Fig. 1. EBSD inverse-pole-figure (IPF) microstructure maps exhibit the morphology of grains in the ultrafine-grained (UFG) steel, and the inset showing (001), (101), and (111) orientations in red, blue, and green, respectively (a). SEM picture showing numerous cementite nanoparticles coexisted with ultrafine grains (b), the corresponding statistical results presenting that the average grain size is ~ 500 ± 30 nm (c) with a mean size of 80 ± 10 nm for nanoparticles (d). Orientation distribution functions (ODFs, φ2 = 45° sections) showing the crystallographic textures for as-prepared steel (e).

Fig. 2. Engineering stress-strain (a, b, c) and true strain-stress curves (a1, b1, c1) taken at various strain rates of 0.17 s-1, 0.017 s-1, and 0.0017 s-1 under 25 °C, 150 °C, 300 °C, 450 °C, and 600 °C for the UFG ferritic steel (D6AC).

Fig. 3. EBSD inverse pole figure (IPF) microstructure maps revealing the morphology of grains in the UFG ferritic steel at 0.17 s-1 (a), 0.017 s-1 (b), and 0.0017 s-1 (c) at 600 °C, which had an average grain size of ~500 ± 30 nm, coexisting with nanosized Fe3C particles (d = 80 ± 10 nm). (d) The variation of grain sizes and volume fraction of high-angle grainboundary (FHAGB) of the as-prepared UFG ferritic steels as a function of strain rate tested at 600 °C.

Fig. 4. TEM micrographs showing the morphologies of the nanosized Fe3C particles in the UFG steel after deformation at different strain rates and temperatures: (a) 0.17 s-1 @300 °C; (b) 0.0017 s-1 @300 °C; (c) close observation reveals the dissolution of the nanosized Fe3C particles during deformation at 300 °C, supported by the white circles marked by green arrows; (d) 0.17 s-1 @600 °C;(e) 0.0017 s-1 @600 °C; (f) the corresponding diffractogram of fine particle (yellow arrow) indicates the particle as VC and close observation shows the wavy band (purple arrows) and the dissolution of the nanosized Fe3C particles (marked by green arrows) at 600 °C; (g) close observation exhibits the slip step and the dissolution thickness of the nanosized Fe3C particles (yellow dotted line) at 600 °C. Representative 3D Atom probe tomography (APT) analysis nanosized Fe3C particles delineated with a 5 at.% C iso-concentration surface (brown): (h) 0.17 s-1 @300 °C; (i) 0.0017 s-1 @300° C; (j) 0.17 s-1 @600 °C;and (k) 0.0017 s-1 @600 °C. Figures are cropped to 80 nm × 80 nm × 300 nm, 60 nm × 60 nm × 360 nm, 80 nm × 80 nm × 320 nm, and 60 nm × 60 nm × 600 nm volumes for 4 specimens. The calculated volume fractions of Fe3C particles can be seen in Table 2.

Fig. 5. SEM micrographs showing the morphologies of fracture surfaces the UFG ferritic steel (D6AC) tested at 600 °C with strain rates of 0.17 s-1 (a, b) and 0.0017 s-1 (c, d).

Fig. 6. Dimple size, λ, on the fracture surface of the deformed UFG ferritic steel (D6AC) (D = ~500 nm, dFe3C = 80 ± 10 nm) as a function of temperature and strain rate (a) and elongation and together with dimple area fraction at 600 °C (b).

Fig. 7. Mechanical properties of the as-prepared UFG ferritic steel (D6AC) tested at 600 °C, compared with martensitic (M) steel [20], oxide-dispersion-strengthened (ODS) steel [24,54,55], low-activation martensitic (LAM) steel [25], reduced activation ferritic-martensitic (RAFM) steel [26,56], ferritic stainless (FS) steel [57], and cold-formed (CF) steel [58].

Fig. 8. TEM image showing the traces of grain boundary sliding of the UFG ferritic steel deformed at 600 °C and 0.0017 s-1, and a number of fine VC particles (circled) and large Fe3C particles occurring in the matrix and grain boundaries. Double yellow arrows indicate the shifting distance of grain boundaries while double red arrows indicate the partial dissolution of Fe3C.

Fig. 9. Schematic illustration showing the fundamental fracture mechanism of the UFG ferritic steel (D6AC) (D = ~500 nm, dFe3C = 80 ± 10 nm) tested at different temperatures and strain rates.