The martensitic transformation (MT) lays the foundation for microstructure and performance tailoring of many engineering materials, especially steels, which are with > 1.8 billion tons produced per year the most important material class. The atomic-scale migration path is a long-term challenge for MT during quenching in high-carbon (nitrogen) steels. Here, we provide direct evidence of ($1\overline{1}2$) body-centred tetragonal (BCT) twinned martensite in carbon steels by transmission electron microscopy (TEM) investigation, and the increase in tetragonality with the C content matches X-ray diffraction (XRD) results. The specific {1$\overline{1}$2}_BCT twin planes which are related to the elongated c axis provide essential structural details to revisit the migration path of the atoms in MT. Therefore, the face-centred cubic (FCC) to BCT twin to body-centred cubic (BCC) twin transition pathway and its underlying mechanisms are revealed through direct experimental observation and atomistic simulations. Our findings shed new light on the nature of the martensitic transition, thus providing new opportunities for the nanostructural control of metals and alloys.

马氏体转变 (MT) 为许多工程材料的微观结构和性能调整奠定了基础，尤其是钢，这些材料的年产量超过 18 亿吨，是最重要的材料类别。在高碳（氮）钢淬火过程中，原子级迁移路径是 MT 面临的长期挑战。在这里，我们提供了（$1\overline{1}2$) 通过透射电子显微镜 (TEM) 研究碳钢中的体心四方 (BCT) 孪晶马氏体，四方性随 C 含量的增加与 X 射线衍射 (XRD) 结果相匹配。具体的{1$\overline{1}$2}与拉长的c轴相关的_BCT双平面为重新审视原子在 MT 中的迁移路径提供了必要的结构细节。因此，通过直接实验观察和原子模拟揭示了面心立方 (FCC) 到 BCT 孪晶到体心立方 (BCC) 孪晶过渡途径及其潜在机制。我们的研究结果揭示了马氏体转变的性质，从而为金属和合金的纳米结构控制提供了新的机会。