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

The temporal evolution of non-equilibrium gamma' 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 gamma' solvus temperature, the supersaturated single phase gamma 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 gamma' domains which exhibit a far from equilibrium composition. Upon isothermal annealing, the gamma/gamma' interface sharpens and the compositions of both gamma and gamma' phases approach equilibrium. The influence of a non-classical mechanism of gamma' 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. (C) 2014 Elsevier Ltd. All rights reserved.