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Aircraft engines; Carbides; Grain boundaries; Isotherms; Superalloys; Thermal cycling; Turbine components; Turbomachine blades, Aero-engine turbines; Component design; Directionally solidified superalloy; Interfacial dislocation networks; Mechanical degradation; Micro-structural; Microstructural degradation; Minimum creep rates, Creep
Abstract:
Aero-engine turbine blades may suffer overheating during service, which can result in severe microstructural and mechanical degradation within tens of seconds. In this study, the thermal cycling creep under (950°C/15 min+1100°C/1 min)-100 MPa was performed on a directionally solidified superalloy, DZ125. The effects of overheating and thermal cycling on the creep properties were evaluated in terms of creep behavior and microstructural evolution against isothermally crept specimens under 950°C/100 MPa, 950°C/270 MPa, and 1100°C/100 MPa. The results indicated that the thermal cycling creep life was reduced dramatically compared to the isothermal creep under 950°C/100 MPa. The plastic creep deformation mainly occurred during the overheating stage during the thermal cycling creep. The thermal cycling creep curve exhibited three stages, similar to the 1100°C isothermal creep, but its minimum creep rate occurred at a lower creep strain. The overheating events caused severe microstructural degradation, such as substantial dissolution of γ' phase, earlier formation of rafted γ' microstructure, widening of the γ channels, and instability of the interfacial dislocation networks. This microstructural degradation was the main reason for the dramatic decrease in thermal cycling creep life, as the thermal cycling promoted more dislocations to cut into γ' phase and more cracks to initiate at grain boundaries, carbides, and residual eutectic pools. This study underlines the importance of evaluating the thermal cycling creep properties of superalloys to be used as turbine blades and provides insights into the effect of thermal cycling on directionally solidified superalloys for component design. © 2021
