Characterization of γ' Precipitates in Cast Ni-Based Superalloy and Their Behaviour at High-Homologous Temperatures Studied by TEM and in Situ XRD.

In situ X-ray diffraction and transmission electron microscopy has been used to investigate René 108 Ni-based superalloy after short-term annealing at high-homologous temperatures. Current work is focused on characterisation of γ' precipitates, their volume fraction, evolution of the lattice parameter of γ and γ' phases and misfit parameter of γ' in the matrix. Material in the initial condition is characterised by a high-volume fraction (over 63%) of γ' precipitates. Irregular distribution of alloying elements was observed. Matrix channels were strongly enriched in Cr, Co, W and Mo, whereas precipitates contain large amount of Al, Ti, Ta and Hf. Exposure to high-homologous temperatures in the range 1100-1250 °C led to the dissolution of the precipitates, which influenced the change of lattice parameter of both γ and γ' phases. The lattice parameter of the matrix continuously grew during holding at high temperatures, which had a dominant influence on the more negative misfit coefficient.