Effect of long-term aging on the microstructure, stress rupture properties and deformation mechanisms of a new cast nickel base superalloy

The microstructure, stress rupture properties and deformation mechanisms of a new alloy during long-term aging were investigated. The microstructure evolution mainly included the coarsening of γ′ phase and the precipitation of η phase. During aging at 700 °C, 750 °C and 800 °C, the coarsening rate k of γ′ phase was about 7.818 nm3/h, 42.927 nm3/h and 178.226 nm3/h, respectively. The activation energy Q for γ′ coarsening was about 279.98 kJ/mol, which meant that element diffusion controlled the coarsening of γ′ phase. After aging at 750 °C for 3000 h and 800 °C for 2000 h, a new needle-like η phase was precipitated near GBs and around MC carbides, then grew into the grain interiors. The quantity of η phase increased with the increase of aging time. With the growth of η phase, γ′ depleted zones were found around η phase. This was principally because both γ′ phase and η phase were enriched in Ni and Ti, and the growing up of η phase absorbed γ′ phase. After aging at 700 °C for 300-3000 h and 750 °C for 300-2000 h, the stress rupture life at 750 °C/430 MPa remained at a high level and most of them were higher than 100 h. That was because the slightly grown-up γ′ phase acted as strong obstacles to the dislocation motion, Orowan bypassing combing stacking fault shearing acted as the dominant mechanism during stress rupture deformation. After aging at 750 °C and 800 °C for 3000 h, the stress rupture life decreased to 45.7 h and 7.68 h, respectively. One reason was that the seriously grown-up γ′ phase was very hard to impede the dislocation motion, almost all γ′ phases were cut by dislocations with leaving stacking faults inside them during stress rupture deformation. The other reason was that needle-like η phase promoted dislocation pile-up and contributed to the nucleation of micro-cracks, meanwhile the γ′ depleted zones could be conductive to the propagation of micro-cracks.