Experimental investigation on creep strengthening phenomenon of a Ni-based single crystal superalloy under cyclic loading and unloading conditions

The complex failure behavior of turbine blades under cyclic creep is of great significance to the safety of aeroengines. In this study, the deformation regulation and fracture mechanism of a single-crystal Ni-based superalloy DD6 under cyclic loading at 980 celcius are investigated, and the strengthened rupture life and fracture deformation are clearly exhibited. Scanning electron microscopy and electron backscatter dif-fraction observations were conducted to reveal the deformation and fracture mechanisms. A smaller par-ticle spacing of the gamma/gamma' interface was observed under cyclic loading/unloading, which restricted dislocation movement and reduced the creep rate. This led to an extended creep life of steady creep stage, and the delay of the accelerated creep stage. Additionally, the creep deformation distribution and slip system activation affected the fracture toughness under the accelerated creep stage, leading to an increase in the creep fracture strain under cyclic loading/unloading. Finally, the fracture mechanism affected by cyclic loading is discussed based on inner and surface cracks. Cyclic loading/unloading promotes the multisource initiation of surface cracks, which improves loading capacity. Furthermore, cyclic loading/unloading promotes the generation of secondary cracks deflected from crack tips. The high residual compressive stress at the crack tip delays creep-crack initiation during unloading, leading to a decrease in the creep-crack propagation rate. These factors led to cyclic strengthening at different scales. (c) 2023 Elsevier B.V. All rights reserved.