| Ni-based single crystal superalloys with superior properties at high temperature are widely used as the key materials for turbine blades in aircraft engines. Fourth generation single crystal superalloys containing Re and Ru additions have been developed since the beginning of this century. Interactive effects of multiple alloying elements on microstructure and properties are complex due to the extremely high alloying degree in single crystal superalloys. For the development of advanced Ni-based single crystal superalloys with low density and cost, it is of great challenge to optimize the alloying effects and partially replace expensive Re and Ru additions.Based on preliminary works in our group, a series of single crystal superalloys containing different levels of Co (～7.0/～15.0wt.%), Cr (～3.5/～6.0wt.%), Mo (～1.0/～2.5wt.%) and Ru (～2.5/～4.0wt.%) additions were investigated in this study. To acquire a better understanding of the individual and synergetic alloying effects, elemental partitioning behavior and lattice misfit between y and y’phases, microstructural stability (γ/γ’dual-phase structure change and TCP phase formation), high-temperature low-stress creep behavior as well as dislocation configurations were analyzed.Compositions of γ and γ’phases in experimental alloys after the standard heat treatment were obtained. It was suggested that the high level of Co and Ru additions decreased γ/γ’partitioning ratios of elements with large atomic radius (Re, W, Mo and Cr), while increasing the Cr and Mo contents induced the opposite effect. Combined with the analyses of equilibrium interfacial dislocation networks, different γ/γ’lattice misfit were revealed due to various alloying additions and their effects on the elemental partitioning behavior. Increasing the Cr, Mo and Ru additions resulted in more negative lattice misfit. However, the lattice misfit magnitude was decreased by more Co additions.Long-term thermal exposure at elevated temperature (950℃and1100℃) was performed. It was indicated that the high level of Co and Ru additions retarded the γ’ phase growth. It could be attributed to the decreased alloying diffusional rates due to more Co and Ru additions. Additionally, since γ/γ’lattice misfit was decreased with increasing the Co content, interfacial energy between y and γ’phases was reduced and then γ’evolution was slowed down. As TCP-forming elements, Cr and Mo additions enhanced the supersaturation of y matrix, which promoted the TCP phase formation. Compared with Mo additions, more Cr content led to much stronger tendency to form TCP phases in experimental alloys. On the contrary, Co and Ru additions suppressed the TCP phase formation. The reason for decreased supersaturation was probably that the high level of Co and Ru additions increased the solubility of refractory elements in y matrix. Ru was shown to more effectively inhibit TCP precipitates compared to Co.Co, Cr and Mo additions significantly affected the type and composition of TCP phases, while Ru additions showed a negligible effect. Alloys containing the high level of Co additions had a tendency for the formation of R phase after thermal exposure at1100℃for500h. Meanwhile, σ and P phases were prone to form in alloys containing the low level of Co additions, σ phase existed in alloys with higher Cr/Mo ratio in comparison with P phase.γ/γ’interfacial dislocation network structure and TCP phase formation were considered as two main factors affecting the high-temperature low-stress (1100℃/140MPa) creep property in this study. Creep resistance was enhanced with increasing the Cr and Mo additions, which resulted in larger γ/γ’lattice misfit and denser interfacial dislocations. However, the harmful effect on creep property induced by inferior microstructural stability in alloys with higher Cr and Mo additions overweighed the interfacial strengthening effect. With increasing the Co additions, γ/γ’lattice misfit and interfacial dislocation density were decreased. This explained the worse creep property due to the high level of Co additions. More Ru additions improved both microstructural stability and interfacial dislocation density, which led to better creep property at1100℃/140MPa.Interactive effects of Co, Cr, Mo and Ru additions affected the high-temperature low-stress creep property. The synergetic effects between microstructural stabilizers Co and Ru (especially Ru) and creep strengtheners Cr and Mo could significantly improve creep property. With increasing both Ru and Cr (or Mo) additions, dense interfacial dislocation networks formed during the creep deformation and the microstructural instability due to the high level of Cr or Mo additions was balanced by Ru additions. |
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