CRYSTAL STRUCTURE AND MORPHOLOGY OF THE ORDERED PHASE IN IRON-CARBON MARTENSITE.

Abstract

An unstable ordered phase formed in martensite tempered around 70 degree C can be identified with Izotov-Utevskiy's Fe//4C, but different from their observation of superstructure spots. They are split in two along the left bracket 001 right bracket alpha prime direction in the reciprocal lattice of martensite alpha prime . The ordered phase takes an orthorhombic lattice with parameters a equals b does not equal square root of 2a//0 and c equals 12c//0 (a//0 and c//0 are the lattice parameters of martensite), in which interstitial carbon atoms or vacancies make a long period of ordered arrangement. The space group is Pmna. 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 multiplied by 10 nm**3 randomly distributed in the matrix. The crystal structure and related diffraction phenomena are discussed.