Fig. 1 OM image a, the schematic illustration of the lenticular martensite b, XRD profile c, the hardness measurements d of the quenched Fe-33Ni sample. Lenticular martensite of bcc structure (aα' = 0.2864 nm) is coexisted with fcc retained austenite of fcc structure (aγ = 0.3599 nm). Dashed line in d shows the boundary between martensite and austenite

Fig. 2 Electron microscopy characterization of the quenched Fe-33Ni: the inverse pole figure image a, [001] pole figure b, bright field TEM image c, the SAED pattern from lenticular martensite d, austenite e and the indexing f, the dark filed TEM image by using the diffraction spot of V1 g, double diffraction h, V2 i. The calculated orientations of martensite from the orientation of the austenite via KS, NW and GT OR agree well with experimental ones from the [001] pole figure in b. Crystal overlapping is underpinned by g-i with V1 on the top of V2, leading to typical diffraction for twinned variant in f characterized by mirror symmetric diffraction patterns with strong double diffraction

Fig. 3 Bright field TEM image a, the experimental b1, b2 and indexed SAED patterns b3, the dark field TEM image c, the HRTEM image d of the twinned structure within lenticular martensite in quenched Fe-33Ni. High density of twinned lamellae is observed at the midrib and grows into the twinned region, having the origin related to the twinned KS variants

Fig. 4 Correlation of variant pairing and the strength of austenite at Ms point ($\sigma_{{M{\text{s}}}}^{\gamma }$) a, the variation of the strength (HV/3) as a function of the boundary spacing (tb) b for the Fe-33Ni. Bain pairs are gradually replaced by twinned pairs as $\sigma_{{M{\text{s}}}}^{\gamma }$ exceeds ~ 200–250 MPa in a wide range of Ms point (800–100 K). The linear function for Fe–C: $\sigma_{{\text{Fe - C}}}^{\gamma } { = 610 - 0}{\text{.6Ms}}$, and for Fe–Ni: ${\sigma }_{{\text{Fe - Ni}}}^{\gamma } { = 306 - 0}{\text{.23Ms}}$, was obtained by fitting the $\sigma_{{M{\text{s}}}}^{\gamma } { - X}_{{\text{c}}}$ and $\sigma_{{M{\text{s}}}}^{\gamma } { - X}_{{{\text{Ni}}}}$ from [23]. Data of Fe25Ni30Co, Fe14.5Ni2Mn0.55C, Fe27Ni10Co, Fe21.5Ni2Mn0.18C and Fe25Ni2Mn were inserted in a for comparison [24]. The data for IF steel subjected to ARB [25], HPT [26], ECAP [27] and HPMT [13] were plotted in b for comparing the strengthening mechanism