TEM observations of variation of dislocation cell structures along the building direction in SLM-316L stainless steel

Abstract

Ultra-fast cooling and cyclic thermal heating associated with the additive manufacturing by selective laser melting (SLM) often lead to the formation of multi-scale microstructures in manufactured metallic materials. It has been frequently reported that a dislocation cell structure develops within the grains in SLM-316L stainless steels (ss), which has important effects on their mechanical properties and thermal stability. However, the formation mechanism of the dislocation cell structure is still under debate. In this study, we used transmission electron microscopy (TEM) techniques to characterize the variation of dislocation structures along the building direction in a SLM-316L ss sample. It exhibits various dislocation structures in the very surface melt pools. The distribution of dislocations at shallower locations is relatively discrete. While the dislocation cell structures are completely formed at deeper locations, which are similar to the bulk interior of the sample. The thermal cycle is not the main contribution to the formation of dislocation cell structures, but plays the role in enhancing the annihilation of dislocations inside the cells and sharpening the cell boundaries.