Temperature dependence of MC decomposition behavior in Ni-base superalloy GTD 111

Microstructural evolution including MC decomposition during thermal exposure in GTD 111 has been investigated. After standard heat treatment, polycrystalline GTD 111 specimens were thermally exposed up to 10,000 h in the temperature range of 871-982 °C. Quantitative and qualitative metallographic analyses have been carried out to identify distribution and composition of the existing phases. Growth rate of γ′ of the alloy was proportional to the 1/3 power of the exposed time. The activation energy calculated for γ′ coarsening was 256 kJ/mol. MC carbides decomposed into η and M23C6 during thermal exposure at 871 and 927 °C rather than M23C6 and γ′. Precipitation of sigma (σ) phase was observed in the vicinity of MC or M23C6 carbides, especially in the specimen exposed at 871 °C. Continuous γ′ and M23C6 film precipitated along the grain boundary and fine η within γ-γ′ eutectic pool dissolved during thermal exposure at 982 °C.