The martensitic transition pathway in steel

Abstract

• BCT twinned martensite and its tetragonality were studied in carbon steels by TEM. • A new transition pathway and its underlying mechanisms are revealed. • The FCC to BCT transition includes an inhomogeneous shear and a pure distortion. • The C ordering in the BCT reach a maximum after the single < 1 ¯ 10>{110} shear in FCC. 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 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 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 were 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.