Influence of composition and cooling rate on constrained and unconstrained lattice parameters in advanced polycrystalline nickel–base superalloys

The influence of composition on the lattice parameters of γ and γ′ has been studied in four advanced polycrystalline nickel–base superalloys using high energy synchrotron X-ray diffraction. Measurements were carried out on small bulk samples and extracted γ′ powder, which allowed distinction between constrained and unconstrained lattice parameters and misfits. It was found that decreasing the cooling rate from the solution temperature led to deviations from sphericity of the secondary γ′ particles and an increase of the unconstrained misfit. The change of unconstrained misfit is believed to be due to a change of γ′ precipitate chemistry. High cooling rates will probably result in γ′ precipitates slightly enriched with Al and correspondingly smaller γ′ lattice parameters when compared to specimens subjected to slower cooling rates, where more Ta can be expected to be contained within the γ′ phase. Compared to the unconstrained misfit the constrained misfit strain only changes relatively little with cooling rate due to the γ′ phase developing a compressive elastic strain for slow cooling rates. It was found that the compressive elastic strain in γ′ coincides with a complex γ′ morphology. Additional measurements of the γ and γ′ lattice parameters at 800 °C demonstrated that the coefficient of thermal expansions (CTE) of each phase are very similar avoiding any thermally induced elastic strain during heat treatment. The complex interactions between chemistry, process history, γ′ morphology and coherency strains are assessed in this paper.