Strengthening and anisotropy induced by cell structures and grain triaxial stress in additive manufactured Inconel 718 superalloy

The anisotropic microstructure and strengthening mechanisms of laser powder bed fused (L-PBF) Inconel 718 (IN718) superalloy were comprehensively investigated using experimental and theoretical analyses. Due to the complex thermal gradient and history, the cell structure evolved heterogeneously along the building direction (BD), exhibiting coarser dimensions with increasing distance from the baseplate. This unique microstructural feature played a dominant role in the anisotropic tensile properties (i.e. yield strength and strain hardening behavior) between the loading direction (LD) parallel and normal to BD. To deeply explore the correlation between the directional flow behavior and the various cellular configurations, in-situ high-energy X-ray diffraction (HEXRD) during tensile loading together with multiscale microstructural characterization was performed. It was revealed that the different yield strengths were primarily induced by the heterogeneous cell structures that experienced distinct thermomechanical histories, rather than by crystallographic texture. Furthermore, the more pronounced strain hardening capability observed in the horizontal specimen (loading normal to BD) was attributed to the strong dislocation-boundary interactions, inhibited microvoid formation and growth in the < 200 > //LD grains, and enhanced micro stress responses in the < 111 > //LD and < 200 > //LD grains. These findings offer new insights into the fundamental mechanisms governing the anisotropic mechanical behavior of L-PBF alloys.