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

Abstract

A transition of ω-fe 3 c → ω′fe 3 c → θ′-fe 3 c in fe-c martensite carbon steel is strong primarily because of carbides with the most well-known one being θ-fe 3 c type cementite. However, the formation mechanism of cementite remains unclear. in this study, a new metastable carbide formation mechanism was proposed as ω-fe 3 c → ω′-fe 3 c → θ′-fe 3 c based on the transmission electron microscopy (teM) observation. Results shown that in quenched high-carbon binary alloys, hexagonal ω-fe 3 C 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 θ′ = a ω′ = a ω = 2 a αfe = 4.033 Å, b θ′ = 2 × b ω′ = 2 × c ω = 3a α-fe = 4.94 Å, and c θ′ = c ω′ = 3a ω = 6.986 Å for a α-fe = 2.852 Å). the θ′ unit cell can be constructed simply by merging two ω′ unit cells together along its b ω′ 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-fe 3 c cementite unit cell. the proposed theory in combination with experimental results gives a new insight into the carbide formation mechanism in fe-c martensite.

Formation of θ-Fe3C Cementite via θ′-Fe3C (ω-Fe3C) in Fe–C Alloys

“…Apparently, our experimental results (Figures and ) do not support the traditional formation mechanism of nucleation and grain growth; moreover, θ-Fe 3 C of several nanometer size has the structure highly dependent on its size and deviating from the ideal bulk structure. Interestingly, the morphology and size of the θ-Fe 3 C fine particles are quite similar to those of the θ′-Fe 3 C carbide particles …”

“…θ′-Fe 3 C evolved from ω-Fe 3 C, which has been proved to be a possible precursor of cementite. − As shown in Table , both the θ′-Fe 3 C and θ-Fe 3 C carbides are orthorhombic with 12Fe and 4C atoms in the unit cell and have close lattice parameters with the difference of 0.5, 0.1, and 0.2 Å for a , b , and c axes, respectively. We have reconstructed the crystal structure of θ-Fe 3 C (Figure a–e), in order to facilitate a detailed comparison with that of θ′-Fe 3 C (Figure f).…”

“…The dark-contrast lines in Figure are the carbide layers, which are composed of ultrafine carbide particles. The carbide particles can be ω′-Fe 3 C, θ′-Fe 3 C, θ-Fe 3 C, or their mixture in the quenched pearlite-like structure, while the metastable ω-Fe 3 C carbide can only be seen in the twin-boundary region of the twinned martensite structure. − …”

“…Several kinds of metastable carbide structures (ω′-Fe 3 C, θ′-Fe 3 C, and the corresponding variants) have been explored in the quenched Fe–C pearlite structure. These new carbides have been analyzed to be strongly related to the twin-boundary ω-Fe 3 C phase, and a transition of ω-Fe 3 C → ω′-Fe 3 C → θ′-Fe 3 C has been proposed recently; the θ′-Fe 3 C carbide was found to have an orthorhombic structure with lattice parameters of a = 4.033 Å, b = 4.94 Å, and c = 6.986 Å. − Some structural parameters of θ′-Fe 3 C and θ-Fe 3 C carbides are listed in Table for comparison. One can find that both θ′-Fe 3 C and θ-Fe 3 C have the same contents of Fe and C in their unit cells, which are orthorhombic with quite similar lattice parameters.…”

HRTEM Investigations on the Substructures of the Quenched Pearlite and Martensite

“…Further tempering causes the two twinned boundaries to merge completely, resulting in the disappearance of the twinned portion and leaving assembled ω-Fe 3 C grains in the original twinning boundary regions, as shown in Figure 8c. 40 The microstructural evolution depicted in Figure 8c 8d), 41 as explained by the transformation principle shown in Figure 11. The microstructure in Figure 8d corresponds to the pearlite structure, thus explaining the formation of quenched pearlite.…”