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

Abstract

Carbon steel is strong primarily because of carbides with the most well-known one being θ-Fe₃C type cementite. However, the formation mechanism of cementite remains unclear. In this study, a new metastable carbide formation mechanism was proposed as ω-Fe₃C → ω'-Fe₃C → θ'-Fe₃C based on the transmission electron microscopy (TEM) observation. Results shown that in quenched high-carbon binary alloys, hexagonal ω-Fe₃C 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 = [Formula: see text]a_α-Fe = 4.033 Å, b_θ' = 2 × b_ω' = 2 × c_ω = [Formula: see text]a_α-Fe = 4.94 Å, and c_θ' = c_ω' = [Formula: see text]a_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-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.

Figure 1

Atomic structure of various carbides. (a) Unit cell of ω-Fe₃C crystal structure. (b) Two ω-Fe₃C and two ω-Fe unit cells projected along their c axes. (c) Coarsening of the four unit cells of the ω-Fe₃C and ω-Fe in (b) results in the formation of new carbide (ω′-Fe₆C) outlined by red dashed lines. (d) The ω′-Fe₆C unit cell can have an orthorhombic structure and lattice parameters (a_ω′ = 4.033 Å, b_ω′ = 2.47 Å, and c_ω′ = 6.986 Å for a_α-Fe = 2.852 Å), and C atom at (0.5 0 0). (e) Four ω-Fe₃C unit cells. (f) Coarsening of the four ω-Fe₃C unit cell in (e) results in the formation of a new carbide (ω′-Fe₆C₂ or ω′-Fe₃C) with the same crystal structure and lattice parameters as the ω′-Fe₆C. (g) The ω′-Fe₆C₂ atomic structure in one unit cell.

Figure 2

Atomic structure of various carbides. (a) Unit cell of one ω′-Fe₆C variant with one C atom at (0 0 0.5). (b) New θ′ variant (θ′-Fe₁₂C₂ or θ′-Fe₆C) formed by merging the ω′-Fe₆C variant with one C atom at (0.5 0 0) (Fig. 1(d)) and the ω′-Fe₆C variant with one C atom at (0 0 0.5) in (a) along b axis. (c) New θ′ (θ′-Fe₁₂C₃ or θ′-Fe₄C) variant. (d) New θ′ variant of θ′-Fe₁₂C₄ or θ′-Fe₃C formed by doubling the ω′-Fe₆C₂ in Fig. 1(f) along b axis. All θ′ have an orthorhombic unit cell with lattice parameters of a_θ′ = 4.033 Å, b_θ′ = 2 × 2.47 Å = 4.94 Å, and c_θ′ = 6.986 Å for a_α-Fe = 2.852 Å).

Figure 3

Electron diffraction patterns of the θ′ variants: (a) Simulated [100] zone axis pattern of the θ′-Fe₁₂C₃ carbide. (b) Experimental pattern consisting of the diffraction spots from the [011] α-Fe zone axis and [100] θ′-Fe₁₂C₃ carbide. (c) Experimental pattern consisting of three sets of diffraction spots: [011] α-Fe zone axis, [100] θ′-Fe₁₂C₃ carbide and [100] zone axis of [100] θ′-Fe₁₂C₂ carbide. (d) Simulated [100] zone axis pattern of the θ′-Fe₁₂C₂ carbide. (e) Simulated [110] zone axis pattern of the θ′-Fe₁₂C₃ carbide. (f) Experimental electron diffraction patterns consisting of the spots from the [012] α-Fe zone axis and [110] θ′-Fe₁₂C₃ carbide.

Figure 4

Simulated θ-Fe₃C electron diffraction patterns: (a) [101]θ and (b) [111]θ. The corresponding experimental patterns observed along the zone axes of (c) [011]α and (d) [113]α.

Figure 5

Schematic electron diffraction patterns between α-Fe and the (a) ω, (b) ω′, (c) θ′, and (d) the θ-Fe₃C carbides. All patterns are along the [011]_α zone axis.(e) Dark field TEM image revealing the twinned structure, which corresponds to the ω existing region. (f) TEM bright field image showing the pearlite-like structure corresponding to the existing region of the ω′, θ′, and θ fine carbide region.