Tuning strength-ductility combination on selective laser melted 316L stainless steel through gradient heterogeneous structure

Abstract

We report a strategy of gradient heterogeneous structure (GHS) architecture to achieve a superior strength-ductility combination in additively manufactured (AM) 316L stainless steel prepared by selective laser melting (SLM) and ultrasonic severe surface rolling (USSR). The GHS is a microstructure gradient system, which is composed of a gradient structured outer layer and a hierarchically heterostructured interlayer. The gradient structured outer layer exhibits a variation in grain size, phases, dislocations, twins, and cellular structure with depth and has a large thickness of approximately 1000 µm. The average grain size is approximately 93.9 nm in the topmost surface layer and increases with increasing depth from the treated surface. The hierarchically heterostructured interlayer has the heterogeneous grain structure, cellular structure, high-density dislocations, and nanoprecipitates, with length scales spanning several orders of magnitude. Due to the high microstructural heterogeneity arising from the combination of two types of heterogeneous structures, the GHS strengthens the synergistic effect, and thus results 316L stainless steel with a superior strength and high ductility. Furthermore, the mechanical properties of the GHS 316L stainless steel can be tuned by tailoring the microstructure: the larger the volume fraction of the gradient structured outer layer is, the higher the strength, but the lower the ductility. Molecular dynamics simulations indicate that deformation twinning, in addition to dislocation slipping, is an important deformation mechanism for nanocrystalline 316L samples at a large strain and thus plays an important role in maintaining strain hardening and ductility. We also discuss the potential application of USSR to AM parts to improve their mechanical properties and surface integrity. Our work demonstrates the potential of the proposed highly heterogeneous microstructure design strategy to prepare high-performance AM parts.