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

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 fromthe fccmatrix 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