In-situ characterization method for the delta phase precipitation kinetics in the nickel-base alloy 718 using high-temperature X-ray diffraction (HT-XRD) techniques

Abstract

An innovative characterization approach for determining the precipitation evolution of the delta phase in nickel-base alloy 718 during thermal processing has been developed, where the phase fraction of the evolving delta phase was determined in-situ by high-temperature X-ray diffraction technique (HT-XRD). This method is based on the evaluation of the peak shift corresponding to the matrix phase, which is independent from effects of texture and peak overlap. The evaluation can be performed without the need of complex Rietveld refinement procedures. Isothermal annealing experiments have been performed in the temperature range between 850 °C and 950 °C for annealing times up to 30 h. The delta phase fraction was evaluated indirectly by quantification of the shift of the diffraction angle of a distinct matrix phase peak (200) which exhibits a strong correlation with the evolving delta phase fraction during isothermal annealing. A nomogram could be constructed from the results which shows the relation between the delta phase fraction and the shift of the diffraction angle of distinct matrix phase peaks. Data deduced by conventional determination from the intensity of diffraction peaks of the delta phase were recorded for comparison. Thus, the application of the Rietveld method for textured materials is not necessary. Finally, experimental results obtained by metallography were used to validate the innovative indirect characterization approach.