A transition of ω-Fe3C→ω′-Fe3C→θ′-Fe3C in Fe-C martensite

Carbon steel is strong primarily because of carbides with the most well-known one being theta-Fe3C type cementite. However, the formation mechanism of cementite remains unclear. In this study, a new metastable carbide formation mechanism was proposed as omega-Fe3C ->omega '-Fe3C ->theta '-Fe3C based on the transmission electron microscopy (TEM) observation. Results shown that in quenched high-carbon binary alloys, hexagonal omega-Fe3C fine particles are distributed in the martensite twinning boundary alone, while two metastable carbides (omega ' and theta ') coexist in the quenched pearlite. These two carbides both possess orthorhombic crystal structure with different lattice parameters (a(theta ') = a(omega ') = a(omega)=root 2 alpha(alpha-Fe)=4.033 angstrom, b(theta ') = 2 x b(omega ') = 2 x c(omega)=root 3 alpha(alpha-Fe)=4.94 angstrom, and c(theta ') = c(omega ') = root 3a(omega)=6.986 angstrom for a(alpha-Fe)=2.852 angstrom). The theta ' unit cell can be constructed simply by merging two omega ' unit cells together along its b(omega)' axis. Thus, the theta ' unit cell contains 12 Fe atoms and 4C atoms, which in turn matches the composition and atomic number of the theta-Fe3C cementite unit cell. The proposed theory in combination with experimental results gives a new insight into the carbide formation mechanism in Fe-C martensite.