Creep performance in a CoNi-based single crystal superalloy with super-high γ′ volume fraction at 760 °C and equivalent high stress

The creep performance at low temperature and high stress is one crucial property for the single crystal superalloys widely used for turbine blades. In this study, the creep behavior of a CoNi-based single crystal superalloy with super-high γ′ volume fraction (Vγ′ = ∼90%) was investigated at 760 °C/550 MPa based on microstructural (γ and γ′ phases) and deformation substructural (dislocations, anti-phase boundaries (APBs) and stacking faults (SFs)) analyses to reveal its creep mechanism under low temperature/high stress conditions. The super-high Vγ′ contributes to the rapid transformation of creep mechanisms from the matrix dislocation movement in the initial decelerating stage to the APB-coupled dislocation pairs and their evolving non-planar SF ribbons inside γʹ precipitates during the accelerating stage. In the following second decelerating stage, the interactions between SFs and APBs improve the deformation resistance of γ′ precipitates and decrease the creep rate. Subsequently, the high density of APBs is believed to play a significant role in microstructural stability and promotes the accumulation of matrix dislocations, which is responsible for the steady and final accelerating stages. Appropriate Vγ′ and increasing the APB energy are assumed to improve the creep resistance of CoNi-based single crystal superalloys by enhancing the strengthening effect of dislocation accumulation in γ matrix and critical shear stress of γ′ phase.