Multiscale Mechanical Characterization of Additively Manufactured Inconel 718

Abstract

Additive manufacturing of alloys is attractive for its ability to fabricate complex and tailored structures. However, unusual thermal history during the additive manufacturing process creates unique microstructures, which leads to distinctive mechanical behavior. Here we report the room temperature and high temperature mechanical properties of Inconel 718 fabricated by direct laser deposition. Interestingly, failure strain at all tested temperatures exhibited noticeable scatter. Titanium nitride (TiN) inclusions, a unique microstructure with internal micro-cracks, was observed in additively manufactured Inconel 718. Effect of TiN inclusions on elongation was confirmed by comparing the microstructure and mechanical characteristics of additively manufactured IN718 and conventionally manufactured IN718 specimens after heat treatment. Notable scatter in the elongation was only observed at additively manufactured Inconel 718. In order to understand the origin of this scatter, strain mapping using digital image correlation (DIC) technique, fracture surface observation, and X-ray tomography was performed. X-ray tomography revealed that cracks propagated through the TiN inclusions, and the strain map implies that strain concentrated in these cracks. Finite element analysis estimated that a compressive stress of approximately 800 MPa can be induced in TiN inclusions during cooling due to thermal expansion mismatch between the matrix and the inclusions. In addition, a non-uniform shear stress can be induced in these TiN inclusions during loading. Through the mesoscale test, it was observed that micro-cracks inside the TiN inclusions propagated at a stress lower than the macroscopic yield strength, introducing rapid fracture and delamination between the matrix and the TiN inclusions. We suggest that rapid local fracture at the TiN inclusion lead to premature failure of additively manufactured Inconel 718.