Dissolution kinetics of δ phase and its influence on the notch sensitivity of Inconel 718

The dissolution kinetics of δ phase in Inconel 718 at 980 deg. C, 1000 deg. C and 1020 deg. C and its influence on high temperature notch sensitivity have been studied using a quantitative X-ray diffraction (XRD) method and high temperature stress rupture life tests of notched specimens. The amount of δ phase decreases gradually during holding time at 980 deg. C, 1000 deg. C and 1020 deg. C. The δ phase will be fully dissolved in the austenitic matrix at 1020 deg. C for more than 2 h. A certain amount of δ phase still exists after holding at 980 deg. C and 1000 deg. C for times up to 6 h; the amount remaining are 3 wt.% and 0.6 wt.%, respectively. The dissolution rate remains at a high level at the beginning, and then decreases gradually with an increase of holding time. A dynamic equilibrium state can be approached after holding at 980 deg. C for more than 30 min and at 1000 deg. C for more than 2 h. The alloy with δ phase amounts higher than 0.62 wt.% does not exhibit notch sensitivity, whereas serious notch sensitivity exists if the concentration is below 0.43 wt.%