Implications of solute super-saturation in growth of vaporisation-induced recrystallised grains during heat treatment in Ni-base superalloys

The role of vaporisation in the evolution of surface microstructural instability has been investigated in Ni-base superalloys during heat treatment below the solvus temperature. Initial nucleation of beta (beta) grains at the surface with random orientations arose from vaporisation of Ni, Co and Cr with subsequent de-stabilisation of gamma (gamma) phase. Further vaporisation and inter-diffusion resulted in a discontinuous reaction (secondary reaction zone -gamma prime (gamma') matrix + topologically close packed phases with needle-like morphology). On the other hand, advancement of the secondary reaction zone grain boundary was shown to be dependent only on the solute supersaturation ahead of the boundary. The driving force for growth of the secondary reaction zone was calculated from the Gibbs energy change and nearly all of this force was available to pull the grain boundary.