Microstructural stability and lattice misfit characterization of nimonic 263

Nimonic 263 has been selected as a candidate header/piping material of advanced ultra-supercritical (A-USC) boilers for the next generation of fossil fuel power plant. Experimental assessments on the microstructural stability of this material are presented in this paper. Microstructural evolution has been quantified by high resolution field emission SEM and TEM. Electron diffraction and the combined XRD and Gaussian peak-fitting have been applied to investigate the coherency and lattice misfit between the gamma prime precipitates and the gamma matrix. The microstructure subjected to solution and hardening treatment consists of gamma-matrix and a network of carbide precipitates along the grain boundaries. Large quantities of fine gamma prime-Ni3(Ti,Al) precipitates were observed, with an average size of 17 nm and coherent with the matrix lattice. The overall misfit has been quantified to be 0.28%. After long term aging at 700 and 725 °C for various periods up to 20,000 hours, gamma prime was still the predominant precipitate and mostly coherent with the matrix. A few needle-shape eta-Ni3Ti intermetallic precipitates were found in the grain boundary regions. The gamma prime size has grown progressively to 78 nm, accompanied by the gamma-gamma prime constrained misfit increasing to 0.50%. Moreover, the M23C6-type grain boundary carbides were found to have experienced morphological evolution, including the nucleation of Widmanstatten-type needles and their initial growth towards the matrix.