Abnormal stress rupture property in K465 superalloy caused by microstructural degradation at 975 °C/225 MPa

Abstract

Degradation of microstructures and mechanical properties of turbine blades during service resulted in premature failure of aircraft engines. However, few investigations about the significant decrease of mechanical properties were reported after thermal exposure at relative lower service temperature. In this study, cast superalloy K465 was thermally exposed at 900 °C and 1000 °C for 300–1000 h, and then stress rupture tests were carried out under 975°C/225 MPa. An abnormal phenomenon was found that the stress rupture lives of specimens after thermal exposure at 900 °C were obviously lower than those at 1000 °C, which contained lower volume fraction of γ′ precipitates with coarsened and coalesced morphology. Microstructural features, including the coarsening and coalescence of γ′ precipitates, stability of carbides and precipitation of plate-like phases, were identified to characterize microstructural degradation. Microstructural observation of stress ruptured specimens indicated that cracks initiated at the interface of γ′ eutectic phase, decomposed MC carbides and plate-like μ phase or M6C carbides with γ matrix. Large amounts of plate-like μ phase weakened the strength of γ matrix by the depletion of solid solution strengthening elements, while the precipitation of discrete M23C6 carbides along grain boundaries were attributed to the strengthening of grain boundaries after thermal exposure at 900 °C in comparison with those at 1000 °C. The precipitation of plate-like μ phase with large amounts at 900 °C combined with the stress concentration between the brittle phases and the weakened γ/γ′ matrix were attributed to the abnormality of stress rupture property in K465 superalloy. This study is helpful to understand the abnormal stress rupture property caused by microstructural degradation at different temperatures and to optimize the design and operation of turbine blades made of K465 superalloy.