The heterogeneous microstructure in laser powder bed fabricated Inconel 718 pillar and its influence on mechanical properties

Abstract

The dimensions of small components have a strong influence on the microstructure and properties of structural material fabricated by the laser-based manufacturing approach. In this study, we analyzed the microstructure characteristics of Inconel 718 tiny pillars fabricated using laser powder bed fusion. We identified microscale heterogeneous microstructure with varying substructure sizes, arrangements of precipitates, dislocation structures, and segregation of solute atoms from the pillar center to the edge. The pillar center and edge owe very low-density dislocations, while the transition zone indicates high-density dislocation tangles on the substructure boundaries. For the first time, it is found the pillar center presents polygonal substructure with straight sub-boundaries in the absence of precipitates, which is entirely different from the typical microstructure in bulk Inconel 718 alloys manufactured using the same method. To evaluate the influence of the heterogeneous microstructure on the mechanical properties, nanoindentation and micropillar compression tests are conducted. The transition zone exhibits higher hardness and yield stress than the center and edge of the pillar. By contrast, the edge and the center region show similar mechanical responses.