Effect of ruthenium on microstructure and high-temperature creep properties of fourth generation Ni-based single-crystal superalloys

The effects of Ru on microstructure and creep properties of the two alloys were investigated in detail. According to the creep curves of the two alloys at 1140 °C/137 MPa, the creep rupture life was significantly improved with the increase of Ru and the mechanism of each stage during the creep deformation was different. Thus, the evolution of the γ' phase, the dislocation configuration, and the effect of Ru on solution strengthening were discussed. The γ/γ' lattice misfit of the initial microstructure presented increasingly negative as the content of Ru increased, which resulted in smaller and more regular γ' particles in the initial state. Meanwhile, more consistent and denser dislocation networks on the γ/γ' interface during creep deformation were examined. Hence, the addition of Ru decreased the minimum creep rate and prolonged the secondary creep stage. Moreover, the so-called “reverse partitioning” behavior and enhancement of γ matrix strength appeared with the increase of Ru. When the alloy contained 2.5 wt% Ru, the rapidly increasing in creep strain rate induced by the topological inversion appeared. When the alloy included 3.5 wt% Ru, the needle-like and rod-like topologically close-packed (TCP) phases were observed occasionally. The stress and the supersaturation of refractory elements resulted in the precipitation of the TCP phase. The dramatic increase of the creep strain rate was principally related to the propagation of micro-cracks around the casting and creep voids in the necked regions.