Achieving superior resistance to high–temperature creep in laser powder bed fusion of Inconel 718 via grain boundaries serration

Grain boundary serration and its effect on creep property in LPBF-built Inconel 718 alloy was investigated systematically. By considering the variation of the wavelength and amplitude with processing parameters, this study elucidates the evolution of the serration degree with changes in the intermediate solution treatment and cooling rate. A processing map for the serrated grain boundaries is presented. Three distinct formation mechanisms arise which rely upon grain boundary interaction with δ phase and element segregation: (i) specific direction of δ phase growth induced serration, (ii) the pinning of separated δ phase which hinders the grain boundaries movement, and (iii) lattice distortion resulting from element segregation induced serration. The creep tests under 650 °C/750 N have been performed on samples with different microstructure, one featuring non-serrated and the other with serrated grain boundaries, while keeping other aspects of the microstructure invariant. Compared to non-serrated grain boundary samples, the creep life of serrated grain boundary samples is increased by 56%, the creep strain is increased by 19%, and creep strain rate is decreased by 26%. Grain boundary serration can improve both creep life and creep ductility. The failure modes and accumulated damage were performed using SEM and EBSD technique. It can be found that serrated grain boundaries play a more crucial role in delaying cracks initiation due to it can alleviate the intensity of stress concentration.