Heterogeneity and Solidification Pathways in Additively Manufactured 316L Stainless Steels

A unique microstructural feature often referred to as “fish-scale” has been reported in 316L austenitic stainless steel parts made by laser powder bed fusion (L-PBF) technique. Because the final microstructure is predominantly austenitic, with a face-centered cubic ( γ -fcc) crystal structure, the “fish-scale” structures were originally assumed to be based on etching response due to crystallographic orientations of the solidified γ grains. This research evaluated this assumption through multi-length scale and site-specific characterization using optical microscopy, hardness mapping, X-ray diffraction, electron back-scattered diffraction imaging, and scanning transmission electron microscopy. The nanoscale compositional measurements suggest that the “fish-scale” structures are related to a phase selection phenomenon that occurs during solidification due to spatial and temporal variation of thermal gradients and liquid–solid interface velocity. This phenomenon triggers the transition from γ -fcc to body-centered cubic (bcc) δ -ferrite solidification and then subsequent solid-state phase transformations of this bcc to fcc at low temperature. The significance of these phase transformation pathways with reference to deployment of additively manufactured 316 stainless steel components for harsh environments relevant to power generation is discussed. Graphical Abstract This research elucidated the mechanism for the “fish-scale” microstructure evolution in 316L additively manufactured stainless steel builds based on phase selection—either body- or fcc solidification—as a function of spatially varying thermal gradient and liquid–solid interface velocity within a single melt pool based on multi-length scale characterization and computational modeling.