Cyclic behaviour and microstructural evolution of metastable austenitic stainless steel 304L produced by laser powder bed fusion

Abstract

en

The present study delivers an insight into the cyclic behaviour of LPBF processed austenitic stainless steel 304L and its relation to observed microstructure evolution and strain-induced martensitic transformation. The combination of electron microscopy observations and feritscope measurements enabled to follow dislocation microstructure evolution and the onset of martensite nucleation. The excellent cyclic strength of stainless steel 304L is a direct consequence of cell microstructure containing high dislocation density walls and elemental microsegregation, which effectively inhibit dislocation motion. Cyclic softening was linked with cyclic strain localization into slip bands of decreased dislocation density and heavily altered dislocation cell walls. These bands have been observed for the first time in LPBF processed metals. This microstructural feature seems to be a variant of frequently observed dislocation arrangement, persistent slip bands (PSBs), typically observed in conventionally produced materials. PSBs present the areas of intensive cyclic plasticity where the strain-induced martensitic transformation occurs preferentially. The increasing α’- martensite volume fraction, accompanied by a formation of intermediate ϵ- martensite and deformation twinning, resulted in recorded cyclic hardening. The martensite nucleation sites are strongly determined by the cell microstructure, especially cell walls dislocation density and chemical segregation, which is tightly related to utilized L-PBF process parameters.

Fatigue; Laser powder bed fusion; Stainless steel; Strain induced martensitic transformation; Cyclic behaviour

2024-09-04

Alloys for Additive Manufacturing Symposium 2024, September 4-6, École Polytechnique, France

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