Abstract: Nickel-base single crystal superalloys (NBSX) exhibit microstructure evolution and mechanical property degradation during service. To distinguish the roles of nanomechanical properties and morphology of γ/γ′ phases in influencing macroscopic mechanical performance in a second-generation single crystal superalloy DD6, systematic experimental tests, theoretical analyses, and numerical simulations were conducted. Grid nanoindentation techniques were applied to characterize the microscopic elastoplastic properties of constituent phases and revealed that the element composition and elastoplastic properties of the γ/γ′ phases are stable during aging treatments. The reduction of macroscopic material properties is largely attributed to the evolution of the γ/γ′ morphologies. A multi-scale constitutive model for NBSX was developed by utilizing a dual-phase framework and crystal plasticity. Stress-strain responses, cyclic behavior, and stress partitioning in the γ/γ′ phases were simulated and matched well with the experiments, capturing alterations of the hysteresis stress-strain loops. Analysis of resolved shear strain, stress fields, and evolution of total accumulative strain revealed stress/strain partitioning behaviors in γ/γ′ phases. The plastic deformation is achieved by crystal slip mainly in γ while the γ′ bears larger stress. Degradation in deformation behavior can be described by the coarsening γ network and the increasing γ volume fraction. This study elucidates the influence of nanomechanical properties and the microstructural morphology of γ/γ′ phases on the mechanical performance of NBSX following long-term aging. This approach enhances performance evaluation and facilitates structure integrity assessment of NBSX components after extensive service.

Key words: Nickel-base single crystal superalloy, Nanoindentation, Crystal plasticity, Microstructure, Material degradation