On the origin of cyclic softening in fatigued 316L austenitic stainless steel with unique cellular solidification/dislocation substructure manufactured additively by selective laser melting

Austenitic stainless AISI 316L steel manufactured by selective laser melting (SLM) was fatigued in the as-built state at room temperature with three constant strain amplitudes of 0.4%, 0.5% and 0.8% until failure. Cycling at all strain amplitudes resulted in a mild cyclic hardening followed by permanent cyclic softening, the intensity of which decreased with decreasing applied strain amplitude. The amount of cyclic softening in SLM 316L steel is comparable to that of its conventionally produced counterpart, cyclically strained after ∼10% cold-working. The mechanisms responsible for cyclic softening in both materials are different due to the difference in the nature of the initial dislocation arrangement. The virgin, non-equilibrium hierarchical SLM-process-induced microstructure exhibited high mechanical stability, with no destruction or rearrangement of the unique, sub-micron rod-like dislocation network. Distinct lamellae of localized intensive cyclic slip – persistent slip bands (PSBs), running crystallographically within individual grains across melt pool and subgrain boundaries, were found using a special color etching technique. The principal mechanism of cyclic softening was convincingly demonstrated through detailed high-resolution characterization of dislocation arrangements using transmission electron microscopy (TEM) and the detection of sharp surface persistent slip markings (PSMs) consisting of extrusions and intrusions. A plausible 3D model of the PSB structure reflecting the virgin SLM-process-induced dislocation arrangement and consisting of condensed multipolar dislocation-rich walls alternated by dislocation-poor cell cores is proposed. This most crucial feature essential for suggested dislocation mechanisms and sources of cyclic slip irreversibility within PSB lamellae is analogous to the classical ‘ladder’ structure of PSBs typical for fcc single crystals and conventionally produced polycrystals. The relevance of the present experimental findings for some other SLMed metallic materials is highlighted.