A transition of ω-FeC → ω'-FeC → θ'-FeC in Fe-C martensite.

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