Quantitative microstructural evolution and corresponding stress rupture property of K465 superalloy

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 degrees C to 1050 degrees C for 100 h to 1500 h, then the microstructural evolution were characterized and the stress rupture tests were conducted under 975 degrees C/225 MPa. The experimental results indicated that gamma' precipitates showed high microstructural stability at 900 degrees C and below, while the decomposition of MC carbides into M6C carbides proceeded gradually in the temperature range from 900 degrees C to 1210 degrees 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 degrees C to 950 degrees C and 1000 degrees C to 1050 degrees C were identified as mu phase and M6C carbides, respectively. The amount of mu phase was much higher than that of M6C carbides. The precipitation of mu 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 degrees C to 950 degrees C, compared with those after thermal exposure in the temperature range from 1000 degrees C to 1050 degrees C, which contained even lower gamma' 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. (c) 2015 Elsevier B.V. All rights reserved.