Effects of Heat Treatment on Microstructural Evolution and Stress Rupture Properties of a Directionally Solidified Nickel-Based Superalloy during Long-Term Thermal Exposure

Abstract

Effects of a low-temperature solution heat treatment (SHT) below γ′ solvus temperature and a high-temperature partial solution heat treatment (SHT) above γ′ solvus temperature on microstructural evolution and stress rupture properties of a directionally solidified nickel-based superalloy during long-term thermal exposure at 900 °C were investigated in detail. The two heat treatments generated different microstructures, e.g., different γ′ sizes and surface fractions of residual γ–γ′ eutectic phase. Carbides in the two heat-treated samples exhibited different microstructural stability during the subsequent long-term thermal exposure. The creep lifetime at 870 °C/310 MPa of the samples treated by the high-temperature SHT is more than two times of that treated by the low-temperature SHT, whereas the creep lifetime of the samples treated by the high-temperature partial SHT exhibited a sharp reduction after 1000-h exposure and then decreased by a steady rate up to 10,000-h exposure. In comparison, the creep lifetime of the samples treated by the low-temperature SHT exhibits a slower reduction rate from 0- up to 10,000-h exposure.