On microstructure evolution and damage onset in 316 L steel produced by laser-powder bed fusion during the early stages of low cycle fatigue loading

Contrarily to conventionally manufactured metals, the development of persistent slip bands (PSBs) and persistent slip markings (PSMs) in additively manufactured ones has been overlooked despite its fundamental role in fatigue crack initiation. To close this gap, this study focuses on the early microstructure evolution in 316 L steel produced by laser-powder bed fusion (L-PBF) cyclically loaded with 0.4 % strain amplitude. The aim is to gain insight into the role of microstructure during the onset of fatigue damage and provide qualitative observations on the formation of PSBs and PSMs. Strain-controlled tests were carried out on cylindrical-shaped and rectangular-shaped specimens. The tests were interrupted at different life fractions to perform microstructural observations using optical microscopy, SEM, and TEM. The results show the very early appearance of well-defined PSBs within the stable L-PBF process-induced cell structure in the bulk of fatigued material. Concurrently, on the specimen surface, the cyclic strain localisation starts after only five cycles as fine slip markings characterised by slip steps. With continuing cycling, PSMs, characterised by extrusions and, eventually, intrusions, are developed. The density of PSBs and PSMs increases with the number of cycles, leading to progressive cyclic softening. Since the crack nucleation from shallow surface intrusions is a relatively slow process, L-PBF process-induced defects (keyhole porosity) present at the specimen surface became more effective crack initiation sites as they represent the areas of stress and strain concentration. Nevertheless, PSBs developed prior to keyhole crack initiation in neighbouring grains play an important role in the subsequent crack growth.