X-ray line profile analysis study on the evolution of the microstructure in additively manufactured 316L steel during severe plastic deformation

Abstract 316L stainless steel was manufactured by additive manufacturing (AM), and then, the samples were severely deformed by the high-pressure torsion (HPT) technique. The evolution of the microstructure was monitored by X-ray line profile analysis. This method gives the crystallite size and the density of lattice defects, such as dislocations and twin faults. The AM-processing of the HPT disks was performed in two different modes: the laser beam was parallel or orthogonal to the normal direction of the disks. The subsequent HPT deformation was carried out for ½, 1, 5 and 10 turns. The microstructure and hardness evolution during HPT were similar regardless of the laser beam direction. For both sample series, the minimum achievable crystallite size was about 30 nm, while the dislocation density and the twin fault probability got saturated at the values of 300–350 × 10 14 m −2 and 3.5–4%, respectively. The microstructure evolution during HPT of the AM-prepared 316L steel was compared with the HPT-induced changes in an as-cast counterpart. It was found that while the AM-prepared 316L steel remained a single-phase face-centered cubic γ-structure during HPT, in the as-cast samples a body-centered cubic (bcc) martensitic α-phase became the main phase with increasing the imposed strain of HPT due to the lower Ni content. In the saturation state achieved by HPT the initially as-cast 316L steel had a considerably higher hardness (about 6000 MPa) than that for the AM-prepared samples (~ 5000 MPa) due to the large fraction of the hard bcc phase formed during HPT.