Fig. 1 XRD profile a, OM image b of the water quenched Fe-1.0C alloy. Note that both lath martensite and plate martensite were induced, and the diffraction peaks can be indexed to be body-centered tetragonal (bct) martensite (α′) with a = 0.2855 nm and c = 0.2914 nm, and fcc austenite with a = 0.36 nm, respectively. See Table 1 and the text for the determination of the lattice parameters

Fig. 2 IPF image a, orientation distribution in {001} pole figure b, distribution of misorientation angles c, TEM observation d, the corresponding SAED pattern from the circled area e, f for the water quenched Fe-1.0C alloy. In b, the orientations calculated according to K-S relationship from the orientation of austenite and an arbitrarily selected V1 are indicated by red and blue dots, respectively

Fig. 3 TEM observations a, SAED patterns b, c, the calculated patterns of three zone axis with different tetragonality m d-f for the lath martensite in Fe-1.0C alloy. Such twinned structure gives rise to mirror symmetric diffraction patterns as the electron beam is parallel with the (112) plane, but including extra diffraction spots that are deviated from the n/3 of (112) due to the tetragonality

Fig. 4 TEM observations a, SAED patterns b, c, the HRTEM images d, d2 for the plate martensite in Fe-1.0C alloy. Fast Fourier transform (FFT) images (d1, d3) from the boundary region for twinned variants and {112} twin-matrix, respectively, showing great resemblance to that of the SAED pattern in b and c