Investigation of Creep Damage Accumulation Under Variable Loading Conditions in Nickel-Based Single Crystal Superalloys

Nickel-based single crystal (SX) turbine blades typically experience varying load conditions during operation. It is essential to consider the effects of changing stress and temperature when evaluating creep damage. The commonly used time-fraction method, based on the linear damage rule (LDR), has been demonstrated to be inconsistent with experimental results under certain loading conditions. There is a lack of research on damage accumulation under variable loading conditions specifically for SX superalloy. This paper conducted two-stage creep experiments for SX superalloy DD6 at 980℃ with stress levels of 230MPa and 320MPa, and at 1050℃ with stress levels of 135MPa and 230MPa. The evolution of the microstructure under variable loading was observed using scanning electron microscope (SEM) and transmission electron microscope (TEM). Afterwards, a novel creep damage model for SX superalloy incorporating the influence of microstructural degradation was developed and verified using experimental results. The result of creep experiments revealed that the damage accumulation behavior of the SX superalloy does not conform to the Linear Damage Rule (LDR). Furthermore, in two-stage loading tests, the creep damage accumulation behavior differs from the classical sequence effect under fatigue loading. SEM and TEM analyses suggested that this phenomenon might be associated with the varying degradation rates of microstructures in SX superalloy. The creep rupture life predicted result of the established damage accumulation model aligns with the experimental results.