The role of cell boundary orientation on mechanical behavior: A site-specific micro-pillar characterization study

The characteristic sub-grain cellular structures widely observed in additively manufactured metals and alloys play an important role during plastic deformation. In the present study, the influence of cell boundaries on the mechanical response of a directed energy deposited 316 L stainless steel was investigated using site-specific micro-compression tests of single and bi-crystal micro-pillars with various cell boundary orientations relative to the compression direction. In an effort to distinguish between grain boundary and cell boundary effects, single crystal micro-pillars were fabricated from strategically selected regions within a single grain, but with variable cell boundary orientations. Bi-crystal pillars that cross grain boundaries were also investigated. While the stress strain curves for single-crystal micro-pillars exhibit serrations and a mild strain hardening, those for bi-crystal micro-pillars are absent of serrations and show higher strain hardening rates. Moreover, despite differences in cell boundary orientation, single-crystal micro-pillars with the same crystal orientation (i.e., from the same grain) show similar yield strengths and strain hardening behavior, indicating a limited effect of cellular structure on plastic anisotropy. The results further demonstrate that although cell boundaries can temporarily impede the motion of dislocations, corresponding to a strengthening effect, they ultimately allow dislocation transmission when the applied stress is sufficiently high. While our results suggest that cell boundaries and grain boundaries have distinct interaction mechanisms with dislocations, they also indicate that manipulation of the cellular texture may provide a pathway to engineer the properties of additively manufactured metals and alloys.

Automated summary

The study on single and bi-crystal micro-pillars reveals that cell boundary orientation significantly affects the mechanical response of steel, with single-crystal micro-pillars showing serrations and mild strain hardening, while bi-crystal micro-pillars exhibit smoother behavior with higher strain hardening rates. Despite differences in cell boundary orientation, single-crystal micro-pillars from the same grain exhibit similar plastic properties, indicating limited effect of cellular structure on plastic anisotropy. The results suggest that cell boundaries and grain boundaries have distinct interaction mechanisms with dislocations, and manipulating the cellular texture could be a pathway to tailor the properties of additively manufactured metals and alloys.