Temporal evolution of non-equilibrium γ' precipitates in a rapidly quenched nickel base superalloy

The temporal evolution of non-equilibrium γ' precipitates in a rapidly quenched and isothermally annealed commercial nickel base superalloy has been investigated by coupling transmission electron microscopy and atom probe tomography. When subjected to rapid quenching from above the γ' solvus temperature, the supersaturated single phase γ matrix appears to undergo compositional phase separation possibly via spinodal decomposition to form solute-rich and solute-depleted regions. The regions that have a depletion in Cr and Co undergo an ordering process resulting in the γ' domains which exhibit a far-from equilibrium composition. Upon isothermal annealing, the γ/γ' interface sharpens and the compositions of both γ and γ' phases approach equilibrium. The influence of a non-classical mechanism of γ' precipitation on the size distribution of precipitates as well as the precipitate and matrix compositions, and its subsequent evolution during isothermal annealing has been discussed.