Enhanced Strength and Microstructure of AA7075 Matrix Composites Reinforced with SiC and TiC Particles

Stir casting is a prevalent and economical liquid-phase method for producing metal matrix composites (MMCs). AA7075 is a high-strength aluminum alloy widely used in aerospace, automotive, and defense sectors due to its excellent strength-to-weight ratio, fatigue resistance, and machinability. However, its relatively low wear resistance and limited thermal stability restrict its performance in demanding applications. To overcome these shortcomings, the alloy is often reinforced with ceramic particles to form metal matrix composites (MMCs) [1]. Silicon carbide (SiC) is selected as a reinforcement because of its exceptional hardness, high elastic modulus, low density, and superior wear resistance. Its incorporation is expected to improve the composite’s hardness, stiffness, and resistance to abrasion. Titanium carbide (TiC), on the other hand, offers high melting temperature, chemical stability, and strong interfacial bonding with aluminum. TiC additions are particularly beneficial for enhancing load-bearing capacity, thermal stability, and resistance to deformation at elevated temperatures [2, 3].

During this procedure, ceramic reinforcing particles, typically silicon carbide (SiC) and titanium carbide (TiC), are physically agitated into the molten aluminium matrix to improve the composite's mechanical and tribological characteristics. Stir casting (liquid metallurgy) remains the most economical and scalable method for producing particulate-reinforced aluminum composites. In a typical procedure, AA7075 is melted (660-780℃), ceramic powders are preheated (300-500℃) to remove moisture and improve wettability, and then introduced into a vortex generated by a graphite or ceramic impeller (200-600 rpm, 10-15 min). Inert gas degassing or fluxes are applied to minimize porosity before pouring into preheated molds [4].

Prabu et al. [5] investigated the influence of stirring speed and preheating temperature on particle dispersion in Al-SiC composites, concluding that preheating reinforcements to 400-500℃ and stirring at 500 rpm for 10 minutes yielded optimal results. These insights also apply to AA7075-SiC/TiC systems. Silicon carbide (SiC) is widely used due to its high hardness, thermal stability, and good interface bonding with aluminum alloys. Rajan et al. [6] investigated the mechanical behavior of AA7075 reinforced with varying percentages of SiC using the stir casting process. They reported significant improvements in hardness, tensile strength, and wear resistance, particularly at 5-10 wt% SiC. Ficici [7] fabricated AA7075-SiC composites (SiC 5-20 wt%) via stir casting and reported a nearly uniform particle distribution. Enhanced hardness and wear resistance were observed, with optimal performance at 10 wt% SiC. SR and Suresh [8] produced AA7075-5 wt% SiC by stir casting. SEM showed homogeneous SiC scattering, and the composite exhibited 15% higher tensile strength and ~30% higher hardness than the unreinforced alloy.

A recent work [9] on AA7075 + (2.5, 5 wt%) TiC reported that 5 wt% TiC yielded a 25% increase in hardness and 30% reduction in wear rate, with a minor effect on corrosion resistance (polarization current 8%). Incorporating TiC particles into the AA7075 matrix transforms the microstructure from a dendritic to a more equiaxed morphology. This transformation is attributed to TiC particles acting as potent nucleation sites during solidification, leading to finer grains and enhanced mechanical properties. Similarly, adding SiC particles results in a uniform dispersion within the aluminum matrix, contributing to grain refinement. The extent of refinement depends on the particle size and weight percentage of SiC used. A modified stir casting study produced AA7075 + 5 wt% SiC + 5 wt% TiC plates. Compared to base AA7075 (UTS = 116 MPa, BHN = 95), the hybrid composite achieved UTS = 155 MPa (+34%) and BHN = 229 (+141%). The addition of SiC/TiC also increased yield strength and moderately decreased toughness [10]. A two-step stir casting route varied SiC/TiC total volume up to 15 vol%. Tensile strength increased linearly up to ~10 vol% reinforcing particles before plateauing, attributed to particle clustering at higher loadings [11].

Combining SiC and TiC as hybrid reinforcements in AA7075 has been explored to leverage the benefits of both particles. Such hybrid composites exhibit improved mechanical properties due to the combined effects of grain refinement, enhanced load transfer, and increased resistance to dislocation movement. However, it's important to note that while strength and hardness are improved, there may be a trade-off with ductility and impact resistance, necessitating a balanced approach in reinforcement selection and processing parameters. Nano-SiC (0.3 vol%) by Stir-Squeeze Casting & Cold Forging (2023) AA7075 + 0.3 vol% nano-SiC exhibited a refined, equiaxed grain structure with SiC well-dispersed at grain boundaries. Ballistic impact strength improved from 5 J to 7.7 J, linked to crack-arresting by SiC networks [12]. Microstructure and Thermal Properties (2023) AA7075 + 5-15 wt% SiC, produced by semi-solid stir casting, exhibited a homogeneous dispersion of SiC particles, 20-30 µm in size, and a 16.5% increase in thermal conductivity. Grain size reduced by 25% compared to the base alloy [13]. The ternary hybrid MMC (AA7075 + SiC + TiC), as fabricated by Khan et al. [14], consists of AA7075 reinforced with 3 wt% SiC and 2 wt% TiC via conventional stir casting. Fine, equiaxed grains; SiC at grain boundaries; TiC finely dispersed in matrix, and UTS increased by 34% (to 155 MPa), hardness doubled (225 BHN), but elongation dropped 15% due to increased particle-matrix interfaces. Reinforced AA7075 Composites summarizes the mechanical, tribological, and corrosion behaviour across Al-7075 MMCs with various ceramic particulates. It highlights that stir-casting remains the most industrially viable route but requires tight control of degassing and fluxing to avoid porosity and particle clustering [15]. In another reported study on Al7075 reinforced via stir casting with SiC, TiC, TiB₂, and others [16]. It identifies key microstructural features, including grain refinement through particle-induced nucleation, modification of intermetallic precipitates, and agglomeration limits, that govern the improvements in hardness and strength [17]. By combining SiC and TiC, the hybrid reinforcement approach leverages the stiffening and wear resistance imparted by SiC along with the grain refinement and interfacial stability introduced by TiC.

The hybrid use of SiC and TiC in AA7075 is intended to combine the advantages of both reinforcements. While SiC contributes to improved hardness and wear resistance, TiC strengthens interfacial bonding and enhances thermal and mechanical stability. Together, these reinforcements are expected to produce a composite with superior mechanical strength, wear resistance, and durability compared to AA7075 reinforced with a single ceramic phase.

The available data consists of limited reports on hybrid particle systems in aluminum alloys; systematic studies on AA7075 reinforced simultaneously with SiC and TiC remain limited. Most of the available work addresses either SiC or TiC alone, or hybrid combinations with other ceramics (TiB₂, Gr, B₄C), but fails to comprehensively evaluate how the complementary strengthening mechanisms of SiC and TiC interact when used together in AA7075. Furthermore, the correlation between microstructural refinement, uniform dispersion, porosity reduction, and their direct impact on tensile strength, hardness, and fracture behaviour has not been thoroughly established for such hybrid systems.

This work introduces a hybrid reinforcement strategy (SiC + TiC, 2-6 wt%) in AA7075, utilizing the stir-casting method, which consciously combines the high hardness and wear resistance of SiC with the grain-refining and strong interfacial bonding capability of TiC. Unlike prior single-particle studies, our approach ensures homogeneous dispersion, minimized porosity, and Zener pinning-induced grain refinement, resulting in a marked improvement in ultimate tensile strength and hardness, with acceptable trade-offs in ductility.

In this study, the specific objectives are explicitly defined as follows:

•Fabricate AA7075-(SiC+TiC) hybrid composites at 0, 2, 4, and 6 wt% using a controlled stir-casting route and standardized specimen preparation.

•Establish phase constitution and confirm particle retention using X-ray diffraction across all reinforcement levels.

•Quantify microstructural evolution (grain size, particle dispersion, and porosity) using optical microscopy and SEM/EDS, and relate these to reinforcement fraction.

•Tensile strength of the fabricated parts, behavior, and map their dependence on reinforcement content.

•Correlate microstructure-property relationships to identify the dominant strengthening contributions.

•Characterize fracture mechanisms via SEM fractography to track the ductile-to-brittle transition with increasing ceramic content. 

Figure 2(a) shows a coarse-grained structure typical of an unreinforced aluminum alloy. Large dendritic α-Al grains are visible. The absence of any particulate reinforcement leads to a relatively unrefined microstructure and no observable secondary phase. Figure 2(b) shows that the Introduction of 2 wt% SiC and TiC results in grain refinement. Small, dark-phase particles (SiC and TiC) are beginning to appear along grain boundaries. Improved grain boundary definition suggests partial pinning by reinforcement particles. The composite is still relatively coarse compared to higher reinforcement cases. Further grain refinement is visible in Figure 2(c) (AA7075 + 4% SiC and TiC). Reinforcement particles are more densely and uniformly distributed across the matrix. The boundaries are more refined and well-defined, indicating improved interfacial bonding and Zener pinning. Fewer signs of agglomeration, suggesting good dispersion at this reinforcement level. Figure 2(d) (AA7075 + 6% SiC and TiC) shows the finest microstructure among all samples. Dense dispersion of SiC and TiC particles effectively impedes grain growth during sintering. Particles are uniformly distributed with minimal porosity. Likely to correspond to the maximum mechanical performance, as also seen in related test results.

Figure 2. Microstructure of composite (a) AA7075 Pure, (b) AA7075 with 2% SiC and TiC, (c) AA7075 with 4% SiC and TiC, and (d) AA7075 with 6% SiC and TiC

Figures 3(a)-(d) provide microstructural and elemental distribution insights into how hybrid reinforcement (SiC + TiC) affects the morphology and composition of AA7075 alloy. In all the images, (a) shows the SEM structure and (b) shows the EDS analysis. The SEM image, as shown in Figure (a), displays a relatively smooth surface with large, uniform aluminum grains lacking any second-phase particles or reinforcements. EDS spectrum is expected to show high peaks of aluminum, with minor signals of Mg, Zn, or Cu (inherent alloying elements of AA7075). The smooth matrix confirms the absence of ceramic particles, and the microstructure is dominated by α-Al grains, with possibly a few intermetallics present. In Figure 3-sample 2(a), SEM reveals the initial distribution of dark contrast particles, like SiC and TiC. These particles mostly appear along grain boundaries. In Figure 3, sample 2(b), EDS analysis would show characteristic elemental peaks for Si, Ti, and C, in addition to Al, Mg, and Zn. SiC and TiC particles begin to play their role as nucleation centers during sintering, initiating Zener pinning. In Figure 3, sample 3(a), with 4 wt% SiC and TiC, significantly refines the matrix, creating a denser microstructure. Darker particles are well-embedded and appear in both intra- and inter-granular regions. EDS reveals stronger signals of Si and Ti, suggesting higher reinforcement density. In Figure 3, sample 4(a), AA7075 + 6 wt% SiC & TiC, SEM shows a dense network of reinforcement particles, uniformly distributed throughout the matrix, with a well-refined structure. EDS mapping should show pronounced peaks for Si and Ti, along with Al and matrix elements.

(a) Sample -1

(b) Sample -2

(c) Sample -3

(d) Sample -4

Figure 3. As-fabricated SEM/EDS analysis (a) Sample-1 AA7075 pure, (b) Sample -2 AA7075+ 2% SiC and TiC, (c) Sample -3 AA7075 + 4% SiC and TiC, and (d) Sample -4 AA7075+ 6% SiC and TiC

Figure 4 presents the X-ray diffraction (XRD) patterns of (a) pure AA7075 and AA7075 reinforced with (b) 2 wt%, (c) 4 wt%, and (d) 6 wt% hybrid SiC + TiC combinations. All four patterns, Aluminium Matrix (α-Al), exhibit the characteristic FCC aluminium diffraction peaks at roughly 2θ = 38.4° (111), 44.7° (200), 65.1° (220), and 78.2° (311), confirming retention of the primary AA7075 phase without transformation. In reinforced Silicon Carbide (SiC) samples, weak peaks at 35.6° and 60.0° correspond to the (111) and (220) planes of cubic SiC, indicating successful incorporation of SiC particles without decomposition. The addition of Titanium Carbide (TiC) low-intensity peaks around 41.8° and 73.7° matches the (111) and (220) reflections of TiC, indicating that TiC remains stable and well-distributed during sintering. The relative intensity of the α-Al peaks gradually decreases from (a) to (d) as reinforcement content increases. As SiC + TiC content rises, the corresponding carbide peaks become more pronounced, most notably in the 6 wt% sample, confirming higher phase volume and good particle retention without significant dissolution.

Figure 4. XRD of as-fabricated samples

Hybrid (SiC + TiC) reinforced composites exhibit higher hardness than the unreinforced matrix, and this hardness generally increases with the reinforcement weight fraction. Prior investigations [18] have demonstrated that introducing hard particles enhances matrix hardness and resistance to plastic deformation by hindering dislocation motion [19]. In our results, the overall gain from 0 to 6 wt.% is modest (~ +9 %) because many micro-indentations still probe the aluminum matrix predominantly; additionally, very low additions (≈ 2 wt.%) can introduce slight inhomogeneity that temporarily depresses the measured hardness.

As shown in Figure 5(a), hardness increases steadily up to a total (SiC + TiC) content of 6 wt.%. At higher particle number densities, reduced inter-particle spacing and occasional clustering can lessen the effective contribution of the SiC and TiC to the measured hardness [20]. Overall, a 6 wt.% hybrid reinforcement yields hardness and corresponding tensile strength superior to the unreinforced alloy. Aluminium is an ideal option for aviation and automotive applications due to its lower density than other materials. The diminished strength and stiffness of aluminium alloys limit their potential uses. The tensile strength parameters of Al7075 hybrid metal matrix composites enhanced with SiC and TiC are shown in Figure 5(b). Hybrid composites made with Al7075 exhibit a tensile strength significantly higher than that of the base metal. The primary factor contributing to the subsequent strengthening effect in the composites is the incorporation of rigid reinforcing particles. The uniform distribution of ceramic particles, such as SiC and TiC, within the aluminum alloy matrix effectively hinders dislocation motion, thereby enhancing the material's strength and reducing its susceptibility to fracture.

The use of ceramic particles significantly improves the tensile and fracture strength of the composite material. In this manner, stress is transmitted from the ductile aluminium matrix to the brittle reinforcing particles. Murali et al. [21] elucidated this phenomenon as the Orowan mechanism, which enables a dislocation to circumvent substantial obstacles when constrained by a particle. This graph is central to understanding mechanical performance. Figure 5(a) shows that pure AA7075 (Base Metal) has a tensile strength of 95 MPa, whereas with reinforcement (2%, 4%, and 6%), the tensile strength is 121 MPa, 159 MPa, and 194 MPa, respectively. Sample 4 had a maximum strength of 194 MPa, reflecting an increase of approximately 30 MPa. The tensile performance improves significantly with increasing hybrid reinforcement, reaching a peak at 6 wt% SiC + TiC. The sharp edges of the hard ceramic particles in samples 2 and 3 provide a nucleation site, resulting in the observed decrease in strength of these samples. Figure 5(c) shows that the failure tensile samples, where fracture will inevitably occur due to the concentration of stress at the nucleation point [22, 23].

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Figure 5. (a) Tensile strength of as-fabricated samples, (b) Tensile specimens before testing, (c) Tensile samples post-fracture

The SEM fractographs illustrate the tensile failure behaviour of stir-casted AA7075 composites with increasing SiC and TiC reinforcement. Figure 6(a), the unreinforced alloy, shows large, deep dimples characteristic of ductile fracture via microvoid coalescence. In Figure 6(b) with 2 wt% reinforcement, large dimples and emerging cleavage facets suggest a mixed ductile-brittle mode. Figure 6(c), containing 4 wt%, displays grain refinement and particle-induced microcracks, with evidence of particle pull-out and fracture initiating at reinforcement-matrix interfaces. Figure 6(d), with 6 wt% reinforcement, exhibits dominant brittle fracture features such as flat facets and minimal dimpling, indicating crack propagation through a rigid ceramic network. The progression from ductile to brittle fracture reflects the increasing constraint on plastic deformation and stress concentration due to ceramic particles.

Figure 6. SEM fractography of tensile failure of stir-cast AA7075 samples