Microstructural evolution and creep behavior of a novel Ni-Fe-based superalloy GH2070P for 700 °C class A-USC steam pipes

The study investigates the microstructural evolution and creep behavior of a GH2070P (HT700P) Ni-Fe-based superalloy, which has the potential for advanced ultra-supercritical coal-fired power plant units at 725 degrees C/ 180 MPa and 725 degrees C/150 MPa. The experimental results reveal that the phase constitution of GH2070P alloy comprises matrix gamma, gamma ' ' phases, MC, and M 23 C 6 carbides. During the creep, the alloy's phase structure remains stable without the precipitation of other phases. The sizes of MC and M 23 C 6 grow slightly, M 23 C 6 carbides tend to form a continuous morphology, and the gamma ' ' phases coarsen. As the creep time increases, the fracture of the alloy undergoes a transition from intergranular to transgranular mode. Transmission electron microscopy investigations indicate that the interaction of dislocations with gamma ' ' phases is governed by the Orowan looping mechanism, and the superlattice stacking faults appear with the increase of creep time. The electron back- scattered diffraction analysis shows that, due to the impediment of Sigma 3 and Sigma CSL <= 29 grain boundaries, cracks tend to accumulate and interconnect along high-angle grain boundaries, leading to the eventual fracture.