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

The microstructural evolution , deformation behavior of a new Ni-Fe-based superalloy HT650T for 650 degrees C ultra-supercritical coal-fired power plant boiler tubes under creep conditions of 700 degrees C/250 MPa and 725 degrees 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 gamma' phases, MC and M23C6 carbides. No gamma'-free regions were observed. With the increase of creep temperature and the prolongation of duration of creep, gamma' 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 gamma' precipitates at 700 degrees C/250 MPa and 725 degrees 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 Sigma 3 and Sigma CSL (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.

Automated summary

The microstructure evolution and deformation behavior of the Ni-Fe-based superalloy HT650T under creep conditions were investigated using scanning electron microscopy with electron backscattered diffraction and transmission electron microscopy. It was found that the precipitates in the alloy consisted of gamma' phases, MC and M23C6 carbides, and no gamma'-free regions were observed. With temperature and duration of creep increasing, the gamma' precipitates grew gradually but maintained their spherical shape, while the M23C6 carbides coarsened and exhibited continuous morphology in some regions. The interaction between dislocations and gamma' precipitates was controlled by the Orowan looping mechanism. Stress concentration primarily occurred at high-angle grain boundaries, and crack propagation was hindered by specific grain boundaries. The excellent creep rupture resistance of HT650T alloy was attributed to the morphology and microstructural stability of phase constituents.