Fig. 1. Overview figure for the twinning and transformation processes in steels and related alloys. There are two possible pathways for the phase transition from FCC to BCC: the deformation pathway, with a transition from FCC to HCP and then to BCC (the BBOC model) [17,18] and the quenching pathway, with a transition from FCC to a BCT/BCT twin and then to a BCC/BCC twin.

Fig. 2. XRD (a) and APT results (b-d) of as-quenched Fe-C specimens with different nominal bulk C contents (0.8 C, 1.0 C, and 1.4 C (wt.%)). The results show that C atoms are not uniformly distributed inside the martensite, and the average C contents of the entire APT tips are far below these nominal values. The black scale bar in (b-d) is 20 nm.

Fig. 3. TEM images of twinned martensite in the Fe-1.4C specimen. (a) SAED pattern of BCT twinned martensite along the [110] ZA. Diffraction spots from the matrix and twin are indicated by the white and red dashed lines, respectively. (b) SAED pattern of BCC twinned martensite along the [110] ZA as reported in previous studies. (c) SAED pattern of BCT twins along the [131] ZA tilted from (a). (d) SAED pattern of BCC twins along the [131] ZA as in previous studies. (e) HRTEM image of BCT twinned martensite. (f) FFT pattern of the region outlined by the dotted frame in (e). (g) FFT pattern of the region outlined by the dotted-dashed frame in (e). (h) Inverse FFT image of (e) displaying the twin lattice. The white row of lines, the black row of lines, and the white dotted-dashed line indicate the (1$\bar{1}$0)m planes, (1$\bar{1}$0)t planes, and (1$\bar{1}$2)m/t twin boundary, respectively. (Subscripts m and t indicate matrix and twin.)

Fig. 4. SAED patterns of twinned martensite for materials with different C contents and heat treatment observed in-situ inside the TEM. (a–c) SAED patterns of BCT twin in the 1.4 C, 1.0 C, and 0.8 (wt.%) C specimens at 300 K. (a, e, and f) in-situ heating TEM observation of the 1.4 C specimen from 300 K to 673 K at a heating rate of approximately 15 K/min. Images of (e and f) were observed after keeping the temperature at 423 K and 673 K for 20 min. Insets in (a, e, and f), which show the morphology of twinned martensite, are the corresponding dark-field images from the (1$\bar{1}$0) diffraction spot. The scale bar in the insets represents 100 nm. (d) SAED pattern of BCT martensite aged at room temperature for three days. (g, h, and i) calculated diffraction patterns of BCT twins with c/a ratios of 1.056, 1.046, and 1.031 without considering the double diffraction of twins.

Fig. 5. Sketches of the atomic shuffling pathways for the individual atoms, as gathered from the experiments and considerations related to the underlying shear and distortion, to better explain the atomic position shifts from FCC to BCT and BCT twin (c/a = 1.056). (a) Atomic arrangement projection of the FCC lattice (a = 3.61 ?) along the [001]FCC direction; the illustration on the right is the stereogram of one FCC crystal cell. Blue and red dots indicate the atoms in two subsequent atomic layers (A and B layers), and the red dotted-dashed line indicates the 0th (110)FCC plane. (b) Atomic arrangement projection of FCC to BCT' with a [$\bar{1}$10](110)FCC shear; the illustration shows the stereogram of the BCT' structure. (c) Projection of FCC to BCT' twin. (d) Projection of BCT’ growth. (e) Pure distortion of BCT' to BCT. (f) Inverse FFT image of an HRTEM image of the BCT twin lattice. (g) Atomic arrangement projection of the BCT' lattice as it is formed by the [$\bar{1}$10](110) shear from the hosting FCC lattice under consideration of the occupation distribution of the C atoms. The stereogram of BCT' shows that the C atoms are located at the c-octahedral site of the BCT' lattice. (h) Symmetric double shears on the (110)FCC and ($\bar{1}$10)FCC planes in the FCC lattice.

Fig. 6. MD simulation of the phase transition process for comparison with experiments. (a) Atomic arrangement projection of the BCC, ideal BCT, BCT', and FCC lattices along [110]BCC/BCT and [001]FCC. (b) MD simulation image of crystal lattices after the first relaxation step viewed along [001]FCC//[110]BCC using EAM potential. (c) MD simulation image after the second relaxation step. (d) HRTEM image of the BCT twin lattice for comparison with (c). The white scale bar in (d) represents 1 nm. (e and f) MD simulation images after two relaxation steps considering C atoms (Fe16C, 5.9 at.% (1.33 wt.%)) in octahedral interstitial sites using the MEAM potential. (Large yellow circles in (e and f) denote C atoms)