Tensile Deformation Behavior of a Novel Nickel-Iron-Based Superalloy after Long-Term Thermal Exposure at 700 °C

The GH4650T alloy, a prime candidate material for high-temperature superheater/reheater applications in 650 °C class advanced ultra-supercritical coal-fired power units, was subjected to long-term thermal exposure at 700 °C. The thermal stability of the main strengthening γ′ phase and the impact of the γ′ phase’s microstructural characteristics on tensile properties and related deformation mechanisms were investigated by microstructure analysis and mechanical property testing. Experimental results indicate that the size of γ′ phase increases slowly and follows the Lifshitz–Slyozov–Wagner ripening law during thermal exposure, demonstrating good microstructural stability. As the exposure time increases, the tensile strength shows a trend of initial increase followed by a decrease, while the tensile plasticity exhibits the opposite pattern. Notably, after 10,000 h of thermal exposure, the elongation at 700 °C high-temperature tension can still reach about 33%. Transmission electron microscopy was used to observe the dislocation configurations formed after plastic deformation at room and high temperatures with different thermal exposure durations. It was found that with increased exposure time and growth in the size of the strengthening phase, the dominant mechanism of plastic deformation changes. When the exposure exceeds 5000 h, due to the coarsening of the particle size, the critical resolved shear stress required for dislocations to bypass the γ′ phase particles is lower, initiating a Orowan looping mechanism, and dislocations in both room temperature and high-temperature plastic deformation predominantly exhibit this Orowan looping mode.