alloying elements is necessary to be added.