Abstract
High resolution electron microscopy and selected area electron diffraction have been applied to investigate an unstable ordered phase formed in iron-carbon martensite tempered around 70°C. This phase can be identified with Izotov-Utevskiy’s Fe4C, but different from their observation of superstructure spots, the present observation has revealed that they are splitted in two along the [001]α′ direction in the reciprocal lattice of martensite α′. Electron diffraction structure analysis shows that the ordered phase takes an orthorhombic lattice with parameters a=b=√2a0 and c=12c0 (a0 and c0 are the lattice parameters of martensite), in which interstital carbon atoms or vacancies make a long period of ordered arrangement. The space group is Pmna. It has also been shown that the carbon content of this phase is not much different from that of matrix martensite. The ordered phase takes a needle-like shape with a size of about (1.5)2×10 nm3 and are randomly distributed in the matrix. The crystal structure and the related diffraction phenomena are discussed.
