Microstructural evolution and strengthening of selective laser melted 316L stainless steel processed by high-pressure torsion

The microstructural evolution and deformation mechanisms of 316L stainless steel fabricated by Selective Laser Melting (SLM) and then processed by high-pressure torsion (HPT) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction (XRD). TEM observations and XRD line broadening analysis reveal that the deformation in the HPT-processed alloy can be categorised into three deformation stages, related to the microstructural features and the corresponding equivalent strain values, epsilon(eq). Twinning, dislocation generation and multiplication, and the formation of twin-matrix lamellae are the main deformation mechanisms in stage 1 (epsilon(eq). = similar to 0-10). Shear banding of the twin-matrix lamellae and formation secondary nanotwins contribute to the deformation process in stage 2 (epsilon(eq). = similar to 10-40), while an equilibrium or saturation stage indicated by the formation of equiaxed nano-sized grains is reached in stage 3 (epsilon(eq). > 40). A model based on the linear additive theory was then established to evaluate the contribution of solid solution, dislocation, grain boundary, and twinning to the hardness increase and overall strengthening attained by this alloy.