Evolution of γ′ precipitation during the early stages of industrial forging of a nickel-based superalloy

A sample of the Ni based superalloy AD730 was heat treated at a supersolvus temperature (1160 °C) then slowly cooled through the solvus temperature (1110 °C) at 10 °C/hr down to 1080 °C, i.e., a rate representative of the cooling conditions of an industrial scale billet undergoing controlled cooling. The γ′ precipitate distribution which forms during this cooling was investigated, and a mix of continuous and discontinuous precipitation was found. The discontinuous γ′ precipitates were imaged using 3D tomography, and were shown to present very different sizes, morphologies and aspect ratios when observed in different 2D imaging planes. The interaction between different populations of γ′ precipitate and recrystallization was investigated, and it was found that the discontinuous precipitates present more of a barrier to recrystallization than the continuous ones. This has been explained based on the different inter-precipitate spacing observed for the two populations. In addition to these γ′ precipitates which form during slow cooling, a fine and dense distribution of approximately spherical γ′ precipitates was found to form dynamically, during subsequent subsolvus forging, within unrecrystallized grains.