Abnormal creep property degradation in a directionally solidified superalloy DZ406 after suffering overheating

Highlights: • The microstructural degradation of γ' precipitates, γ/γ' eutectic, carbides in DZ406 superalloy are investigated after suffering overheating • The fracture mode, macro and micro fracture morphologies, dislocation configurations in DZ406 superalloy after stress rupture are investigated • The rejuvenation of γ' precipitates in DZ406 superalloy during stress rupture tests are confirmed • The abnormal degradation mechanism of stress rupture property of DZ406 superalloy is revealed after suffering overheating Turbine blades may suffer overheating and the resulting severe microstructural degradation or even premature failure during service. At present, the research on the effect of overheating on creep properties of directionally solidified superalloys is still very limited. In this study, directionally solidified superalloy DZ406 was subjected to overheating treatments at 1050 °C, 1150 °C, 1180 °C and 1220 °C for 30 min, and the corresponding stress rupture properties were tested under 980 °C/275 MPa. The results show that overheating resulted in the obvious dissolution of strengthening γ' precipitates and γ/γ' eutectic, for example the area fraction of γ' precipitates decreased to 24.0% after overheating at 1220 °C, and was only about 1/3 of the initial value in the as-received microstructure. However, overheating has negligible influence on microstructural stability of carbides. Moreover, it is abnormal that the creep rupture properties of all samples after various overheating treatments were similar, and the creep voids were all located at the interfaces of MC carbides and γ matrix, and the alloy finally suffered interdendritic fracture. The main reason for this phenomenon is that the seriously degraded γ' phase will recover before loading during the stress rupture tests, and its area fraction will basically return to the initial value, and thus leads to the recovery of the stress rupture property. Our results indicate that turbine blades will suffer obvious microstructural degradation during sudden short-term overheating, while this degradation may be restored in the subsequent normal service process, overheating does not necessarily lead to the damage of blade performance.