dislocation configurations of high-temperature long-term exposed polycrystalline gtd-111 nickel-based superalloy

This research investigates the effect of very long-term service exposure on the microstructure of a gtd-111 precipitation-hardened ni-based superalloy after 75,000 hours. gtd-111 is a precipitation strengthened nickel-based superalloy introduced in the eighties, having both polycrystalline and directionally solidified structures, and known for its creep, thermo-mechanical fatigue, oxidation and corrosion resistances. the gtd-111 mainly comprises three phases including γ phase as the matrix, γ́ (ni3al) phase as hardener precipitations, and mc carbides which two former phases are in charge of principal strengthening and creep resistance. due to their long-term operation at high temperatures, the alloy is subjected to harsh environments characterized by high pressure (centrifugal and gas pressure), high temperature, substantial temperature gradients, and oxidizing and corrosive elements. as a result, various damage mechanisms can occur during service, such as local creep, oxidation, thermo-mechanical fatigue, corrosion, bowing, and grain boundary oxidation. besides the microstructural degradation, including coarsening and spheroidizing, carbide decomposition (mc to m23c6), and grain boundary widening, dislocation formation can also affect alloys' high-temperature mechanical properties and reliability. the coarsening and spheroidizing of the primary γ́, which is a strengthening phase in many alloys, can significantly impact the creep behavior of the exposed alloy. as the γ́ particles become larger and more rounded, they lose their effectiveness in impeding dislocation movement, leading to a decrease in the alloy's creep resistance. this phenomenon is often observed in high-temperature applications, where creep deformation is a primary concern. therefore, it is essential to carefully monitor and control alloys' microstructural evolution to ensure optimal service performance. the coarsening and spheroidizing of the primary γ́ can affect on the creep behavior of the exposed alloy. furthermore, topological closed packed (tcp) like σ phase, η phase, in the gtd-111 after long-term exposure, were observed in the interdendritic area close to the eutectic and mc carbides. based on the transmission electron microscopy’s observations, although the dislocation movement are limited in the γ channels and interface between γ/γ′ phases, some dislocations cut into γ′ phases. the long-term exposure leads to the formation of 3d dislocation networks in the γ channels through consequent cross slip and climb. the study's results confirmed that the primary and secondary γ′ phases could hinder the movement of dislocations under stress and temperature conditions experienced during operation. as the stress and temperature increase, the dislocation mechanism changes from the network in γ channels and the interface between γ/γ′ phases to shearing the γ′ phases. this results in the formation of apb and sfe zones. furthermore, the creep test results confirmed that the exposed alloys' creep behavior decreased dramatically compared to unexposed alloys. fracture analysis indicated that the crack initiation and propagation in the interdendritic zone, comprising grain boundary oxidation, tcp and eutectic phases. crack initiation sites can be located on the specimen surface or in the grain boundary area close to the eutectic and tcp phases.