My research focuses on exploring novel pathways to enhance the properties of as-cast spheroidal graphite cast iron, a material widely valued for its excellent castability and balanced mechanical properties. Traditionally, improvements in strength and ductility of as-cast grades are sought by alloying with elements like Cu, Ni, or Mo and by carefully balancing the ferrite-to-pearlite ratio. However, these methods often increase cost or lead to a trade-off where gains in ductility come at the expense of strength. This investigation examines an alternative approach: the exogenous addition of ceramic particles. Specifically, I studied the influence of titanium carbide (TiC) particles on the graphite morphology, matrix structure, and resulting mechanical properties of spheroidal graphite cast iron. The hypothesis was that well-dispersed, stable TiC particles could act as potent heterogeneous nucleation sites and modify the solidification and solid-state transformation processes, leading to a refined and improved microstructure without the traditional trade-offs.
The core material in this study was a standard spheroidal graphite cast iron melt. To introduce the TiC phase, commercially pure TiC powder with an initial particle size of 3-5 μm was used. A critical challenge in exogenous particle addition to liquid metals is particle agglomeration and poor wettability, leading to inhomogeneous distribution and defects. To overcome this, I employed a high-energy ball milling process to modify the TiC powder. The TiC powder was milled with iron powder for 24 hours. This process successfully coated and embedded the fine TiC particles within larger iron powder agglomerates, as confirmed by energy-dispersive X-ray spectroscopy (EDS) mapping which showed Ti and C signals within the iron particles. This modification aimed to improve particle density to reduce flotation and enhance compatibility with the iron melt.
Two heats of spheroidal graphite cast iron were prepared in a medium-frequency induction furnace. The base iron was melted from high-purity pig iron with ferromanganese and ferrosilicon additions. For the treated sample, a small amount (targeting 0.1 wt.%) of the ball-milled TiC-Fe powder mixture was placed in the pouring ladle beneath the inoculant but above the spheroidizing agent (a rare-earth magnesium alloy). A conventional sandwich treatment (cover method) was then performed. Both the reference (untreated) and the TiC-added melts were poured into resin-bonded sand molds to produce Y-block castings. Chemical composition analysis confirmed similar base chemistry for both heats, with the TiC-added sample showing a slight increase in titanium content, corresponding to an estimated 0.02 wt.% TiC incorporation with an absorption efficiency of approximately 20%.
Microstructural characterization was performed using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with EDS. Graphite parameters such as nodule count, size, and spheroidization rate were quantified according to standard methods using image analysis software. The volume fraction of ferrite and pearlite was also measured. The ferrite grain size was determined using the linear intercept method. Mechanical properties were assessed via tensile testing at room temperature and Charpy V-notch impact testing at both room temperature and -20°C.
Influence of TiC Particles on Microstructure
Graphite Morphology
The most immediate and pronounced effect of adding the modified TiC particles was a significant refinement and improvement of the graphite phase in the spheroidal graphite cast iron. A comparative analysis is summarized in the table below.
| Sample | Spheroidization Rate (%) | Graphite Nodule Count (N/mm²) | Graphite Area Fraction (%) | Average Nodule Diameter (μm) |
|---|---|---|---|---|
| Reference | 70.2 | 42.5 | 9.30 | ~35 |
| TiC-Added | 79.3 | 59.7 | 11.39 | ~28 |
The data shows a 40.5% increase in nodule count and a 9.1% improvement in spheroidization rate for the TiC-added spheroidal graphite cast iron. The graphite area fraction also increased. This can be explained by the theory of heterogeneous nucleation. For effective nucleation, the crystallographic mismatch (lattice disregistry) between the nucleant substrate and the nucleating phase should be minimal. The Bramfitt two-dimensional lattice disregistry model provides a framework for this, calculated as:
$$
\delta^{(hkl)_n}_{(hkl)_s} = \sum_{i=1}^{3} \frac{|d_{[uvw]_s}^i \cos \theta – d_{[uvw]_n}^i|}{d_{[uvw]_n}^i} \times \frac{100}{3}
$$
Where \((hkl)_s\) and \((hkl)_n\) are low-index planes of the substrate and nucleus, \([uvw]_s\) and \([uvw]_n\) are low-index directions in those planes, \(d\) are the interatomic spacings along these directions, and \(\theta\) is the angle between the directions. A disregistry value (\(\delta\)) below approximately 12% indicates good potency for nucleation. The (111) plane of TiC and the (0001) basal plane of graphite have a calculated disregistry of about 8.33%, confirming that TiC can act as an effective heterogeneous nucleation site for graphite. Therefore, the introduced TiC particles provided additional nucleation sites during the eutectic solidification of the spheroidal graphite cast iron, leading to a higher number of graphite nodules. The increased nucleation rate also reduces the growth time and undercooling for each graphite sphere, resulting in smaller, more uniform nodules and a lower probability of graphite malformation, hence the higher spheroidization rate.
Matrix Structure
The modification of the graphite structure indirectly influenced the matrix. The increased nodule count in the TiC-added spheroidal graphite cast iron shortened the diffusion path for carbon atoms during the subsequent solid-state, austenite-to-ferrite transformation. This enhanced the growth kinetics of the ferrite halo (bull’s-eye ferrite) around each graphite nodule. Consequently, the final ferrite content in the matrix increased from 61.5% in the reference sample to 66.0% in the TiC-added sample.
More directly, the TiC particles incorporated into the austenite matrix during solidification profoundly affected the pearlitic transformation and ferrite grain boundaries. SEM-EDS analysis confirmed the presence of TiC particles, typically 1-5 μm in size, within the matrix. Their behavior can be categorized into two primary effects:
1. Modification of Pearlite Morphology: TiC particles were frequently observed at the boundaries of pearlite colonies. In some instances, clusters of TiC particles appeared to delineate a region within the matrix, inside which the cementite morphology was altered from typical lamellae to a broken, short-rod, or even granular form. This unique “pearlite colony” structure can be rationalized by the role of TiC as a preferential site for cementite nucleation during the eutectoid reaction. The disregistry between the (001) plane of TiC and the (001) plane of cementite (Fe$_3$C) is favorable (~9.54%). Cementite nucleating on TiC particles may grow with different orientations, disrupting the coordinated lamellar growth with ferrite and leading to a degenerate pearlite structure. The localized heat release from the transformation around clustered particles might further alter the growth kinetics, promoting spheroidization.
2. Refinement of Ferrite Grains: TiC particles located at ferrite grain boundaries acted as pinning agents, hindering grain boundary migration during the austenite-to-ferrite transformation. This Zener pinning effect resulted in a measurable refinement of the ferrite grain structure. Ferrite grain size measurements yielded a grain size number of 8.17 for the reference spheroidal graphite cast iron and 8.62 for the TiC-added material. According to the ASTM grain size relationship, this corresponds to a significant increase in the number of grains per unit area. Grain refinement is a well-established strengthening and toughening mechanism, often described by the Hall-Petch relationship for yield strength:
$$
\sigma_y = \sigma_0 + k_y d^{-1/2}
$$
where \(\sigma_y\) is the yield stress, \(\sigma_0\) is the friction stress, \(k_y\) is the strengthening coefficient, and \(d\) is the average grain diameter. While primarily for strength, finer grains also improve toughness by impeding crack propagation.
Influence of TiC Particles on Mechanical Properties
The microstructural modifications induced by the TiC particles led to substantial changes in the mechanical properties of the spheroidal graphite cast iron, particularly its ductility and toughness.
Tensile and Plastic Deformation Behavior
The room-temperature tensile properties are summarized below:
| Sample | Yield Strength (MPa) | Tensile Strength (MPa) | Total Elongation (%) |
|---|---|---|---|
| Reference | ~310 | ~480 | 14.1 |
| TiC-Added | ~305 | ~475 | 16.8 |
The key finding is the 19.1% increase in total elongation, achieved without a detrimental loss in yield or tensile strength. Fractographic analysis of the tensile specimens provided insights. The fracture surface of the reference spheroidal graphite cast iron showed a mix of dimples around graphite nodules and areas of quasi-cleavage with river patterns. The TiC-added specimen exhibited a fracture surface with more numerous, finer, and deeper dimples, indicating a greater volume of material undergoing plastic deformation. The cleavage areas were reduced and were often associated with larger, irregular particles (likely undissolved Fe-TiC agglomerates). The enhanced ductility is attributed to the synergistic effect of multiple factors: the refined and more spherical graphite nodules better distribute stress concentration; the refined ferrite grains allow for more uniform plastic deformation across more grains; and the modified, less lamellar pearlite structure presents less resistance to dislocation movement, facilitating slip.
Impact Toughness
The effect on toughness, both at room and sub-zero temperatures, was even more pronounced, as shown in the following table.
| Sample | Impact Toughness at 25°C (J/cm²) | Impact Toughness at -20°C (J/cm²) |
|---|---|---|
| Reference | 8.36 | 7.65 |
| TiC-Added | 10.96 | 9.32 |
The TiC-added spheroidal graphite cast iron exhibited a 31.1% improvement in room-temperature impact toughness and a 21.8% improvement at -20°C. The fracture surfaces of the impact samples showed a trend similar to but more severe than the tensile samples, with a higher proportion of cleavage in all cases due to the higher strain rate and lower temperature. However, the TiC-added specimens consistently displayed a larger area fraction of dimpled rupture. The mechanisms behind the improved toughness are multifaceted. The primary role is played by the refined graphite structure, which reduces the effective size of pre-existing defects (graphite nodules) from which cracks can initiate. Furthermore, the finer ferrite grain size increases the grain boundary area, which acts as a barrier to propagating cleavage cracks, forcing them to change direction and absorb more energy. The TiC particles themselves, when well-bonded and finely distributed, can also contribute to toughness by promoting the formation of fine micro-voids around them during plastic deformation, a process that blunts advancing cracks and leads to a higher energy-absorbing, ductile fracture mode.
Conclusions
My investigation into the exogenous addition of modified TiC particles to as-cast spheroidal graphite cast iron demonstrates a viable and effective method for microstructural refinement and mechanical property enhancement. The key findings are:
- Graphite Refinement: TiC particles act as potent heterogeneous nucleation sites for graphite due to a low crystallographic disregistry. This results in a significant increase in graphite nodule count (40.5%), a reduction in nodule size, and an improvement in spheroidization rate (9.1%).
- Matrix Modification: The increased nodule count indirectly raises the ferrite content. More directly, TiC particles within the matrix pin ferrite grain boundaries, leading to grain refinement, and can alter the morphology of surrounding pearlite, promoting degenerate or spheroidized cementite formations.
- Mechanical Property Enhancement: The combined microstructural effects lead to a substantial improvement in ductility and toughness without compromising strength. The total elongation increased by 19.1%, room-temperature impact toughness by 31.1%, and low-temperature (-20°C) impact toughness by 21.8%.
This work confirms that the strategic incorporation of ceramic particles like TiC offers a promising alternative to traditional alloying for developing high-performance grades of spheroidal graphite cast iron. The approach leverages particle-induced refinement across multiple scales—from the graphite phase to the ferrite grains—to achieve a superior combination of properties. Future work could optimize the particle size, distribution, and volume fraction to target specific property profiles for demanding applications of spheroidal graphite cast iron.