Quantitative microstructural evolution and corresponding stress rupture property of K465 superalloy

Abstract

The damage assessment and life prediction of cast turbine blades are closely related to various microstructural degradation caused by thermal exposure at service temperatures. However, limited systematic investigations about quantitative characterization of microstructural evolution in superalloys were reported. In this paper, cast superalloy K465 was thermal exposed from 900°C to 1050°C for 100h to 1500h, then the microstructural evolution were characterized and the stress rupture tests were conducted under 975°C/225MPa. The experimental results indicated that γ′ precipitates showed high microstructural stability at 900°C and below, while the decomposition of MC carbides into M6C carbides proceeded gradually in the temperature range from 900°C to 1210°C. Moreover, blocky M6C and M23C6 carbides precipitated in the interdendritic region and along grain boundary in different temperature ranges. The plate-like phases precipitated in the temperature range from 900°C to 950°C and 1000°C to 1050°C were identified as μ phase and M6C carbides, respectively. The amount of μ phase was much higher than that of M6C carbides. The precipitation of μ phase enriched in W and Cr were mainly attributed to the decrease of stress rupture lives for K465 superalloy after thermal exposure in the temperature range from 900°C to 950°C, compared with those after thermal exposure in the temperature range from 1000°C to 1050°C, which contained even lower γ′ volume fraction with more degradation. This study is helpful to better understand the various microstructural evolution at different temperatures and to optimize the design and assessment of service induced degradation of turbine blades made of K465 superalloy.