Coarsening Behavior of Precipitates in a Conventional Cast Nickel Base Superalloy during Long Term Thermal Exposure

Abstract

A long term aging treatment at 900oC for 3000h was carried out on a conventional cast nickel base superalloy with standard solution and aging treatment. The microstructural evolution including the phases within grains and at grain boundaries (GBs) during thermal exposure in the alloy was observed using OM and SEM. It was shown that the major phases in the alloy after standard heat treatment were γ' precipitates and coarse blocky MC carbides, both of which were distributed not only within grains but also at GBs. During aging, the size of MC carbides and γ' precipitates both within grains and at GBs increased with increasing aging time. It was found that two characteristics of MC carbide and γ' precipitates evolved with aging time: the one is that the coarsening ratio of carbides and γ' precipitates at GBs were significantly higher than that of carbide and γ' precipitates inside the grains; the other is that the coarsening ratio of carbides was markedly higher than that of γ' precipitates. M6C carbide and η phase initially formed in the edge of MC carbide at the expense of MC carbides and γ' precipitates in the vicinity of MC during the early stage of aging. After 3000h, MC carbides inside the grains were covered by η phase films, while those distributed at GBs were completely decomposed into η phase and M6C particles.