Non-monotonic plasticity and hardness evolution of an additively manufactured 316L stainless steel at very high shear strains

Abstract

The present work demonstrates the unique micro-mechanisms of deformation of an additively manufactured (AM) 316L stainless steel (SS) fabricated using laser powder bed fusion (L-PBF) when subjected to very high strain levels (up to an equivalent strain of ∼216.5) using high-pressure torsion (HPT) technique. A non-monotonic transition in the deformation mechanism(s) was exhibited during the HPT processing of the L-PBF 316L SS. In contrast to the general evolution trend from slip→TWIP→TRIP with increasing strain, we report a shift from initial dominance of slip-based mechanisms to extensive deformation twinning followed by a resurgence of dislocation glide and subsequent detwinning during HPT of AM 316L SS. For the first time, the contribution of the compression stage of HPT on the microstructure and hardness evolution is revealed for AM 316L SS. The compression stage of HPT processing itself produced a significant alteration in microstructure, texture, and hardness, reflected as an order of magnitude increase in the dislocation density compared to the as-printed condition. A four-stage hardness evolution as a function of increasing strain is observed and explained based on the evolving nature of strengthening and softening mechanisms. While complete nano-structuring was achieved after a strain of 9.2, a saturation in grain size (∼42 nm) and dislocation density (∼4 × 10¹⁶ m⁻²) were achieved after an equivalent strain of ∼54.1. This study can promote a better understanding of the deformation mechanisms of the non-equilibrium cellular microstructure of AM alloys when subjected to extreme deformation.