Nested size effects in the nanoindentation response of additively manufactured 316L stainless steel

Recent work has shown that the presence of cellular substructures in selectively laser melted (SLM) 316L stainless steel coincides with significant increases in yield stress and ultimate strength. These substructures have been associated with dislocation cell formation, with cell walls and cell interiors corresponding to regions of high and low dislocation density, respectively. Motivated by this finding, the present effort seeks a more comprehensive understanding of this phenomenon through spatially resolved hardness mapping via nanoindentation. Initially it was assumed that a spatially varying cellular hardness pattern would emerge, corresponding to the geometry and the characteristic features (cell walls and cell interior) of the cell structure. However, due to multiple, simultaneously operating hierarchical size effects, the apparent hardness deviates from this anticipated behavior. Direct evidence is provided in the present work indicating that these size effects are due to the indenter, characteristic sub-structural length, and grain size. Analysis of the load/displacement behavior enabled observation of each of these size effects, independent of the local anisotropy due to crystal orientation. Published by Elsevier B.V.