Effects of alloying elements on microstructure and mechanical properties of Co–Ni–Al–Ti superalloy

The effects of the relative content of Al and Ti, and the addition of 2 at.% Cr or Mo to partly substitute Co on the microstructure and mechanical properties of Co–Ni–Al–Ti superalloy were investigated. The microstructures of five as-cast superalloys showed that γ'-(Co,Ni)3(Al,Ti) precipitates segregated on γ-Co matrix and exhibited dendritic morphology. After aging, nano-sized γ'-(Co,Ni)3(Al,Ti) precipitates uniformly precipitated on γ-Co matrix. Ni, Al, Ti and Mo atoms partitioned preferentially to γ′ phase, while Co and Cr atoms partitioned preferentially to γ matrix phase. The increase in Ti content, and the addition of 2 at.% Cr or Mo all increased the volume fraction and the average size of γ′ phase. The lattice misfit increased with the concentration of Ti and the addition of 2 at.% Mo, but decreased with the addition of 2 at.% Cr. With increasing aging time, the size of γ′ phase and lattice misfit increased, while γ′ phase volume fraction increased firstly, and then decreased after reaching the maximum at 100 h. The lattice misfit plays a key role on vicker's hardness in the time of 20 h–100 h aging, while γ′ phase volume fraction plays a decisive role in vicker's hardness after 100 h aging.