Scheme 1. (a) Schematic illustration of experimental jet-polishing procedure for nanoindentation experiments and (b) nanoindentation performed in the center of a austenite grain.

Fig. 1. Micrographs of three steels: (a) 2Al steel; (b) 4 Al steel; (c) 6Al steel.

Fig. 2. Measured ferrite and austenite fraction in 2Al, 4Al and 6Al steels.

Fig. 3. Engineering stress-strain curves of 2Al, 4Al and 6Al steels.

Fig. 4. Work hardening rates of 2Al, 4Al, and 6Al steels. (S1: stage 1; S2: stage 2; S3: stage 3).

Fig. 5. Transmission electron micrographs of tensile deformed region in vicinity of fracture surface: (a) 2Al steel; (b) 4Al steel; (c, d) 6Al steel with presence of martensite and twins, respectively; (e) diffraction pattern of twins occurring in (d).

Fig. 6. Representative load-displacement plots for 2Al (a), 4Al (b) and 6-Al (c) steels acquired via nanoindentation experiments in load-controlled mode with a Berkovich indenter of radius 20 nm (maximum load of 0.5 mN) and loading rate of 2 μN s-1.

Fig. 7. Representative TEM images in different regions illustrating strain-induced martensite in plastic zone surrounding indentation after indentation experiments at a loading rate of 2 μN s-1 for 2Al steel.

Fig. 8. Representative TEM images in different regions illustrating strain-induced martensite in plastic zone surrounding indentation after indentation experiments at a loading rate of 2 μN s-1 for 4Al steel.

Fig. 9. Representative TEM images in different regions illustrating strain-induced martensite and twinning in plastic zone surrounding indentation after indentation experiments at a loading rate of 2 μN s-1 for 6Al steel.