Microstructure and stress-rupture life of high W-content cast Ni-based superalloy after 1000–1100 °C thermal exposures

Abstract

High W-content cast Ni-based superalloy which can be served as isothermal forging die materials over 1050 °C was investigated in this paper. Based on the microstructure evolutions, stress-rupture life of the alloy were examined under 1100 °C /70 MPa, where the alloy was evaluated after thermally exposed at 1000, 1050, and 1100 °C for 100–1000 h, respectively. Results showed that increasing of exposure temperature can cause coarsening and coalescing of γ′ precipitates along with a decreasing in their volume fraction, especially at 1100 °C. The coarsening rate of the γ′ precipitates increased with the exposure temperature and followed by the modified Lifshitz-Slyozov-Wagner coarsening theory of Ostwald ripening. The coarsening rate coefficient (k) at 1000, 1050, and 1100 °C was calculated to be 5.72, 7.40, and 8.71 nm/s1/3, respectively. In addition, at those temperatures, it found the blocky M6C carbides in the interdendritic area to be precipitated and coarsened. Furthermore, the M6C carbides linked with each other like a chain wrapped around the grain boundary and along the eutectic γ′ phase. Because of the degradation of the microstructure, the stress-rupture life decreased after thermal exposure, from 94.8 h at 1000 °C to 49.3 h at 1100 °C for 1000 h. Finally, in view of the experimental results, a relationship between the microstructural evolution and the stress-rupture life of the alloy was discussed in details.