Electron diffraction analysis of quenched Fe–C martensite

Martensite has a body-centered tetragonal (bct) structure in high carbon steels. However, body-centered cubic (bcc) {112} 〈111〉-type twins instead of bct twins always be observed as the substructure of martensite in high carbon steels. In this paper, martensitic substructure in a quenched high carbon Fe-1.4C (wt%) alloy has been investigated in detail using selected area electron diffraction (SAED) technique in a conventional transmission electron microscopy. The reciprocal lattice of martensite has been built based on the experimental SAED patterns. Two sets of diffraction spots (one face-centered cubic lattice and one hexagonal lattice) in the built reciprocal lattice suggest that two crystalline phases with bcc (or α-Fe) and hexagonal (ω-Fe) structure actually coexist in the twinned martensite. The two-phase diffraction spot patterns from the reciprocal lattice can match perfectly with the experimental results. The fact that the {0001}_ω diffraction spot at the 1/3{222}_α position and the {0002}_ω at 2/3{222}_α can support the ω-Fe existence in the twinned martensite.