Defect-associated microstructure evolution and deformation heterogeneities in additively manufactured 316L stainless steel

Abstract

Previous research mainly focused on heterogenous microstructure modification to improve mechanical properties, but how the microstructure evolves during deformation has not been clarified yet when metallurgical defects are involved. In this work, austenitic 316L stainless steel was additively manufactured using selective laser melting (SLM), exhibiting typical heterogeneous microstructure with variation in grain structure, texture and metallurgical defects. Besides, the quasi-in-situ microstructural observation was conducted to reveal three competitive plastic deformation mechanisms to accommodate the deformation heterogeneities, i.e., lack-of-fusion pores, twinning and deformation bands. With presence of large lack-of-fusion pores (>1%), they accommodated the main strain and acted as preferential crack initiation sites, contributing to the pre-mature failure. In terms of near-full dense materials, microstructure and texture became significant with prevalent twinning and deformation bands. Twinning was activated in grains with <111> and <101> orientations parallel to the tensile direction, which promotes twin-dislocation and dislocation-cellular wall interactions and thus stabilizes the strain hardening. Deformation bands occurred along the columnar grain bands due to the deformation heterogeneities, which are likely to give rise to the initiation of micro-cracks with continuous strain. These findings offer insight into the microstructure modification and heterogeneous deformation behavior and provide key guidance for additively manufacturing stainless steels with high strength and ductility.