Effect of Ru addition on γ/γ′ partitioning behavior of alloying elements, γ/γ′ lattice misfit and 1200 °C creep properties of Ni-based single-crystal superalloy

The influence of Ru addition on γ/γ′ partitioning behavior of alloying elements, solid solution strengthening degree of γ and γ′ phases, γ/γ′ lattice misfit and 1200 °C/80 MPa creep properties of Ni-based single-crystal superalloys was detailed investigated. Ru addition increased 1200 °C/80 MPa rupture life of Ni-based single-crystal superalloy from 125.93 ± 5.61 h to 179.87 ± 4.43 h. The experimental single-crystal superalloys exhibited a high γ′ phase volume fraction at 1200 °C, which contributed to the superior 1200 °C/80 MPa creep properties of both Ru-free and Ru-containing single-crystal superalloys. In this study, Ru addition increased Mo, Cr, and Re contents in γ phase, and thus increased both solid solution strengthening degree and lattice constant of γ phase. These effects collectively decreased γ/γ′ lattice misfit and γ/γ′ interfacial dislocation network spacing, which increased γ/γ′ interfacial strengthening effect, and ultimately increased 1200 °C/80 MPa creep life. Furthermore, it was revealed that the 1200 °C/80 MPa creep/stress rupture lives of single-crystal superalloys with low thermal stability of γ′ phase, creep/stress rupture lives are dominated by volume fraction of γ′ phase. In contrast, for single-crystal superalloys with high thermal stability of γ′ phase (such as the two single crystal superalloys in this study), the 1200 °C/80 MPa creep/stress rupture lives were primarily governed by γ/γ′ lattice misfit. Therefore, in order to design single-crystal superalloys with excellent 1200 °C/80 MPa creep performance, it is essential to simultaneously improve thermal stability of γ′ phase and decrease γ/γ′ lattice misfit of single crystal superalloys.