Introduction to Cementite
Iron exhibits a strong attraction to carbon in various forms, including the compound known as cementite. Equilibrium diagrams of the Iron Carbon Phase Diagram illustrate the stable phases of iron with graphite, diamond, and cementite. However, the concept of stability is relative, as there may be other Fe-C compounds that are even more stable. When iron, graphite, and cementite coexist, it will eventually yield a more stable equilibrium between graphite and iron.
Although these equilibria are metastable, the annual production of around 50 million tonnes of cementite within 1.6 billion tonnes of steel greatly enhances our quality of life. Its hardness at ambient temperature, resulting from its crystal structure with lower symmetry than other iron forms, remains significant under normal conditions.
Cementite’s Metastability and Multidisciplinary Significance
Despite its metastable nature, they are found in cooled meteorites and deep within diamonds. It potentially played a key role in the formation of carbon nanotubes and shows promise in biomedicine for targeted drug delivery. The chemical composition was established through early metallography experiments. The name “cementite” originated from the idea that it cements iron in a cellular structure.
Beyond metallurgy, these hold relevance in diverse fields such as astrophysics, planetary science, and biomedicine. Coexisting with graphite, cementite remains stable, while ferrite favors a mixture with graphite. Its stability under heat treatment and its presence in iron-rich meteorites raise intriguing questions. Its presence in deeply-mined diamonds also suggests metastability.
Cementite’s Stoichiometry
Stoichiometry is the study of the quantitative relationship between reactants and products in a chemical reaction, determining the precise ratios and amounts of substances involved.
Small deviations from stoichiometry in cementite can affect its cohesion and lattice structure. The Curie temperature (TC) of this element varies with its carbon concentration, with pressure influencing TC as well. Atom probe techniques have limitations in measuring carbon concentration, but conventional field ion microscopy revealed deviations in severely deformed cementite particles.
However, the orthorhombic crystal structure was not retained in these cases. Claims of significant deviations in carbon concentration in undeformed cementite are debated due to conflicting experimental data. Temperature quenching in cast iron may alter their stoichiometry, but the influence of other solutes is not fully understood.
Crystal Structure of Cementite
It is a compound of iron and carbon and possesses an orthorhombic crystal structure. Its unit cell comprises 12 iron atoms and 4 carbon atoms, with lattice parameters set at
a = 0.50837 nm, b = 0.67475 nm, and c = 0.45165 nm. Within the crystal structure, four iron atoms are positioned on mirror planes while the remaining eight occupy general positions.
Their structure includes various interstitial sites such as prismatic, octahedral, and tetrahedral. Prismatic interstices, located on mirror planes, are fully occupied by carbon atoms. Octahedral sites, on the other hand, remain empty unless the carbon concentration exceeds 25%. They exhibit anisotropic elastic moduli due to their orthorhombic nature. It undergoes elastic deformation, transitioning to a monoclinic structure (space group P21/c) with enhanced covalent bonding and increased shear strength.
Slip systems in them involve specific planes and directions, such as
(001)[100], (100)[010], (100)[001], (010)[001], and (010)[100]. Slip is hindered by large Burgers vectors, making them highly resistant to deformation at ambient temperatures. Planar defects, like stacking faults, can appear in cementite, while point defects manifest as vacancies or additional carbon atoms. Irradiation-induced cascades lead to vacancy formation and iron atom migration to interstitial positions.
There is a debated hexagonal form also that is less stable than the orthorhombic variety. Its structure and chemical composition are not definitively determined, with differing proposals and limited experimental evidence. Overall, their crystal structure, interstitial sites, defects, deformation behavior, and slip systems contribute to their unique properties and mechanical characteristics.
Magnetic and Thermal Properties of Cementite
Magnetic Properties
Cementite is a metallic ferromagnet at room temperature, exhibiting paramagnetic behavior beyond the Curie temperature. The Curie temperature has been reported to be around 186°C, with varying claims ranging from 180°C to 240°C. The magnetization decreases with temperature, and calculations show local magnetic moments for iron atoms within specific ranges. It undergoes a transition from ferromagnetic to nonmagnetic states under high pressure. Its magnetization is anisotropic, with specific magnetization directions. Alloying with elements like nickel and manganese affects the magnetic properties, resulting in changes in saturation magnetization and coercivity. They also demonstrate a magnetocaloric effect, and there is limited evidence of coexisting ferromagnetic and paramagnetic modifications at ambient temperature.
Thermal Properties
Polycrystalline form exhibits a change in average thermal expansion coefficient as the temperature surpasses the Curie temperature. The lattice parameters are affected by the ferromagnetic to paramagnetic transition. The transition is accompanied by a contraction in the parameter due to the dominant effect of spontaneous magnetization. The orthorhombic structure of cementite remains intact throughout the transition. Experimental data and calculations demonstrate the temperature-dependent behavior of the lattice parameters, while pressure influences these parameters in a predictable manner.
Surface Energy and Elastic Properties of Cementite
The surface energy of cementite, which contributes to its fracture behavior, exhibits anisotropy. Experimental findings suggest that it cleaves on certain crystal planes, such as {101}, (001), and {102}, despite inconsistencies in surface energy values. Additional localized plasticity may be involved in this process, even though it is brittle at room temperature. Calculated surface energy data indicate that the (001) plane has the highest surface energy compared to {010} and {100}. The elastic properties of cementite, determined through first-principles calculations and experiments, reveal anisotropy. Certain calculations indicate negative values for the C44 modulus at 0 K, implying mechanical instability, although they remain stable in practice.
Other calculations show positive values, suggesting sensitivity to atomic positions and potential changes at finite temperatures. Experimental measurements of elastic moduli in single-crystal form demonstrate lower values compared to calculated ones. Polycrystalline cementite’s bulk modulus is influenced by pressure, affecting its behavior at the Earth’s core. The elastic properties of polycrystalline form can be estimated using single-crystal data, providing values for Young’s modulus and shear modulus. The presence of alloying elements can further influence these properties.
Electrical Conductivity of Cementite
Electrical conductivity information was obtained from calculations and various measurements on cementite. The significant disparity between calculated and measured values can be attributed to the presence of defects in real materials, which tend to reduce conductivity. The included data for pure cementite in its polycrystalline state and measurements from different studies show variations in conductivity, and it remains unclear why certain measurements indicate lower conductivity. However, the observed increase in electrical resistance with temperature confirms that cementite exhibits metallic conductivity rather than behaving as a semiconductor.
Advantages of Cementite
- High hardness
- High melting point
- Chemical stability
- High thermal conductivity
Disadvantages of Cementite
- Brittle nature
- Lack of ductility
- Poor electrical conductivity
Applications of Cementite
- Steel strengthening
- Metal alloys
- Carbides and ceramics
- Scientific research
Conclusion
In conclusion, cementite demonstrates a complex crystal structure and exhibits unique electrical and thermal characteristics. With high surface energy, it tends to cleave along specific crystal planes. Although it is brittle and lacks ductility, it offers advantages such as high hardness, a high melting point, and good thermal conductivity. Despite its poor electrical conductivity, it remains stable even at low temperatures and displays ferromagnetic properties. Consideration of these properties is essential for the appropriate utilization of them in industries that prioritize hardness, stability, and thermal conductivity while acknowledging their brittleness and limited ductility.
