Microstructure evolution and stability of directionally solidified superalloys with varying γ′ phase forming elements

The microstructure evolution and stability of the directionally solidified superalloys with various contents of γ′ phase forming elements were investigated. The increase of mass fraction of γ′ phase forming elements from Al: 5 %, Ti: 1 % to Al: 6 %, Ti:2 % dramatically increased the area fraction of eutectic and the inhomogeneity. However, the segregation ratio decreased with the increase of γ′ phase forming elements, owing to the discharge of W and Mo. After heat treatments, the addition of γ′ phase forming elements led to the formation of a four-layer layered structure, and the inhomogeneity could not be eliminated. The increased contents of γ′ phase forming elements were detrimental to the structural stability, promoted the coarsening of the γ′ phase, and increased the mismatch stress between the two phases in the dendrite.