Mechanical and Microstructural Characterization of Hematite-Reinforced LM6 Aluminum Alloy Composites

Abstract

Metal matrix composites based on aluminum are frequently utilized in sophisticated engineering applications because of their improved mechanical performance. This study used the stir-casting method to create hematite (Fe₂O₃)-reinforced LM6 aluminum alloy composites with reinforcement ranging from 0 to 12 weight percent. At 3–6 weight percent reinforcement, microstructural examination showed uniform particle distribution and good interfacial bonding; at higher levels (9–12 weight percent), clustering and porosity were seen. While ductility declined with increasing hematite content, mechanical characteristics demonstrated a notable improvement in hardness and compressive strength, reaching maximum values at 12 weight percent reinforcement. At lesser amounts of reinforcing, heat treatment increased strength even more and partially recovered ductility. The range of 6–9 weight percent hematite was found to have the best balance between strength and ductility. These findings demonstrate the potential for enhanced structural performance of hematite-reinforced LM6 composites.

1. Introduction

A metal matrix reinforced by one or more secondary phases, such as particles, fibers, or whiskers, makes up metal matrix composites (MMCs). MMCs are attractive materials for a variety of applications because they can produce much better mechanical and functional qualities than their monolithic metallic equivalents. A metallic matrix is reinforced with secondary phases like particles, fibers, or whiskers in metal matrix composites (MMCs), an advanced class of engineering materials used in order to improve mechanical and functional properties. This configuration enhances properties such as wear resistance, specific strength, elastic modulus, corrosion resistance, and specific stiffness [1,2]. These improved properties make MMCs highly significant from technological, commercial, and scientific perspectives. MMCs can be fabricated through solid-state processing (powder metallurgy and foil diffusion bonding) and liquid-state processing (squeeze casting, stir casting, and reactive processing). MMCs are used in various industries, including automotive, aerospace, construction, and chemical sectors. Reinforcement, a crucial component of composite materials, imparts strength and stiffness to two-phase systems. By enhancing both the physical and mechanical properties, it plays a vital role in significantly extending the service life of the composite [3,4]. Hematite (Fe₂O₃), an iron oxide mineral, is the most important iron ore in the world. It is a widespread constituent of rocks and soils and has major economic significance in iron and steel production. Stir casting is an economical and straightforward liquid-state technique used for fabricating metal matrix composites (MMCs) [5].

In recent decades, aluminum-based metal matrix composites (AMMCs) have garnered considerable attention because of their exceptional blend of strength, stiffness, and wear resistance, while maintaining a low density. These attributes make AMMCs particularly well-suited for structural, aerospace, and automotive components, where high strength-to-weight ratios are essential. Among the various aluminum alloys, LM6, a high-silicon casting alloy, is notable for its excellent castability, corrosion resistance, and ability to maintain mechanical integrity under challenging conditions. Its widespread use in the marine, aerospace, and automotive sectors highlights its significance as a matrix material for developing composites. Incorporating ceramic reinforcements is an effective strategy for further enhancing the mechanical and tribological properties of LM6 alloys. Reinforcing agents such as SiC, Al₂O₃, B₄C, and TiC have been extensively studied for their ability to improve the hardness, strength, and wear resistance of composites. However, these reinforcements often present challenges related to cost, density mismatch, and interfacial bonding with the Al matrix.

Micro-sized particles are highly cost-effective owing to their affordability and ease of dispersion during manufacturing. Aluminum matrix composites enhanced with micron-scale particles hold significant potential for commercial applications, offering superior mechanical properties at relatively low production costs. Hematite (Fe₂O₃) is a widely used discontinuous dispersion, valued for its low cost, abundance, hardness compared to iron, and distinctive rust-red color [6]. As an effective load-bearing component, Fe₂O₃ reinforcement in MMCs enhances interfacial bonding, reduces porosity, increases structural homogeneity, and improves mechanical strength [7,8]. While the incorporation of hard ceramic reinforcements significantly enhances stiffness, hardness, tensile and compression strength, flexural strength, fatigue strength, and abrasive wear resistance, previous research has also shown that Al–Cu alloys exhibit accelerated age-hardening [9,10,11,12]. In this context, hematite (Fe₂O₃), a naturally occurring iron oxide, is a promising alternative owing to its high hardness, density, thermal stability, and economic availability. Its use as a reinforcement can improve the load-bearing capacity and hardness of the composite while maintaining its cost-effectiveness and environmental sustainability.

The choice of Fe₂O₃ as a low-cost reinforcement is explicitly supported by emphasizing its availability and economic benefit over popular reinforcements like SiC, Al₂O₃, and B₄C.

A comparison of the mechanical and tribological performance of composites reinforced with Fe₂O₃ and those reinforced with more costly ceramic particles is included. It is emphasized that the performance to cost ratio of Fe₂O₃-reinforced LM6 composites makes them very appealing for large-scale and cost-sensitive applications, even though some high-end reinforcements may offer superior absolute attributes. Large-scale industrial use is limited by the higher cost of reinforcements like SiC and B₄C, despite their superior hardness and wear resistance. Fe₂O₃, on the other hand, has a good cost–performance ratio, making it a good substitute for applications where cost is a major factor.

The analysis highlights several practical methods for enhancing the manufacturing efficiency of metal matrix composites (MMCs). Fabrication procedures are primarily categorized into three types: liquid-, solid-, and vapor-state processes, each selected based on the requirements of the final product. Conventional liquid and powder metallurgy processes often face challenges such as poor wetting of reinforcements, particle settling or flotation owing to density variations, and the formation of unwanted intermetallics, which limit their scalability and cost effectiveness [13]. To overcome these limitations, various MMC and AMC fabrication methods, including stir casting, liquid infiltration, chemical vapor deposition, spray deposition, additive manufacturing, and powder metallurgy, have been explored to develop more cost-effective and property-enhancing pathways [14,15]. Stir casting is the most commonly chosen method for hybrid composites because of its simplicity, suitability for large-scale production, efficient melt stirring, and ability to maintain the high superheat required for high-quality castings [16,17]. This process remains one of the most economical and scalable methods for producing particle-reinforced aluminum composites, as it ensures a uniform dispersion of the reinforcements within the matrix and good interfacial bonding when parameters such as stirring speed, temperature, and preheating are optimized. Additionally, heat treatment is crucial for tailoring the microstructure of aluminum alloys by modifying the precipitate distribution and relieving residual stresses, thereby enhancing their strength, hardness, and ductility.

By enabling efficient load transfer from the ductile matrix to the stronger reinforcement phase, the incorporation of reinforcements, typically ceramic particles or fibers, significantly enhances the strength, stiffness, wear resistance, and fatigue performance. Uniformly distributed reinforcements also improve the toughness by preventing crack propagation [18]. Consequently, aluminum metal matrix composites (AMCs) are highly valued in the automotive, aerospace, and defense industries for their superior specific strength, stiffness, wear resistance, corrosion resistance, and creep resistance [19]. Numerous studies have illustrated how the type and content of reinforcement influence the properties of composites. For instance, Kumar et al. found that an Al 6061 alloy reinforced with TiC exhibited improved tensile and wear performance, along with a lower friction coefficient at higher TiC fractions and sliding velocities [20]. Baradeswaran et al. observed that incorporating 5 weight percent graphite and varying the Al₂O₃ concentration (2–8 wt.%) significantly reduced the friction coefficient and enhanced the tensile properties of hybrid composites based on AA7075 [21].

In a related study, Kumar et al. prepared composites by casting A356 Al alloy with hematite reinforcements using copper chills to control the solidification rates and microstructure. They concluded that copper chills promote finer grains and better hematite particle distribution, boosting fatigue life by delaying crack initiation at the matrix-reinforcement interfaces [22]. Radhi et al. reported the improved wear resistance of Al/hematite composites produced through powder metallurgy, with a nearly 66% drop in wear factor as the hematite content increases to 8 wt.%. Optimal performance was observed at 6–8 wt.% that balances mechanical strength with minimal porosity [23]. Studies have been conducted on producing laminated Al/Fe₂O₃ composites fabricated via accumulative roll bonding to develop ultrafine grain structures with enhanced electrical conductivity and magnetic permeability by layering aluminum and Fe₂O₃ sheets. Through controlled process parameters, Daneshmand et al. successfully enhanced the mechanical properties, electrical conductivity, and magnetic permeability of Al/Fe₂O₃ composites through the uniform dispersion of Fe₂O₃ particles and improved wettability of Fe₂O₃ particles with the Al matrix [24]. A review of research publications revealed few investigations on the mechanical properties of aluminum reinforced with hematite. However, there is a lack of research on aluminum alloy LM6 reinforced with hematite particles. The present work aims to develop and characterize Al LM6–hematite (Fe₂O₃) composites fabricated through the stir-casting technique with varying reinforcement levels (3%, 6%, 9%, and 12% by weight).

Hematite (Fe₂O₃)-reinforced LM6 aluminum composites are systematically investigated in this work with a combined focus on post-casting heat treatment and reinforcement variation, which have not been thoroughly addressed in previous Al/Fe₂O₃ composites. The current work demonstrates a direct association between microstructural evolution, elemental distribution (by EDS), and consequent mechanical behavior under both as-cast and heat-treated circumstances, in contrast to previous research that mainly reports mechanical attributes. Additionally, the use of LM6 alloy, a cast-specific alloy with a high silicon concentration, offers new insights into load-transfer mechanisms, property optimization, and reinforcement stability by introducing unique interfacial and phase interaction characteristics with hematite.

2. Materials and Methods

2.1. Matrix Material

Aluminum LM6, a high-silicon casting alloy, is celebrated for its exceptional wear resistance, corrosion resistance, and castability. Its durability under extreme conditions and ability to produce intricate castings make it widely applicable in marine, automotive, and aerospace applications. Consequently, in this study, LM6 was employed as the matrix material, and its chemical composition is detailed in

Table 1. Chemical composition of aluminum LM6 alloy in wt.%.

Cu	Fe	Mg	Mn	Ni	Pb	Si	Al
0.1	0.6	0.1	0.5	0.1	0.1	10–13	Balance

.

2.2. Reinforcement Material

Hematite (Fe₂O₃), a naturally occurring iron oxide mineral, is characterized by its high density, reddish-brown color, and high hardness. Owing to its strength and wear resistance, it is commonly used as a primary iron ore and reinforcement element in composites. In this study, hematite (Fe₂O₃) was selected as the reinforcement, with particle sizes ranging from 100 to 140 µm.

Table 2. Chemical configuration of hematite in wt.%.

Fe2O3	FeO	SiO2	Al2O3	MnO	TiO2	ZnO	CuO	MgO	S	Al
92.6	0.44	0.28	1.35	0.01	0.07	0.01	0.01	0.11	0.001	Balance

presents the chemical composition of the hematite particles used as reinforcements in this study.

2.3. Fabrication Process

The matrix alloy, aluminum LM6, was used alongside hematite as reinforcement to produce composites through the stir-casting process. A 6 kW electric resistance furnace was employed to melt 4 kg of Al LM6 alloy ingots.

Table 3. Composition of prepared composites and their codes.

Sl. No.	Composition	Codes
1	Aluminum LM6	LM6
2	Aluminum LM6 + 3% Hematite	LM6 + 3% Fe2O3
3	Aluminum LM6 + 6% Hematite	LM6 + 6% Fe2O3
4	Aluminum LM6 + 9% Hematite	LM6 + 9% Fe2O3
5	Aluminum LM6 + 12% Hematite	LM6 + 12% Fe2O3

presents the composites created with various compositions, while Figure 1 illustrates the electric resistance furnace used for the melting process. Graphite-based crucibles were used to melt the aluminum LM6 alloy. Once the temperature was raised to 780 °C, the molten Al alloy was maintained in a semi-liquid state. Fe₂O₃ reinforcements were introduced to the molten material in two phases after preheating for 30 min at 200 °C. Following five minutes of stirring at 250–300 rpm, the heated hematite particles were gradually added to the swirling vortex. The molten metal was stirred with a mechanical stirrer rotating at 250–300 rpm to create a fine vortex. The slurry is then poured into the mold after the crucible is removed from the furnace. Figure 2 displays the prepared composite specimens before machining. Table 3 lists the composites made with various compositions and their corresponding codes.

2.4. Solution Heat Treatment

The heat treatment procedure used in this work included a solution heat treatment, air quenching, and artificial aging in order to assure reproducibility. In order to dissolve the alloying materials and homogenize the microstructure, the alloy and composite samples were first solutionized for two hours at a temperature between 500 and 540 °C. The samples were taken out of the furnace as soon as the solution treatment was completed, and they were allowed to cool in the surrounding air (a process known as “air quenching”) until they reached room temperature. The air-quenched samples were then artificially aged for two hours at 120 °C in the same furnace to improve mechanical qualities and encourage controlled precipitation.

2.5. Microstructural Analysis

The evolution of the microstructure after casting and heat treatment was analyzed using an optical microscope. Specimens for microscopic inspections were prepared using standard metallographic procedures, starting with polishing the alloy and composite samples manually using silicon carbide paper with 100–1000 grit size. Subsequently, the samples were polished to a mirror finish using a single platen polisher machine through the application of a diamond paste. The samples were etched using Keller’s reagent for enhanced imaging of the Al LM6 alloy and its composites.

2.6. Density Test

At room temperature, the cast Al LM6/hematite composite specimens and Al LM6 matrix alloy were machined to 10 mm in diameter and 10 mm in height. First, an electronic balance was used to record the weight of each specimen. Subsequently, the samples were submerged in water, and their submerged weights were recorded. The experimental density of the specimen was calculated using Archimedes’ principle [25] illustrated in Equation (1), which is based on the difference between the weights in air and water for both heat-treated and as-cast samples. Using the rule of mixtures, the theoretical density was calculated independently using Equation (2), and the two values were compared for further analysis. Three specimens were tested under each condition to assess the samples’ experimental density. The graphs were plotted using the average values. Error bars are used to show variations in the measured data, and bar charts are used to better illustrate the statistical scatter

$${Density}_{experimental}=\frac{Massofthespecimen}{Volumeofwaterdisplaced}$$

(1)

$${Density}_{theoretical}=\left(\left({\rho }_m\times V_m\right)+\left({\rho }_r\times V_r\right)\right)$$

(2)

where $\rho$ and $V$ represent density in g/cm³ and volume in cm³, while suffixes $m$ and $r$ represent matrix and reinforcement, respectively.

2.7. Hardness Test

The hardness of Al LM6 and its composite was assessed using a Brinell hardness tester. To assess the effect of heat treatment, hardness tests were conducted on both the heat-treated and as-cast composite specimens. To guarantee uniformity and dependability, hardness measurements were performed on three specimens for each circumstance. The graphical representation was based on the average hardness values. Error bars are used to display the variation in the measured values, and bar charts are used to clearly display the statistical scatter and data distribution. The testing process strictly followed ASTM E10-95 [26] guidelines. Round specimens, each with a diameter of 20 mm, were machined and polished using various grits of emery paper. The hardness test was performed using an HB500 tester, applying a 500 kg-f load, utilizing a 10 mm diameter ball indenter and a dwell time of 10 s.

2.8. Mechanical Strength Test

To investigate the effects of varying the reinforcement content in the Al LM6 alloy and the heat treatment process on the tensile and compressive strengths of the composites, both the alloy and its composites were tested in as-cast and heat-treated conditions. Three specimens were tested for each condition to guarantee reproducibility, and the graphical representation was based on the average values. Error bars in the plots show the variability in the experimental data. The tensile strength test was conducted on dog-bone-shaped specimens prepared according to the ASTM E8M-15 [27] a test standard, while the compression test followed the ASTM E9 [28] standard. Both tests were performed on a universal testing machine at a crosshead speed of 1 mm/min. Figure 3 illustrates the dimensions of the tensile specimens and Figure 4 shows the fractured tensile specimens.

3. Results and Discussions

3.1. Microstructure Analysis

Optical micrographs of the as-cast LM6 alloy and its composites are presented in Figure 5. Figure 5a shows the microstructure characterized by the presence of coarse, relatively sparse eutectic Si or intermetallic plates dispersed in a continuous Al-Si matrix. In addition, there is a large plate spacing with more open matrix regions. With 3 wt.% inclusion of hematite, the microstructure in Figure 5b indicates the presence of several eutectic or intermetallic phases with slightly reduced spacing as the hematite particles act as additional nucleation sites, beginning to refine the eutectic morphology and increasing dark-contrast regions. At 6 wt.% addition of hematite, darker areas become more continuous as shown in Figure 5c, which indicates the formation of associated intermetallics around the increased amount of hematite particles within the Al LM6 alloy. The micrograph in Figure 5d indicates that the LM6 alloy matrix is heavily filled with intermetallic precipitates, with evident shrinkage of the light matrix region. At 12 wt.% hematite addition, the microstructure (Figure 5e) is dominated by fine eutectic and/or intermetallic precipitates that are closely packed with the lowest matrix continuity.

The overall analysis of the microstructure of the as-cast LM6 alloy and its composites indicated that the addition of hematite along with the presence of eutectic and/or intermetallic precipitates could lead to an increase in the hardness and stiffness of the material from 0 to 12 wt.% inclusion of the hematite reinforcement. Such improvements can be expected between 0 and 9 wt.% because of the evident refinement in the microstructure. At 9 to 12 wt.% hematite inclusion, the presence of highly interconnected hard phases might lead to a reduction in the ductility of the composite as the dense acicular network might act as sights for crack propagation [29,30].

The basic Al LM6 alloy displays coarse, elongated, needle-like eutectic Si particles dispersed unevenly throughout the matrix in the as-cast condition (Figure 5a). The eutectic Si starts to exhibit enhanced dispersion and little fragmentation when 3 wt.% Fe₂O₃ is added (Figure 5b). A discernible refinement is shown at 6 wt.% Fe₂O₃ (Figure 5c), with smaller Si particles and closer spacing. Even though there may be some clustering, a further increase to 9 wt.% Fe₂O₃ (Figure 5d) produces finer and more evenly distributed eutectic Si. The microstructure is considerably refined at 12 wt.% Fe₂O₃ (Figure 5e), with closely spaced and densely packed eutectic Si and intermetallic phases.

Figure 6 shows elemental compositions from the EDX spectrum of intermediate LM6 + 9% Fe₂O₃ and

Table 4. Elemental compositions of LM6 + 9% Fe₂O_3.

Element	Weight %	Atomic %
AlK	74.75	75.88
SiK	24.22	23.62
FeK	1.03	0.50

shows elemental compositions of LM6 + 9% Fe₂O₃. The EDS spectrum of the chosen region verifies that the microstructure is primarily made up of Al and Si, indicating a typical composition of LM6 alloy, with weight percentages of around 74.75% Al and 24.22% Si. A reasonably homogeneous distribution of the main constituents throughout the examined region is suggested by the elemental peaks’ consistent and well-defined distribution. The homogeneity of the matrix without discernible elemental segregation is further supported by the lack of appreciable variations in peak intensity. Strong metallurgical connection between the silicon phase and the aluminum matrix, a feature of eutectic Al–Si alloys, is indicated by the presence of Si in addition to Al. The spectrum shows no signs of significant Si particle agglomeration or clustering, suggesting that the phases are widely distributed rather than clustered. The creation of iron-based intermetallic compounds is responsible for the detection of a small peak corresponding to Fe (~1.03 wt.%). When combined with oxygen, this Fe presence raises the possibility of Fe₂O₃ or Fe-rich oxide phase production. The overall homogeneity of the alloy is not greatly impacted by these phases, which are typically found in trace amounts and may exist as finely distributed inclusions within the matrix.

Figure 7 shows optical micrographs of the heat-treated LM6 alloy and its composites reinforced with varying hematite contents. Figure 7a shows the fragmented eutectic Si or intermetallic plates that are less interconnected than those in the as-cast LM6 alloy, indicating an improved ductility. Heat treatment of the LM6 composite with 3 wt.% hematite content (Figure 7b) led to the refinement of intermetallic precipitates with uniformly dispersed acicular remnants due to heat treatment. The microstructure in Figure 7c is characterized by a dense fine network with many short laths, balancing the reinforcement density with the reduced sharpness of the individual plates relative to the as-cast state. However, the microstructure depicted in Figure 7d shows acicular features that are considerably blunter than those of the as-cast counterpart. The beneficial effects of heat treatment can be observed in the micrograph in Figure 7e. Despite the high 12 wt.% hematite content, there is the presence of a highly refined grain structure with negligible network of intermetallic precipitates, which is clearly in stark contrast with the microstructure of the as-cast LM6 composite with 12 wt.% hematite. A distinct feature in all of the micrographs of the heat-treated composites, in contrast with the micrographs of the as-cast composites, is the presence of a shorter acicular and/or eutectic network, which is more rounded and uniformly dispersed within the matrix phase. This microstructure clearly indicates the spheroidization or fragmentation of eutectic and intermetallic precipitates due to solution heat treatment followed by aging. Another feature of the microstructure of the heat-treated composites is the appearance of a cleaner matrix region with minimal segregation, suggesting the dissolution of a few coarse phases into the LM6 matrix during solution heat treatment, followed by controlled re-precipitation during aging [31].

The base alloy exhibits partial spheroidization of eutectic Si in the heat-treated state (Figure 7a), although the particles are still somewhat coarse. The Si particles become more equally distributed and rounded with 3% and 6% Fe₂O₃ additions (Figure 7b,c). Enhanced spheroidization and refinement are visible at 9% Fe₂O₃ (Figure 7d), with smaller particles and better uniformity. With fine and tightly spaced spheroidized Si particles, the 12% Fe₂O₃ sample (Figure 7e) has the most refined and evenly distributed microstructure.

3.2. Density

Figure 8 shows the theoretical and experimental densities of the as-cast LM6 alloy and the composite specimens. The hematite particulates used as reinforcement had a density of 4.9 g/cm³. Owing to the higher density of the reinforcement compared to that of the matrix material, the theoretical density increased with an increase in the reinforcement content within the Al LM6 matrix.

The slight drop in the experimental density of the cast samples, as shown in Figure 8, can be attributed to the presence of a few blow holes and pores within the samples owing to gas entrapment during the casting process. The difference between the theoretical and experimental density values is often used to measure the porosity of the cast samples. The existence of a minimum difference between the theoretical and experimental densities indicates a low pore content within the castings, which in turn indicates that the castings produced are of good quality with minimal defects [32,33]. A close match between the experimental and theoretical density values reinforces the prediction of enhanced wettability of the LM6 alloy with the Fe₂O₃ particles, which leads to minimization of the possibility of gas entrapment during casting along with reduction in shrinkage cavity formation at the matrix–particle interface [34]. Generally, a large difference between the theoretical and experimental densities in cast composites arises because of the agglomeration and/or sedimentation of denser reinforcing particles [35]. A smaller deviation between the theoretical and experimental values indicates a uniform dispersion of the hematite particles within the LM6 matrix.

3.3. Hardness

Figure 9 illustrates that the hardness increases as the weight percentage of the reinforcement increases in both heat-treated and as-cast samples. The hardness values are influenced by the bonding between the matrix material and reinforcement in the metal matrix composite [36,37]. As shown in Figure 9, for both the as-cast and heat-treated samples, the hardness increased linearly from the unreinforced LM6 alloy to 12 wt.% Fe₂O₃-reinforced LM6 alloy.

The hardness of the as-cast sample increased from 70 BHN for the unreinforced LM6 alloy to 97 BHN for the LM6/12 wt.% Fe₂O₃ composite. There is an approximately 20%, 29%, 31% and 39% increase in the hardness value for composites with 3, 6, 9 and 12 wt.% Fe₂O₃ inclusion, respectively, in comparison to the hardness of the unreinforced LM6 alloy. This indicates effective load sharing between the uniformly dispersed hard and stiff hematite reinforcement and the soft LM6 alloy matrix. The linear increment in the hardness value with increasing reinforcing content suggests the absence of any saturation and/or any significant particle–particle weakening up to 12 wt.% Fe₂O₃ addition.

It is also evident from Figure 9 that the hardness of all of the heat-treated samples is higher than that of the as-cast samples. The hardness values for heat-treated samples increased from 76 BHN for the unreinforced alloy to 110 BHN for the LM6/12 wt.% Fe₂O₃ composite. This indicates that the addition of 12 wt.% Fe₂O₃ results in improving the hardness of soft LM6 alloy by nearly 45%. This implies that the chosen solutionizing followed by aging heat treatment yields a refined microstructure with optimal redistribution of the eutectic Si and intermetallic precipitates, which results in enhancing the hardness of the LM6 alloy matrix. The addition of the hard ceramic Fe₂O₃ reinforcement further enhanced the hardness of the already hardened heat-treated LM6 alloy, thus increasing the overall hardness of the heat-treated samples in comparison to the as-cast samples. In addition, as the Fe₂O₃ reinforcement increased, the difference between the hardness of the as-cast and heat-treated samples further increased owing to an improved matrix–particle interaction and more effective generation of finer precipitates within the matrix phase restrained around the particles [38]. The hard Fe₂O₃ particles collectively enhanced the resistance of the material to indentation by hindering dislocation motion, acting as effective slip barriers, and promoting grain refinement during solidification [39].

3.4. Ultimate Tensile Strength

The analysis of the tensile strength test results shown in Figure 10 illustrates a clear synergistic strengthening response of the LM6 alloy to both Fe₂O₃ addition and the post-casting heat treatment. The ultimate tensile strength (UTS) increased steadily from 175 MPa (as-cast LM6 alloy) to 238 MPa (as-cast LM6/12 wt.% Fe₂O₃ composite) and also from 210 MPa (heat treated LM6 alloy) to 300 MPa (heat treated LM6/12 wt.% Fe₂O₃ composite). This clearly indicates that increasing the Fe₂O₃ content from 0 wt.% to 12 wt.% raises UTS by approximately 36% in the as-cast condition and by nearly 43% in the heat-treated condition. This strengthening effect may be attributed to the efficient load transfer from the soft LM6 alloy matrix to the stiff Fe₂O₃ reinforcement particles that are uniformly dispersed within the matrix, resulting in a reduction in the effective cross-section available for plastic deformation [40]. Similar trends have been reported for other Fe₂O₃- and iron-oxide-reinforced aluminum alloys, where progressive particle addition increases the hardness and tensile strength owing to Orowan looping, dislocation pile-up around particles, and constrained plastic flow of the matrix [41,42]. As shown in the microstructure (Figure 5), hard Fe₂O₃ particles act as heterogeneous nucleation sites during the solidification of the castings. Thus, a refined dendritic arm spacing with more equiaxed grains within the microstructure was promoted. As such, numerous refined grains with clearly defined grain boundaries are generated, which hinder the movement of dislocations and thus allow the material to resist tensile forces [43,44]. Thus, the presence of uniformly dispersed, well-bonded, hard, and stiff Fe₂O₃ improved the resistance of the composites to localized plastic deformation and inhibited crack initiation. Previous studies on hard ceramic particle-reinforced Al alloys have reported a continuous improvement in the UTS of Al composites up to an optimum wt.% inclusion of the reinforcements when the chances of particle agglomeration and porosity generation are low [45,46,47,48].

Figure 10 clearly indicates that the heat-treated samples possess higher UTS than the as-cast samples. The increment in the UTS due to heat treatment was found to be approximately 35 MPa for the unreinforced LM6 alloy and increased to nearly 62 MPa when the hematite inclusion was at 12 wt.%, highlighting the beneficial effects of heat treatment. For the hypoeutectic LM6 alloy, solution treatment ensures the dissolution of non-equilibrium intermetallics and promotes the redistribution of eutectic Si [49,50]. Thus, the motion of the dislocations is severely hindered, which results in an increase in the UTS of the heat-treated samples [51]. The strengthening effect of heat treatment was more pronounced at higher wt.% inclusion of Fe₂O₃. This trend suggests a beneficial interaction between the hard particles and soft matrix, wherein the rigid particles constrain matrix deformation, amplifying the effects of precipitation hardening and grain refinement. Heat treatment promotes the generation of a healthy interface between the matrix and particles, leading to an improvement in the effective load-bearing area. This, in turn, results in a delay in crack initiation at the particle–matrix interface. A similar healthy synergy between hard particle reinforcement and T6 heat treatment has been reported in studies involving composites of alloy belonging to LM series [48,52,53].

3.5. Ductility

The ductility chart (Figure 11) shows a continuous decrease in elongation with increasing Fe₂O₃ content; however, the heat-treated LM6/hematite composites retained slightly higher ductility than the corresponding as-cast condition at every reinforcement level. The elongation of the as-cast samples decreased from 2.5% for 0 wt.% Fe₂O₃ inclusion to 0.5% at 12 wt.% Fe₂O₃ inclusion, which indicates that ductility of the LM6 alloy drops with increasing content of Fe₂O₃ particle addition. The brittle and non-deformable character of hematite particles, which limit the plastic flow of the ductile aluminum matrix, is one of the elements responsible for this declining tendency. As the particle content increased, the effective volume fraction of the deformable aluminum decreased, and stress concentration sites were created at the particle–matrix interfaces, accelerating the onset of cracks. In contrast, the elongation of the heat-treated samples decreased from 3.1% for the unreinforced LM6 alloy to 0.8% for the LM6/12 wt.% Fe₂O₃ composite, indicating a similar downward trend with increasing particle addition, but with consistently higher values in comparison to their as-cast counterparts. The heat treatment ensured a reduction in residual casting stresses, refined and spheroidized brittle phases in the matrix, and redistributed soluble alloying elements, which led to an overall improvement in the ductility of the LM6/Fe₂O₃ composites. For all compositions, heat treatment provides a ductility advantage of approximately 0.6 to 0.3 percentage points (for example, 2.5% vs. 3.1% at 0 wt.% Fe₂O₃ and 0.5% vs. 0.8% at 12 wt.% Fe₂O₃), which indicates that the heat-treated LM6 matrix phase can accommodate more uniform plastic strain before failure despite being constrained by rigid Fe₂O₃ particles. The steep drop in ductility with increasing reinforcement content can be attributed to (i) the presence of a higher volume fraction of hard, non-deforming Fe₂O₃ particles, (ii) an increased number of particle–matrix interfaces that act as microcrack nucleation sites, and (iii) a reduced ligament size of the ductile LM6 matrix phase between the hard Fe₂O₃ particles. A combination of all of these factors limits the uniform plastic deformation of the LM6/Fe₂O₃ composites subjected to tensile loading. The published literature on ceramic particle-reinforced Al alloys reports a similar trade-off between strength and ductility, where increasing particle content and matrix strengthening by T6 heat treatment results in an improvement in the hardness of the composite at the expense of elongation. As such, high particle loading leads to the fracture of brittle particles, resulting in particle–matrix debonding that promotes early crack initiation and propagation [41,54].

Heat treatment, to a certain extent, mitigates these factors by refining the eutectic Si phase morphology, redistributing micro-porosity, and improving the matrix–particle bonding, which together delay the coalescence of micro-voids and cracks. Hence, the heat-treated samples had a slightly better elongation percentage than their as-cast counterparts at identical Fe₂O₃ levels.

3.6. Factography of Tensile Samples

As the Fe₂O₃ content increases, the fracture morphology in the as-cast condition gradually shifts from ductile to brittle behavior. The fine, deep, and evenly distributed dimples that dominate the fracture surface in Figure 12a Al LM6 + 3% Fe₂O₃ are indicative of ductile failure by microvoid nucleation, growth, and coalescence. With little particle pull-out and few defect sites, the comparatively uniform dimple structure indicates strong wettability and bonding between the LM6 matrix and Fe₂O₃ particles. The fracture surface in Figure 12b Al LM6 + 6% Fe₂O₃ displays a mixed-mode behavior, combining brittle cleavage facets and tear ridges with ductile dimples. A decrease in plastic deformation is shown by the dimples being shallower and less consistent. Increased reinforcement content is thought to be the cause of this shift since it causes particle clustering and interfacial debonding, which serve as locations for stress concentration. In contrast to the 3% Fe₂O₃ composite, microcracks may also be seen starting around grouped particles, encouraging earlier failure. The fracture surface in Figure 12c Al LM6 + 12% Fe₂O₃ is mostly brittle, with cleavage planes, river patterns, microcracks, and significant particle agglomeration. Particle pull-out and voids are noticeable, indicating inadequate interfacial bonding and inefficient load transfer. Due to increased heterogeneity and stress localization brought on by the high reinforcing content, ductility is greatly reduced by early fracture initiation and rapid propagation.

The fracture characteristics in the heat-treated state show a general improvement in microstructural integrity and bonding because of stress reduction and microstructural refinement. In comparison to the as-cast counterpart, the fracture surface in Figure 13a Al LM6 + 3% Fe₂O₃ exhibits wider, deeper, and more equiaxed dimples, indicating improved ductility and energy absorption capability. Stronger matrix–particle connection and more even stress distribution are suggested by the better dimple shape, which lowers the chance of early failure. The fracture surface in Figure 13b Al LM6 + 6% Fe₂O₃ is still mixed ductile–brittle, but it has fewer cleavage facets and more evenly distributed dimples than the sample that was not heated. Heat treatment has enhanced particle dispersion and interfacial strength, delaying crack initiation and improving overall mechanical performance, as evidenced by the decrease in particle clustering and interfacial debonding. The fracture surface in Figure 13c Al LM6 + 12% Fe₂O₃ seems more refined and less severe than in the as-cast state, despite the fact that brittle characteristics such as cleavage facets and microcracks are still predominant. Improved particle–matrix adhesion and a discernible decrease in fracture length and propagation routes lead to a minor recovery in ductility. However, brittle fracture mechanisms continue to predominate because of excessive reinforcing and agglomeration. Because of enhanced particle contact, clustering, and stress concentration, the sequence from Figure 12a to Figure 12c amply illustrates how an increase in Fe₂O₃ content changes the fracture mechanism from ductile to brittle. As seen in Figure 13a to Figure 13c, heat treatment greatly improves interfacial bonding, decreases microstructural flaws, and boosts load transmission, resulting in higher ductility and fracture resistance, especially at lower and intermediate reinforcement levels (3–6%).

3.7. Compression Strength

The compressive strength, as shown in Figure 14, increased substantially with an increase in the Fe₂O₃ content for both the as-cast and heat-treated LM6/Fe₂O₃ composites. The heat-treated samples, however, outperformed the as-cast counterparts by possessing higher compressive strengths across all reinforcement levels. The compression strength of the as-cast samples increased from 440 MPa for the unreinforced LM6 alloy to 610 MPa for the LM6/12 wt.% Fe₂O₃ composite, marking an approximately 39% gain. Simultaneously, the compressive strength of the heat-treated samples improved from 470 MPa for the unreinforced LM6 alloy to 730 MPa for the LM6/12 wt.% Fe₂O₃ composite, which is an increment of nearly 55%. This trend highlights that the addition of hard Fe₂O₃ particles improves the strength of the LM6 alloy under compressive loads compared to tensile loads. This is due to the fact that hard Fe₂O₃ particles constrain the matrix flow and bear the external loads, yielding a 20–60% improvement in compressive strength over the unreinforced LM6 alloy.

Heat treatment also proved to be beneficial for boosting the compressive strength by 30 MPa for the unreinforced alloy to 120 MPa for the LM6/12 wt.% Fe₂O₃ composite. This improvement in compressive strength can be attributed to precipitation hardening, refinement of eutectic phases, and shrinkage of micropores after heat treatment. The largest absolute gain in compressive strength was obtained for the composite with the highest Fe₂O₃ inclusion of 12 wt.%. This reinforces the presence of synergy, where rigid particles amplify matrix strengthening by limiting plastic deformation while simultaneously enhancing the work-hardening capacity.

Thus, it is evident that a higher rigid Fe₂O₃ inclusion favors superior tolerance of the LM6/Fe₂O₃ composites to compression rather than tension, as hard, rigid particles resist buckling under external loads by generating several shear bands. Thus, LM6/Fe₂O₃ composites with 9–12 wt.% Fe₂O₃ inclusion can be employed for high-compression applications such as engine pistons and/or structural supports.

4. Conclusions

The study successfully created LM6 alloy composites reinforced with different weight percentages (3–12 wt.%) of Fe₂O₃ and methodically assessed their mechanical and microstructural characteristics using the stir-casting method.

Reduced particle size, closer spacing, and better distribution in both as-cast and heat-treated conditions are microstructural observations from optical micrographs that show a progressive refinement of eutectic Si and intermetallic phases with increasing Fe₂O₃ content. These findings imply that heterogeneous nucleation during solidification, which results in microstructural refinement, may be facilitated by Fe₂O₃ particles. Due to heterogeneous nucleation effects, microstructural investigation showed that eutectic Si and intermetallic phases became refined as the Fe₂O₃ level increased. Improvements in hardness, strength, and stiffness up to 9 wt.% reinforcement were made possible by this modification. However, the development of a densely linked dendritic network and enhanced particle interactions caused a discernible decrease in ductility at 9 wt.% Fe₂O₃. Hardness and compressive strength, on the other hand, kept getting better, reaching a high of 610 MPa at 12 wt.% Fe₂O₃ inclusion. The decrease in elongation as a percentage of Fe₂O₃ clearly shows a trade-off between strength and ductility.

In comparison to as-cast circumstances, heat treatment greatly improved overall performance by encouraging spheroidization and homogeneous dispersion of precipitates, which led to higher hardness and strength as well as better ductility. The LM6/9 wt.% Fe₂O₃ composite provides a balanced combination of strength and ductility when considering the combined mechanical response, making it a better option for applications needing both appropriate deformation capability and load-bearing capacity. On the other hand, in situations where wear resistance and compressive loads predominate and lower ductility is less important, larger reinforcement levels (e.g., 12 wt.%) might be more suitable. As a result, the choice of reinforcing content should be application-specific, considering the needs for both strength and ductility. To reduce brittleness at greater reinforcement levels, future research may concentrate on increasing interfacial bonding, fine-tuning particle size, and optimizing particle dispersion.