Vanadium carbide

H. K. D. H. Bhadeshia

Here we examine the microstructural characteristics and chemical composition of vanadium carbides found in steel. While these particles were historically identified as cubic V₄C₃, modern diffraction analysis reveals they are actually monoclinic V₆C₅ with specific vacancy patterns.

These carbides typically manifest as fine, square platelets that significantly improve steel performance by providing strength and preventing hydrogen embrittlement. Specialised steels may contain V₈C₇ or mixed titanium–vanadium carbides.

Advanced scientific imaging, including transmission electron microscopy and atom-probe analysis, is used to visualise these structures and confirm the absence of iron within the particles.

Audio podcast

Vanadium carbides form as fine, square-shaped platelets that precipitate on the {100}_α planes (Figure 1). The shape at the early stages of precipitation in austenite is octahedral with facets parallel to {111}_γ [Yazawa et al., 2004].

The fine particles that form in ferrite or tempered martensite are associated with coherency strain fields that not only harden the steel but can trap hydrogen, thereby mitigating embrittlement due to the ingress of diffusible hydrogen into the metal. It is diffusible hydrogen that is responsible for the embrittlement of steel [Johnson, 1875; Bhadeshia, 2016].

vanadium carbide platelets — **Figure 1:** Three crystallographic variants of vanadium carbide forming on the cube planes of the ferrite. The plates have square shapes. After Yamasaki and Bhadeshia [2006]. Fine plates of vanadium carbide (about 20 nm length, square shaped) in ferrite. Two variants look like needles normal to each other, the third lies in the plane of the thin foil.

The vanadium carbide that precipitates in steel was thought originally to be V₄C₃ [Baker, 1959] with space group Fm3m and lattice parameter 0.4157 nm. However, a detailed examination of electron diffraction patterns has demonstrated that it is V₆C₅ with a monoclinic structure containing ordered vacancies in the carbon sublattice (Figure 2) [Billingham et al., 1972; Epicier et al., 2008]. The space group is C2/m, with lattice parameters a = 0.509 nm, b = 0.882 nm, c = 1.018 nm, β = 109.47°.

a_{V_{6} C_{5}} = \sqrt{\frac{3}{2}} a_{V_{4} C_{3}} b_{V_{6} C_{5}} = \frac{3}{\sqrt{2}} a_{V_{4} C_{3}} c_{V_{6} C_{5}} = \sqrt{6} a_{V_{4} C_{3}}

Given that the literature is full of interpretations based on V₄C₃, the following conversions apply, noting that the basis symbols 'M' and 'C' refer to the monoclinic and cubic forms respectively, and that [M; u] = (M J C)[C; u] and (h; M^*) = (h; C^*)(C J M):

\begin{array}{l} (MJC) = [\begin{matrix} 0.500365 & 0.500365 & - 0.500365 \\ - 0.333430 & 0.333430 & 0.000000 \\ 0.250182 & 0.250182 & 0.250182 \end{matrix}] \\ (CJM) = [\begin{matrix} 0.499635 & - 1.499563 & 0.999271 \\ 0.499635 & 1.499563 & 0.999271 \\ - 0.999271 & 0.000000 & 1.998542 \end{matrix}] \end{array}

(001)_V₆C₅ || (111)_V₄C₃

[2 0 1]_V₆C₅ || [001]_V₄C₃

(010)_V₆C₅ || (1 1 0)_V₄C₃

[0.500365 0.333430 0.250182]_V₆C₅ || [010]_V₄C₃

(100)_V₆C₅ || (1 1 1)_V₄C₃

[0.500365 0.333430 0.250182]_V₆C₅ || [100]_V₄C₃

Therefore, the classical Baker-Nutting orientation relationship between vanadium carbide and ferrite:

⟨001⟩_V₄C₃ || ⟨001⟩_α, {110}_V₄C₃ || {010}_α
becomes: ⟨2 0 1⟩_V₆C₅ || ⟨001⟩_α, {132}_V₆C₅ || {010}_α

Vanadium carbide at the early stages of precipitation in austenite has the following orientation relationship:

⟨001⟩_V₄C₃ || ⟨001⟩_γ, {010}_V₄C₃ || {010}_γ
becomes: ⟨2 0 1⟩_V₆C₅ || ⟨001⟩_γ, {132}_V₆C₅ || {010}_γ

Monoclinic unit cell of V6C5 — **Figure 2:** (a) Monoclinic unit cell of V₆C₅. The unique axis b is normal to a and c. (b) Projection of the cell on the {001} plane. The carbon atoms are octahedrally coordinated by vanadium atoms.

Another vanadium carbide V₈C₇ has a cubic crystal structure (P4₃32, a = 0.832 nm) when the carbon vacancies are ordered. The order-disorder temperature is about 1125 °C. V₈C₇ occurs only in vanadium-rich steels such as those used in the manufacture of dies [Qi et al., 2018].

Transmission Electron Microscopy

The following micrographs are taken from thin-foil samples studied using a transmission electron microscope.

Titanium-vanadium carbide in steel

A. R. Waugh

An imaging atom-probe study of a mixed carbide particle in a steel containing titanium and vanadium. Bob wanted to confirm that there is a disturbance in the trajectories of the carbon at the edges of the particle, since the carbon distribution goes beyond the boundaries of the carbide particle. The possibility of such an aberration had been suggested to him by P. Turner.

Part 1: Short-Answer Quiz

Instructions: Answer the following questions using 2–3 sentences based on the provided research.

What is the characteristic shape and orientation of vanadium carbide precipitates when they form in ferrite?
How do fine vanadium carbide particles mitigate the effects of hydrogen embrittlement in steel?
What was the historically accepted stoichiometry for vanadium carbide precipitates, and why has this been corrected?
Describe the structural characteristics and space group of the V₆C₅ phase.
What are the specific lattice parameters for the monoclinic V₆C₅ crystal structure?
How is the classical Baker–Nutting orientation relationship updated when transitioning from the V₄C₃ model to the V₆C₅ model?
What are the structural properties and specific applications of the V₈C₇ carbide?
What occurs to the structure of V₈C₇ at its order–disorder temperature?
What observation was made regarding carbon distribution in the imaging atom-probe study of titanium–vanadium carbide?
Briefly define the crystallographic properties of Tungsten Carbide (WC).

Part 2: Quiz Answer Key

Vanadium carbides form as fine, square-shaped platelets. These platelets precipitate specifically on the {100} α planes of the ferrite matrix.
The particles are associated with coherency strain fields that trap diffusible hydrogen, which is the species responsible for embrittlement. By trapping this hydrogen, the particles prevent it from causing damage while simultaneously hardening the steel.
Originally, the precipitates were thought to be V₄C₃ with a cubic structure (Fm $\bar{3}$ m). Detailed examination reveals the structure is actually V₆C₅ with a monoclinic structure containing ordered vacancies in the carbon sublattice.
V₆C₅ possesses a monoclinic structure belonging to the space group C2/m. It is characterised by ordered vacancies within the carbon sublattice and contains alternating planes of carbon atoms with and without these vacancies.
The lattice parameters for the monoclinic form are a = 0.509 nm, b = 0.882 nm, and c = 1.018 nm. The angle β is measured at 109.47°.
The updated relationship for V₆C₅ is defined as ⟨201⟩ V₆C₅ || ⟨001⟩ α and {132} V₆C₅ || {010} α.
V₈C₇ has a cubic crystal structure (P4₃32) with a lattice parameter of a = 0.832 nm when carbon vacancies are ordered. It typically occurs in vanadium-rich steels used for manufacturing dies.
The order–disorder temperature is approximately 1125 °C. At this temperature, the vacancies become disordered, and the resulting cubic form has roughly half the lattice parameter of the ordered state.
The study suggested a disturbance in the trajectories of carbon at the edges of the carbide particle, evidenced by carbon distribution extending beyond the physical boundaries of the particle.
Tungsten carbide (WC) has a space group of P $\bar{6}$ m2 with lattice parameters a = 0.292 nm and c = 0.284 nm, and an angle γ = 120°.

Part 4: Glossary of Key Terms

Austenite (γ): A high-temperature phase of iron; vanadium carbide precipitates here as octahedral shapes with {111} γ facets.
Baker–Nutting Relationship: The classical orientation relationship describing how the crystal lattice of a precipitate aligns with the lattice of the ferrite matrix.
Coherency Strain Field: A region of lattice distortion around a precipitate that remains continuous with the matrix; it contributes to hardening and can trap hydrogen.
Diffusible Hydrogen: Hydrogen atoms capable of moving through the metal lattice; this form is responsible for the embrittlement of steel.
Ferrite (α): The body-centred cubic phase of iron where vanadium carbide precipitates as fine, square-shaped platelets on the cube planes.
Monoclinic Structure: The crystal system of V₆C₅, characterised by three unequal axes and one non-right angle (β = 109.47°).
Ordered Vacancies: A phenomenon in the carbon sublattice of V₆C₅ and V₈C₇ where certain carbon atom sites are consistently empty.