A micropillar compression investigation into the plastic flow properties of additively manufactured alloys

Solidification cells and a high density of dislocations are two common features of additively manufactured (AM) alloys that are processed using techniques such as laser powder bed fusion (L-PBF) and directed energy deposition (DED). A critical assessment of their role in determining the plastic properties (yield strength, σy, and work hardening behavior) was performed via the micropillar compression tests on the austenitic 316L stainless steel (316L) and the Inconel 718 (IN718) alloys manufactured using the L-PBF and DED techniques, and comparing the results obtained with those of the compression and tensile tests on bulk samples. While both the L-PBF alloys contain submicron-scale cells whose boundaries are decorated with the dislocation networks, the DED 316L consists of micron-scale cells (whose boundaries are enriched with elemental segregation) with a uniform distribution of dislocations within them. The variations in σywith the pillar size are similar to those reported on pillars fabricated from pre-strained Ni but are opposite to those reported on pillars of micro/nano-crystalline alloys. The mechanical responses of the DED 316L pillars with and without cell boundaries (CBs) are similar. These observations suggest that the high density of dislocations (arranged in the network fashion or distributed uniformly) —and not the CBs—determine σy of the AM alloys. The stress-strain responses of pillars and transmission electron micrographs obtained on the deformed bulk samples suggest that the dislocation networks significantly enhance dislocation storage, leading to bulk-like deformation behaviors and superior work hardening capability in the L-PBF pillars with larger diameters.