Influence of thermal exposure on γ′ precipitation and tensile properties of DZ951 alloy

The effect of long-term exposure on the γ′ phase and the tensile behavior of a directionally solidified nickel-base superalloy DZ951 was investigated. Alloys after standard heat treatment (SHT) were isothermally aged at 900 °C up to 2000 h and tensile tests were performed in both SHT and aged conditions at various temperatures. The morphology of the γ′ phase changes from cuboid to rafting and the size increases from 300 nm at SHT to 930 nm, and the volume fraction of the γ′ phase decreases from 70% at SHT to 65% during aging at 900 °C for 2000 h. The changing trend of yield stress at different test temperatures is similar. The yield stress decreases slightly at 600 °C. This arises from few dislocations shearing the γ′ precipitates. There is a peak yield stress value at 760 °C, which is attributed to the high strength of the γ′ phase, the homogeneous deformation structure, and dislocation-γ′ precipitate and dislocation–dislocation interactions. The yield stress then decreases rapidly with increased temperature. The low strength of the γ′ phase and γ′ rafting at high temperature contribute to the drop of yield stress. The change of tensile elongation is inverse to that of yield stress. The yield stress continuously decreases with the increase of aging time at 900 °C. This arises from the coarsening of γ′ and a decrease in the γ′ volume fraction.