Deformation mechanisms of the subgranular cellular structures in selective laser melted 316L stainless steel

Selective Laser Melting (SLM) of certain alloys such as Stainless Steel yields a unique microstructure consisting of complex subgranular cell structures produced by ultra-fast cooling during solidification. Owing to the presence of these cell structures, SLM-fabricated 316L Stainless Steel displays a superior yield strength and hardness. Despite the significance of the distinctive cellular structures, their deformation characteristics and the mechanisms responsible for superior mechanical properties are not well-understood yet. In this paper, the mechanical deformation behavior of the complex cellular structures observed in the SLM-fabricated 316L Stainless Steel is investigated by performing a series of molecular dynamics simulations of uniaxial tension tests. The atomic configurations in the computational models of the cell structures are inspired by the SEM images of the microstructure of 316L Stainless Steel. The effects of compositional segregation of alloying elements, distribution of austenite and ferrite phases in the microstructure, subgranular cell sizes, and pre-existing (grown in) nanotwins on the tensile characteristics of the cellular structures are investigated. The highest yield strength is observed when the Ni concentration in the cell boundary drops very low to form a ferritic phase in the cell boundary. However, the flow stress is found to be the highest when both the cell boundary and cell interior are austenitic with higher Ni concentration in the cell interior. Moreover, governed by the initial dislocation density, the subgranular cell size has an inverse relationship with mechanical strength. Finally, the nano-twinned cells exhibit higher strength in comparison with twin-free cells as a result of the dislocation pinning at the twin boundaries. The investigation on the effect of twin spacing reveals a Hall-Petch type phenomenon in which smaller twin spacing strengthens the nano-twinned subgranular cells further.