In the pursuit of enhancing the strength and toughness of nodular cast iron, controlling the pearlite content and refining its lamellar spacing is a critical pathway. Cementite (Fe3C), as a primary constituent of pearlite, directly influences the microstructure through its nucleation and growth mechanisms. Previous studies have shown that elements like Cu and Sb can segregate at Fe3C interfaces, reducing interfacial energy and leading to pearlite refinement, thereby improving mechanical properties. However, the role of silicon carbide (SiC) particles in this context remains underexplored, particularly regarding its potential as a heterogeneous nucleation site for Fe3C. In this work, we systematically investigate the regulatory mechanism of SiC pretreatment on the microstructure and properties of nodular cast iron, combining first-principles calculations with experimental validation to reveal the underlying mechanisms. Our focus is on how SiC particles promote the heterogeneous nucleation of Fe3C in pearlite, ultimately contributing to the high-strengthening and toughening of as-cast nodular cast iron.
The significance of nodular cast iron in industrial applications cannot be overstated, as it offers a unique combination of ductility and strength due to its spherical graphite morphology. The matrix structure, often comprising ferrite and pearlite, plays a pivotal role in determining mechanical performance. Pearlite, consisting of alternating layers of ferrite and cementite, can be optimized through microstructural refinement. Herein, we address a research gap by examining SiC particles as a pretreatment agent, which has been used in melting processes since the 1980s but without a clear understanding of its role in pearlite refinement. We hypothesize that SiC acts as an effective heterogeneous nucleation substrate for Fe3C, reducing the nucleation barrier and leading to finer pearlite lamellae. This study aims to provide theoretical insights and practical optimization strategies for advancing nodular cast iron technology.
To elucidate the atomic-scale interactions, we employed density functional theory (DFT) calculations using the Vienna Ab initio Simulation Package (VASP). The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) was used for exchange-correlation functional, along with the projector augmented-wave (PAW) method. Convergence tests ensured accuracy with energy change rates below $$1 \times 10^{-5} \, \text{eV/atom}$$, atomic forces less than $$0.3 \, \text{eV/nm}$$, and displacements under $$1 \times 10^{-4} \, \text{nm}$$. For bulk properties, the plane-wave energy cutoff (Ecut) was set to 360 eV, and k-point meshes were $$8 \times 8 \times 8$$. Surface and interface calculations used similar parameters, with k-meshes adjusted to $$8 \times 8 \times 1$$. These settings guaranteed reliable results for analyzing the electronic structure, surface energies, and interfacial properties of SiC and Fe3C.
The computational models were constructed based on optimized crystal structures. Fe3C has an orthorhombic lattice (space group Pnma) with lattice parameters: $$a = 0.504 \, \text{nm}$$, $$b = 0.67 \, \text{nm}$$, and $$c = 0.448 \, \text{nm}$$. SiC adopts a face-centered cubic structure (space group Fm-3m) with $$a = b = c = 0.437 \, \text{nm}$$. We analyzed electronic structures through band diagrams and density of states (DOS). For SiC, the band gap near the Fermi level confirms its semiconductor nature, with significant p-p orbital hybridization indicating strong covalent bonding. In contrast, Fe3C exhibits metallic characteristics, as evidenced by bands crossing the Fermi level, and its DOS reveals contributions from metal, covalent, and ionic bonds. These properties are crucial for understanding interfacial behavior in nodular cast iron systems.
To assess the heterogeneous nucleation potential, we calculated the two-dimensional lattice mismatch between SiC and Fe3C using Bramfitt’s theory. The mismatch parameter $$\delta$$ is given by:
$$\delta = \sum_{i=1}^{3} \frac{|d_{[uvw]_s} \cos \theta – d_{[uvw]_n}|}{d_{[uvw]_n}} \times 100\%$$
where $$(hkl)_s$$ and $$(hkl)_n$$ are the crystallographic planes of the substrate and nucleus, respectively; $$d_{[uvw]_s}$$ and $$d_{[uvw]_n}$$ are atomic spacings along specific directions; and $$\theta$$ is the angle between these directions. A mismatch below 6% indicates efficient nucleation. Our calculations focused on the Fe3C (100) and SiC (110) interface, as summarized in the table below.
| Matching Interface | [uvw]SiC | [uvw]Fe3C | θ (°) | dSiC (nm) | dFe3C (nm) | δ (%) |
|---|---|---|---|---|---|---|
| Fe3C (100) // SiC (110) | [1 -1 0] | [0 1 0] | 0 | 0.618 | 0.670 | 5.77 |
| Fe3C (100) // SiC (110) | [1 -1 -1] | [1 1 0] | 1.50 | 0.757 | 0.806 | – |
| Fe3C (100) // SiC (110) | [0 0 1] | [0 0 1] | 0 | 0.437 | 0.448 | – |
The key result is a mismatch of only 5.77% for the Fe3C (100)/SiC (110) interface, satisfying the condition for effective heterogeneous nucleation. This low mismatch suggests that SiC can serve as an ideal substrate for Fe3C formation in nodular cast iron, potentially refining pearlite microstructure.
Surface energy calculations were performed to ensure model convergence. For SiC (110), two stacking configurations (Case 1 and Case 2) were considered, with surface energy $$\sigma_{\text{SiC}(110)}$$ calculated using the Botteger equation:
$$\sigma_{\text{SiC}(110)} = \frac{1}{A} (E_{\text{slab}}^N – N \Delta E)$$
where $$\Delta E = (E_{\text{slab}}^N – E_{\text{slab}}^{N-2})/2$$, $$E_{\text{slab}}^N$$ is the total energy of an N-layer slab, and A is the surface area. Convergence was achieved at 11 layers, with surface energies stabilizing at 2.97 J/m² for both cases. For Fe3C (100), three termination models (Fe-terminated, C-terminated, and Fe-Fe-terminated) were analyzed. The surface energy $$\sigma_{\text{Fe}_3\text{C}(100)}$$ is given by:
$$\sigma_{\text{Fe}_3\text{C}(100)} = \frac{1}{A} (E_{\text{slab}} – N_{\text{Fe}} \mu_{\text{Fe}} – N_{\text{C}} \mu_{\text{C}})$$
where $$\mu_{\text{Fe}}$$ and $$\mu_{\text{C}}$$ are the chemical potentials of Fe and C, respectively, set to 8.3 eV and 9.09 eV under equilibrium conditions. Convergence tests yielded the following results:
| Model | Layers | Surface Energy (J/m²) | Convergence Layer |
|---|---|---|---|
| Fe-terminated | 6–30 | 0.165 (at 12 layers) | 12 |
| C-terminated | 8–32 | 0.172 (at 14 layers) | 14 |
| Fe-Fe-terminated | 7–31 | 0.176 (at 13 layers) | 13 |
These converged models were used to construct interface structures for further analysis, ensuring reliability in subsequent calculations for nodular cast iron applications.
Interface properties were evaluated through adhesion work and interfacial energy. Six interface models were built based on different stacking modes of SiC (110) and Fe3C (100) surfaces. The adhesion work $$W_{\text{ad}}$$, representing the reversible work to separate the interface into two free surfaces, is calculated as:
$$W_{\text{ad}} = \frac{E_{\text{Fe}_3\text{C}} + E_{\text{SiC}} – E_{\text{Fe}_3\text{C}/\text{SiC}}}{S}$$
where $$E_{\text{Fe}_3\text{C}}$$ and $$E_{\text{SiC}}$$ are the total energies of the isolated surface models, $$E_{\text{Fe}_3\text{C}/\text{SiC}}$$ is the interface energy, and S is the interface area. The interfacial energy $$\gamma$$, indicating stability, is given by:
$$\gamma = \sigma_{\text{Fe}_3\text{C}} + \sigma_{\text{SiC}} – W_{\text{ad}}$$
Lower $$\gamma$$ values denote more stable interfaces. The results for the Fe3C (100)/SiC (110) interfaces are summarized below.
| Interface Model | Adhesion Work $$W_{\text{ad}}$$ (J/m²) | Interfacial Energy $$\gamma$$ (J/m²) |
|---|---|---|
| Case 1/Fe | 1.842 | 1.304 |
| Case 1/C | 1.808 | 1.338 |
| Case 1/Fe-Fe | 0.510 | 2.632 |
| Case 2/Fe | 0.507 | 2.628 |
| Case 2/C | 0.513 | 2.629 |
| Case 2/Fe-Fe | 0.520 | 2.615 |
The Case 1/Fe interface exhibits the highest adhesion work (1.842 J/m²) and lowest interfacial energy (1.304 J/m²), indicating strong bonding and stability. This supports the hypothesis that SiC can effectively promote Fe3C nucleation in nodular cast iron, leading to refined pearlite structures. The atomic-scale insights from these calculations provide a foundation for experimental validation in nodular cast iron systems.
To verify the theoretical predictions, we designed experiments with nodular cast iron samples treated with varying SiC content. The base iron composition was 3.8% C, 2.2% Si, and 0.4% Mn (by weight). Melting was conducted in a 50 kg medium-frequency induction furnace, with FeSi75 (75 wt.% Si) used as an inoculant at 1.3 wt.%, and FeSiMg6RE1.8 (6% Mg, 1.8% RE, 43% Si) as a nodularizing agent at 1.4 wt.%. SiC particles (50 nm diameter) were added at the end of melting, followed by mechanical stirring at 60 rpm for 10 seconds to ensure dispersion. The melt temperature was maintained above 1450°C to prevent oxidation or decomposition of SiC. Casting was performed using a sand-coated iron mold to produce Y-shaped blocks, which were machined into tensile specimens according to GB/T 1348-2009. Four experimental groups with SiC additions of 0, 0.05, 0.1, and 0.15 wt.% were prepared, as outlined in the table below.
| Group | SiC Addition (wt.%) | Description |
|---|---|---|
| Control | 0 | Untreated nodular cast iron |
| 1 | 0.05 | Low SiC content |
| 2 | 0.10 | Optimal SiC content |
| 3 | 0.15 | High SiC content |
Microstructural analysis was conducted using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Graphite nodularity, distribution density, and pearlite lamellar spacing were measured according to standards. Tensile tests were performed on a universal testing machine to evaluate mechanical properties. The goal was to correlate SiC content with microstructural evolution and performance in nodular cast iron.
The effect of SiC pretreatment on graphite morphology in nodular cast iron is crucial, as spherical graphite contributes to ductility. Using image analysis software, we quantified nodularity, distribution density, and nodule grade. The results are presented in the following table.
| SiC Addition (wt.%) | Nodularity (%) | Graphite Distribution Density (nodules/mm²) | Nodule Grade |
|---|---|---|---|
| 0 | 84 | 328 | Grade 3 |
| 0.05 | 91 | 434 | Grade 2 |
| 0.10 | 93 | 472 | Grade 2 |
| 0.15 | 88 | 512 | Grade 3 |
With SiC addition up to 0.1 wt.%, nodularity improved from 84% to 93%, and distribution density increased, indicating enhanced graphite nucleation. This is attributed to SiO2 formation from SiC decomposition, which combines with SiC to form effective nucleation substrates for graphite, reducing the nucleation energy barrier. Additionally, free carbon released from SiC increases local carbon concentration, promoting carbon diffusion and maintaining spherical graphite growth. However, at 0.15 wt.% SiC, nodularity decreased to 88% despite higher distribution density, with some graphite appearing as exploded or elliptical shapes. This suggests excessive SiC may disrupt graphite morphology in nodular cast iron, emphasizing the need for optimal content.
Pearlite lamellar spacing was analyzed using SEM after etching with 4% nital. The images revealed that SiC addition refined pearlite structure, with the smallest spacing observed at 0.1 wt.% SiC. Quantitative measurements show a clear trend: as SiC content increases, spacing decreases initially but slightly expands at higher contents. This refinement is linked to SiC particles acting as heterogeneous nucleation sites for Fe3C, increasing nucleation density and forcing finer alternating layers with ferrite. To confirm SiC incorporation, XRD analysis was performed on samples with 0.1, 0.5, and 1 wt.% SiC. Peaks corresponding to SiC were detected in higher-content samples, proving successful introduction without complete decomposition. The presence of undissolved SiC in nodular cast iron matrices supports its role as a nucleation catalyst.
Mechanistic analysis via SEM-EDS identified Si-rich regions near Fe3C areas, suggesting SiC particle presence. XRD patterns for 0.1 wt.% SiC-treated nodular cast iron showed weak SiC peaks at 35.8°, 60.23°, and 71.96°, alongside strong matrix peaks. This confirms that SiC particles survive the melting process and integrate into the nodular cast iron microstructure. The combination of SiC decomposition and undissolved particles creates a dual effect: Si release promotes ferrite formation and solid-solution strengthening, while SiC cores facilitate Fe3C nucleation. The equation for carbon diffusion enhancement can be expressed as:
$$J = -D \frac{\partial C}{\partial x}$$
where J is the diffusion flux, D is the diffusion coefficient, and $$\partial C / \partial x$$ is the carbon concentration gradient. SiC addition locally increases carbon concentration, accelerating diffusion and supporting pearlite refinement in nodular cast iron.
The mechanical properties of SiC-pretreated nodular cast iron were evaluated through tensile tests. The results demonstrate a significant improvement with optimal SiC content, as summarized below.
| SiC Addition (wt.%) | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|
| 0 | ~780 | ~3.5 |
| 0.05 | ~820 | ~4.2 |
| 0.10 | 846 | 4.7 |
| 0.15 | ~830 | ~4.0 |
At 0.1 wt.% SiC, tensile strength reached 846 MPa with 4.7% elongation, representing the best performance. This enhancement stems from microstructural optimization: higher nodularity improves ductility, while refined pearlite lamellae and solid-solution strengthening boost strength. The relationship between strength and microstructure can be approximated by the Hall-Petch equation for pearlite refinement:
$$\sigma_y = \sigma_0 + k \lambda^{-1/2}$$
where $$\sigma_y$$ is the yield strength, $$\sigma_0$$ is the friction stress, k is a constant, and $$\lambda$$ is the pearlite lamellar spacing. Smaller $$\lambda$$ due to SiC-induced nucleation increases $$\sigma_y$$, contributing to the overall strength of nodular cast iron. However, at 0.15 wt.% SiC, properties decline slightly due to graphite deterioration and coarser pearlite, highlighting the importance of controlled addition for nodular cast iron processing.
In discussion, we synthesize computational and experimental findings to explain the role of SiC in nodular cast iron. The first-principles calculations reveal that SiC (110) and Fe3C (100) interfaces have a low lattice mismatch (5.77%), high adhesion work (1.842 J/m²), and low interfacial energy (1.304 J/m²). This confirms SiC as an effective heterogeneous nucleation substrate for Fe3C, reducing the energy barrier for pearlite formation. Experimentally, SiC addition up to 0.1 wt.% optimizes graphite morphology and refines pearlite lamellae, leading to superior mechanical properties. The mechanism involves two synergistic effects: (1) SiC decomposition releases Si, which promotes ferrite and inhibits pearlite, while also providing solid-solution strengthening; and (2) undissolved SiC particles act as nucleation sites for Fe3C, increasing nucleation density and refining pearlite structure in nodular cast iron. This dual action enhances both strength and ductility, offering a novel approach for high-performance nodular cast iron production.
Furthermore, the impact of SiC on carbon diffusion cannot be overlooked. SiC addition elevates local carbon activity, expressed as:
$$a_C = \gamma_C \cdot C_C$$
where $$a_C$$ is carbon activity, $$\gamma_C$$ is the activity coefficient, and $$C_C$$ is carbon concentration. Increased activity accelerates carbon redistribution during solidification, favoring spherical graphite growth and fine pearlite formation. This aligns with observations in nodular cast iron, where SiC pretreatment improves nodularity and matrix homogeneity. The interplay between SiC particles and the iron-carbon system underscores the complexity of microstructural engineering in nodular cast iron.
In conclusion, this study comprehensively investigates the mechanism of SiC particles promoting heterogeneous nucleation of Fe3C in nodular cast iron. Through first-principles calculations, we established that SiC (110) and Fe3C (100) interfaces exhibit a lattice mismatch of 5.77%, with high adhesion work and low interfacial energy, validating SiC as an efficient nucleation substrate. Experimental results demonstrate that adding 0.1 wt.% SiC significantly optimizes the microstructure of nodular cast iron, achieving 93% nodularity, minimized pearlite lamellar spacing, and enhanced mechanical properties—tensile strength of 846 MPa and elongation of 4.7%. The refinement mechanism combines heterogeneous nucleation of Fe3C on SiC particles and carbon diffusion promotion, leading to synergistic strengthening and toughening. These findings provide a theoretical foundation and practical guidelines for optimizing SiC pretreatment in nodular cast iron production, paving the way for advanced applications requiring high strength and ductility. Future work should explore long-term stability and industrial scalability to fully harness the potential of SiC in nodular cast iron technology.
The implications extend beyond laboratory settings, as nodular cast iron is widely used in automotive, machinery, and construction industries. By refining pearlite through SiC addition, manufacturers can achieve better performance without complex heat treatments, reducing costs and energy consumption. This research highlights the importance of interfacial engineering in metallurgy, where nanoscale phenomena dictate macroscopic properties. As we continue to explore novel additives and processes, nodular cast iron will remain a cornerstone material, with SiC pretreatment offering a promising avenue for innovation. Ultimately, this work contributes to the broader goal of developing sustainable and high-performance materials for engineering applications.