TEM Study of High-Temperature Precipitation of Delta Phase in Inconel 718 Alloy

Abstract

Inconel 718 is widely used because of its ability to retain strength at up to 650∘C for long periods of time through coherent metastable 𝛾 Ni3Nb precipitation associated with a smaller volume fraction of 𝛾 Ni3Al precipitates. At very long ageing times at service temperature, 𝛾 decomposes to the stable Ni3Nb 𝛿 phase. This latter phase is also present above the 𝛾 solvus and is used for grain control during forging of alloy 718. While most works available on 𝛿 precipitation have been performed at temperatures below the 𝛾 solvus, it appeared of interest to also investigate the case where 𝛿 phase precipitates directly from the fcc matrix free of 𝛾 precipitates. This was studied by X-ray diffraction and transmission electron microscopy (TEM). TEM observations confirmed the presence of rotation-ordered domains in 𝛿 plates, and some unexpected contrast could be explained by double diffraction due to overlapping phases.