Microstructural evolution of a new Ni-Fe-based superalloy deformed by creep

The microstructural evolution and deformation behavior of a new Ni-Fe-based superalloy HT650T for 650°C ultra-supercritical coal-fired power plant boiler tubes under creep conditions of 700°C/250 MPa and 725°C/150 MPa were investigated by scanning electron microscopy with electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The experimental results show that the precipitates in HT650T alloy before and after creep are composed of γ' phases, MC and M23C6 carbides. No γ'-free regions were observed. With the increase of creep temperature and the prolongation of duration of creep, γ' precipitates grow gradually but keep their spherical shape, while M23C6 carbides coarsen and show continuous morphology in some regions. TEM analysis indicates that the interaction of dislocations with γ' precipitates at 700°C/250 MPa and 725°C/150 MPa is controlled by the Orowan looping mechanism. The EBSD analysis reveals that stress concentration is primarily at high-angle grain boundaries (HAGBs), cracks tend to propagate along HAGBs and are hindered by Ʃ3 and ƩCSL (coincident site lattice) ≤ 29 grain boundaries. The excellent creep rupture resistance of HT650T alloy is attributed to the combined effects of morphology and microstructural stability of phase constituents.