Fig. 1. Microstructures of BAT steels. (a) SEM micrograph; (b) TEM micrograph with electron diffraction pattern of RA [43]; (c) EBSD image combined with band contrast, inverse pole figure (IPF) and pole figure (PF) maps of bainitic ferrite and martensite, the dashed black line showing the boundary of a single prior austenite grain; B: bainite, M/A: martensite/austenite island, BF: bainitic ferrite, RA: retained austenite.

Fig. 2. (a) S-N data and (b) fraction of each fatigue crack initiation mode of failed E-BAT and U-BAT samples. Sur: surface, Sur (Inc): surface inclusion, Int (Inc): interior inclusion, Int (NIICI): interior non-inclusion induced crack initiation.

Fig. 3. (a) SEM image of a typical NIICI fracture surface of E-BAT specimen failed at σa = 745 MPa with Nf= 1.69 × 106 cycles; (b) enlarged window of ICA area; (c) 3D morphology image; (d) the sectional profile of the labeled line in (c); Sur: surface, FiE: fish eye, ICA: initiation characteristic area.

Fig. 4. (a) SEM image of a typical NIICI fracture surface of U-BAT specimen failed at σa = 686 MPa with Nf= 2.16 × 107 cycles; (b) enlarged window of ICA area; (c) 3D morphology image; (d) the sectional profile of the labeled line in (c); Sur: surface, FiE: fish eye, ICA: initiation characterisitic area.

Fig. 5. (a) Stress intensity factor range of inclusions, micro-facet and ICA area of U-BAT and E-BAT specimens, the blue dash lines indicate the mean values; (b) Schematic showing the fish eye, ICA, facet and inclusion in the fracture surface (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 6. SEM images of NIICI fracture surface (a) before and (b, c, d) after etching (E-BAT specimen failed at σa = 785 MPa with Nf= 3.78 × 106 cycles), (c) and (d) are enlarged windows of (b). PAG: prior austenite grain.

Fig. 7. SEM images of NIICI fracture surface. (a) before and (b) after etching (E-BAT specimen failed at σa = 745 MPa with Nf= 7.99 × 106 cycles). PAG: prior austenite grain, M/A: martensite/austenite.

Fig. 8. SEM images of NIICI fracture surface. (a) Before and (b) after etching (E-BAT specimen failed at σa= 706 MPa with Nf= 9.49 × 106 cycles).

Fig. 9. Microstructural characterization of fatigue crack initiation characteristic area of E-BAT specimen failed at σa = 745 MPa with Nf= 2.87 × 106 cycles. (a) SEM image with a tilted angle, the red line shows the position for preparation of FIB sample; (b) TEM image showing the overall microstructure underneath ICA; (c) TKD result showing inverse pole figure and boundary maps, the inset shows slip system trace figure and the numbers are values of maximum Schmid factor of each slip system; (d) TKD results showing local misorientation of bcc and fcc phase; (e) and (f) the bright and dark field TEM images combined with selected area diffraction rings showing fine grains; (g) TEM image and selected area diffraction patterns of RA; (h) TEM image of secondary crack. LD: loading direction, TA: transition area, BF: bainitic ferrite, RA: retained austenite (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 10. Microstructural characterization of fatigue crack initiation characteristic area of U-BAT specimen failed at σa = 745 MPa with Nf= 9.09 × 106 cycles. (a) SEM image with a tilted angle, the red line showing the location of FIB sample; (b) TEM image showing the overall microstructure underneath ICA; (c) TKD result showing inverse pole figure and boundary maps, the inset shows slip system trace figure and the numbers are values of maximum Schmid factor of each slip system; (d) TKD result showing local misorientation of bcc and fcc phase; (e) the bright-field TEM image combined with selected area diffraction rings showing fine grains; (f) enlarged TEM image showing fragmentation of bainitic ferrite laths; (g-1) bright-field TEM image of of RA and newly formed martensite; (g-2) dark- field TEM image and selected area diffraction patterns showing the newly formed martensite; (h-1) TEM image and selected area diffraction patterns of RA; (h-2) HRTEM image showing the presence of stacking faults. LD: loading direction, TA: transition area, RA: retained austenite, M: martensite (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 11. Microstructural characterization near secondary cracks marked in Fig. 10(b). (a) TEM image with the selected area diffraction patterns of RA twins; (b) TKD result showing inverse pole figure and boundary maps with slip system trace figure; (c) enlarged STEM image; (d) TKD result showing local misorientation of bcc and fcc phase.

Fig. 12. Microstructural characterization of fatigue crack initiation characteristic area of U-BAT specimen failed at σa = 680 MPa with Nf= 4.88 × 106 cycles. (a) TEM image showing the overall microstructures underneath ICA; (b) TKD result showing inverse pole figure and boundary maps, the insets show slip system trace figure and the numbers are value of maximum Schmid factor of each slip system; (c-1): enlarged STEM image showing the fine grains and voids along micro-facet; (c-2): selected area diffraction rings of fine grains; (c-3): HRTEM image showing the newly formed martensite; (d-1) enlarged STEM image showing the region without fine grain along micro-facet; (d-2): selected area diffraction patterns of bainitic ferrite and RA; (e) enlarged STEM image and selected area diffraction rings of fine grains; (f) TEM image showing the fine grains along PAG boundary; (g-1) enlarged TEM image showing the formation of fine grains; (g-2): HRTEM image showing the slip band in bainitic ferrite; (h) enlarged TEM and HRTEM images showing the multiple slip bands in RA grains. LD: loading direction, TA: transition area, RA: retained austenite, BF: bainitic ferrite, M: martensite.

Fig. 13. Schematic illustration of scenarios of mechanisms of NIICI failure. (a) Transgranular-cracking-assisted NIICI with fine grains formation; (b) transgranular-cracking-assisted NIICI without fine grain formation; (c) intergranular-cracking-assisted NIICI without fine grain formation; and (d) intergranular-cracking-assisted NIICI with fine grains formation. RA: retained austenite, BF: bainitic ferrite, PAGB: prior austenite grain boundary, FG: fine grain.