The recovery of microstructure and mechanical properties of directionally solidified Ni-based superalloy DZ411 after long-term thermal exposure at different temperatures

Abstract

Directionally solidified (DS) Ni-based superalloys are widely used in gas turbine blades due to excellent mechanical properties for elevated temperature applications. In harsh service environments, the microstructure degrades and mechanical properties deteriorate. In order to extend the service life of gas turbine blades, rejuvenation heat treatment (RHT) is applied. Considering the existence of temperature gradient in a real gas turbine blade during service, the effects of RHT on the different degraded microstructure at different temperatures are necessary to be investigated. However, few investigations concentrated on it. In this study, to simulate the existence of temperature gradient in a real gas turbine blade during service, DS Ni-based superalloy DZ411 was carried out with long-term thermal exposure at different temperatures. The impact of RHT on the different degrees of degraded microstructure and mechanical properties after thermal exposure at different temperatures was explored. Furthermore, the evolution of dislocations after RHT is studied, which has been rarely investigated. It was found that the degenerated microstructure at different aging temperatures can be significantly restored by RHT, including dissolution and reprecipitation of γ′ and M23C6 carbides and annihilation and rearrangement of dislocations. In addition, creep and low cycle fatigue (LCF) properties could also be effectively recovered by RHT. After RHT, the steady-state creep rate is decreased and creep rupture life has been prolonged. The LCF life and the shape of the stabilized hysteresis loop are restored. However, the creep properties are not completely restored, due to the unrecovered microstructure including the undissolved γ′ in interdendritic regions, the residual dislocations, and irreversible decomposition.