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

Abstract EN

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.