Multi-Scale Mechanical Characterization of Additively Manufactured Inconel 718

Additive manufacturing (AM) is an emerging technology due to its superiority in complex and customized design. However, in general, additively manufactured (AMed) materials inherently possess certain undesirable microstructural features, including anisotropy, hot-cracking, and inclusions, to name a few. While previous studies have mainly focused on bulk-scale mechanical testing to correlate apparent mechanical properties with microstructure, these attempts cannot fully explain the independent effect of various defects (e.g., grain/phase boundary, inclusion, pore, micro-cracks).<br/>In this presentation, we will report the effect of titanium nitride (TiN) inclusion on the mechanical properties of AMed Inconel 718 through multi-scale mechanical characterization. First, bulk-scale tensile testing is performed to understand the macroscopic mechanical behavior of AMed Inconel 718. Each test shows a discrepancy in elongation and yield strength, which suggests that TiN inclusions highly affect the mechanical properties of AMed Inconel 718. Based on strain map analysis, X-ray tomography imaging, and finite element analysis, we suggest that the presence of titanium nitride inclusions can lead to significant strain localization, which invokes premature failure. Second, meso-scale testing is performed to maximize the effect of TiN inclusions on mechanical properties by reducing the overall volume of the specimen. Finally, the effect of a single TiN inclusion on the deformation behavior is being investigated via micro-scale experiments. Micro-scale tensile bars are fabricated utilizing femtosecond laser machining followed by focused ion beam milling, and <i>in-situ</i> scanning electron microscopy (SEM) tensile testing is performed using a nanoindenter. Micro-scale testing also provides a direct comparison of the mechanical behavior of specimens with and without TiN inclusions.