Fig. 1. (a) Schematic illustration of the scanning strategy adopted in the experiments. (b) 3D overview of the L-PBF produced 316L SS bars. (c) The geometry of the 316L SS specimen used in fatigue tests.

Fig. 2. Typical microstructures of the as-built SS. (a-d) SEM micrographs of the as-built alloy showing the characteristics of MPBs, grains, and cellular structures. (e) A bright-field STEM micrograph of the cellular structures. (f) The representative locations of STEM-EDS points on the cellular walls, HAGBs, and matrix, as indicated by the blue, violet, and red circler marks, respectively. (g) A HAADF STEM micrograph showing the precipitation of nanoparticles on the cellular walls. (h) A HAADF STEM micrograph showing cell structures widely formed in the microstructure. (i) Enlarged region of the red box in Fig. 2(h) and the corresponding EDS maps showing segregation of Mo and Cr to the cellular walls. The EDS map also illustrates that nanoparticles are rich in Si.

Fig. 3. Microstructural evolution with annealing treatment for the present L-PBF 316L SS. (a1), (b1), and (c1) Grain orientation maps. (a2), (b2), and (c2) Grain boundary distribution maps. (a3), (b3) and (c3) Recrystallization (REX) behavior. (a4), (b4), and (c4) Configurations of cell structures. (a), (b), and (c) The as-built, 900 °C-10 min and 1050 °C-10 min samples, respectively. (d-g) Evolution of grain size, aspect ratio, LAGB concentration, and recrystallization behavior as a function of annealing temperature, illustrating that other microstructural characteristics are roughly the same in different L-PBF 316L SS.

Fig. 4. EBSD-BC (EBSD band contrast) image with boundaries superimposed by different color lines and the corresponding GND density maps of the as-built (a), 900 °C-10 min (b) and 1050 °C-10 min (c) samples, respectively. (d) The probability density of discrete GND density measurements in the present L-PBF alloy after different annealing treatments.

Fig. 5. (a) Effects of cell structures on the fatigue properties of L-PBF 316L SS. (b) Typical hysteresis loops of the as-built sample under the total stress range (ΔσT) of 769 MPa. (c) The strain response as a function of cycle number for the as-built sample under ΔσT of 769 MPa. (d) The fracture surface together with the crack propagation length of the as-built sample under ΔσT of 769 MPa. (e) Magnified image of the fracture surface in the blue box in Fig. 5(d). (f-j) Fracture characteristics of different fatigue stages. (k) Crack length dependence of crack growth rate, obtained from the measurement of striation width, as presented in Figs. 5(g, h).

Fig. 6. Typical dislocation configurations of the as-built 316L SS at different distances from the fracture after failure under a total stress amplitude of 769 MPa. (a) and (b) 5 mm. (c) and (d) 2 mm. (e) and (f) near the crack initiation sites.

Fig. 7. (a) A HAADF-STEM image of a partial dislocation pair in the as-built sample near the crack initiation sites after fatigue. (b-d) High resolution (HR) HAADF STEM images illustrating the SF, leading partial and trailing partial dislocations in (a). (e) The interaction of the two SFs forming a V-shaped configuration. “⊥” within Burgers circuits shows the Shockley partial at one end of each SF. The third “⊥” indicates a Lomer-Cottrell dislocation formed by the combination of the partials at the other ends of the two SFs. (f-h) Typical configurations of stacking faults (SFs) in the as-built sample near the crack initiation sites after fracture. (f) SFs originated from the cell walls. (g) The leading partial dislocation sliding across the cell interior with the increasing applied stress, and stopping by another cell wall. (h) The leading partial dislocation cutting through another cell wall and slipping into the neighboring cell. (i) Schematic showing the mechanisms of cell structure on the motion of dislocations during fatigue deformations.

Fig. 8. TEM micrographs of the as-built sample near the crack initiation sites after deformation. (a) Nanotwins initiated from the cell walls. (b) Nanotwins cutting through the cell walls. (c) Nanotwins stopped by HAGBs, leading to stress concentrations along them. (d) An HR HAADF-STEM image showing a stable nanotwin consisting of three atomic layers, supporting the layer-by-layer growth mechanism of twins in our case.

Fig. 9. (a) The typical microstructures of the 1050 °C-10 min sample before fatigue deformations, showing that cell structures are degraded by annealing treatments. (b) and (c) The TEM image of the 1050 °C-10 min sample after failure under a total stress amplitude of 754 MPa, illustrating that the SFs only can be occasionally observed and some cell-like dislocation substructures are formed during fatigue deformations. (d) The corresponding EDS maps of the red box in (c) showing no element segregation on the formed substructure boundaries. (e) Schematic of the interactions between cell structures and dislocations, showing the significant impediment effects to the dislocation propagation not only from the cell walls but also from the compositional micro-segregation. (f) Deformation mechanisms of the as-built L-PBF 316L SS during the entire fatigue process.