On the dendritic growth and microsegregation in Ni-base superalloys In718, In625 and In939

Abstract

Directional Solidification and Quenching (DSQ) has been used to analyse the solidification, segregation and precipitation behaviour of the Ni-base alloys IN718, IN625 and IN939. The microsegregation in the primary solidified y-phase was analysed by Energy Dispersive Spectroscopy (EDS) and were mainly found to be Nb for alloy IN718 and Ti for alloy IN939. The microsegregation behaviour were analysed by numerical back diffusion models and it was found that the diffusion rate in the samples was higher than expected from the theory. A suggested explanation was that this was an effect of the formation of lattice defects during the solidification process. The coarsening process that occurs during the further cooling of the samples is proved to be by the mechanism of back diffusion i.e. the smaller secondary dendrite arms grow together with the thicker ones into a plate.