Hetero-deformation induced (HDI) strengthening in directed energy deposited SS316L: A nanoindentation-based investigation

Hetero-deformation induced (HDI) strengthening is investigated as the mechanism responsible for the exceptional combination of strength and ductility exhibited by directed energy deposited (DED) stainless steel 316L (SS316L). Recent studies have reported that the exceptional properties are due to the influence of cell wall features, i.e. austenitic cell walls and secondary delta ferrites on the deformation of cell interiors. The influences manifest as barrier effects causing deformation localization and slip band refinement. While these findings allude to the possibility of HDI strengthening, the role of the properties of these individual features is unclear. To investigate the role of individual features in terms of their properties and establish HDI, we performed a nanoindentation-based study. First, using low-load (<1500 μN) nanoindentations, the hardness of the individual features were determined. Contour maps of local hardness generated using these hardness values showed a striking resemblance to its cellular subgrain morphology, revealing a microstructure with hetero-zones. Then, the synergistic influence of the hetero-zones was revealed from the relationship between the hardness of individual features and the effective hardness (determined with high-load indentations of greater than 7 N), thus establishing HDI strengthening. Finally, by analyzing the hardness variation with respect to the distance from the feature interface, the presence of an interface affected zone (IAZ), one that has been hypothesized in recent studies, was experimentally established, and its width estimated. The findings of this study suggest that DED SS316L is a heterostructured material with great promise for deriving exceptional properties by enhancing HDI strengthening.