Evolutions of microstructure and lattice misfit in a γ′-rich Ni-based superalloy during ultra-high temperature thermal cycle

The microstructure and lattice misfit of a γ′-rich superalloy during an ultra-high temperature thermal cycle (25-1200 °C) were studied. The cubic γ' precipitate gradually dissolved as the temperature increased from 900 °C to 1200 °C. Upon subsequent cooling, numerous tiny secondary γ' phases precipitated from the supersaturated γ channel. The microstructure was composed of a bimodal distribution of coarsened primary γ' and secondary γ' after the first ultra-high temperature thermal cycle. In subsequent thermal cycles, the secondary γ' dissolved gradually (or re-precipitated), and the primary γ' remained relatively stable. The dissolution of the γ′ phase led to a smaller absolute value of the lattice misfit. Interestingly, the lattice misfit was strongly dependent on the variation of the γ′ volume fraction during the thermal cycles, which exhibited good inheritance.