Micro-mechanisms underlying enhanced fatigue life of additively manufactured 316L stainless steel with a gradient heterogeneous microstructure

The fatigue life of additively manufactured 316L stainless steel (SS) in an aqueous corrosive medium is shown to improve markedly when the surfaces were processed by surface mechanical attrition treatment (SMAT). SMAT induced surface nano-crystallization owing to severe plastic deformation and yielded a gradient microstructure beneath the surface of additively manufactured SS 316L, which increases the resistance to fatigue crack initiation and propagation. During cyclic deformation, dislocations are generated and accumulated in the cell boundary, resulting in the evolution of persistent slip bands (PSBs) at the surface of the as-manufactured specimen. Continuous intrusions and extrusions of PSBs provide crack initiation sites due to strain localization at the surface of the as-manufactured specimen, whereas the crack initiation region moved from the surface to the sub-surface layer after SMAT. Mainly, the mutual crossing of shear bands or twin boundaries accommodates most of the plastic deformation and provides crack nucleation sites at subsurface regions in this case. To elucidate the fatigue failure mechanism associated with fatigue loading (interactions of stacking faults, slip bands, and deformation twins), electron back-scattered diffraction and transmission electron microscopy analysis of the fatigue-failed alloy after SMAT was performed at different distances adjacent to the fatigue-failed tip. The interfaces between dislocation forest and twin-twin boundaries hinder dislocation motion and disrupt stress concentration. In addition, amorphous bands were formed at the interfaces between twin and shear bands which co-deform with the matrix during dynamic loading and act as a natural sink for dislocations, delaying decohesion and commencement of fracture. The changes in the crack initiation site and fatigue micro-mechanism cause considerable changes in fatigue life. This work provides insight into the failure mechanisms of additively manufactured SS for designing surface modification techniques to enhance the fatigue life of the parts post-printing.