Resume: Inconel 718 is widely used because of its ability to retain strength at up to 650 degrees C for long periods of time through coherent metastable gamma `' Ni(3)Nb precipitation associated with a smaller volume fraction of gamma' Ni(3)Al precipitates. At very long ageing times at service temperature, gamma `' decomposes to the stable Ni(3)Nb delta phase. This latter phase is also present above the gamma `' solvus and is used for grain control during forging of alloy 718. While most works available on delta precipitation have been performed at temperatures below the gamma `' solvus, it appeared of interest to also investigate the case where delta phase precipitates directly fromthe fccmatrix free of gamma `' precipitates. This was studied by X-ray diffraction and transmission electron microscopy (TEM). TEM observations confirmed the presence of rotation-ordered domains in delta plates, and some unexpected contrast could be explained by double diffraction due to overlapping phases.
Equipe: Département SI2M : Microstructures et Contraintes