Fig. 2. Microstructural evolution of the steel processed via the CBE strategy confirming the strong effect of CBs on martensitic transformation.

(A) EBSD image quality map with superposed phase color map of the face-centered cubic (FCC) phase (red region) of the ART-processed steel, showing the equiaxed microstructure of austenite (γ) with ferrite (α), and (C) the ultrafine dual phase microstructure of γ and martensite (α′) of the CBE-processed steel. (B) Sketch of the microstructural evolution of the steels during ultrafast heating and quenching via the CBE strategy to illustrate the role of GBs, PBs, and CBs. (D and E) TEM images with EDS analysis showing the microstructure and Mn concentration profile between γ and α of the ART-processed steel, while (F and G) is that between γ and α′ of the CBE-processed steel. Near-atomic level Mn distribution is revealed by 3D-APT, (H and I) between γ and α, and (J and K) between γ and α′.