Role B₄C Addition on Microstructure, Mechanical, and Wear Characteristics of Al-20%Mg₂Si Hybrid Metal Matrix Composite

Abstract

In the current study, the effect of different B₄C additions (0, 2.5, 5, and 10 wt%) on the microstructural, solidification behavior, mechanical, and tribological properties of Al-20%Mg₂Si composite were studied by means of scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Vickers hardness, tensile, and dry sliding wear tests. The cooling curve thermal analysis (CCTA) approach was utilized to monitor the influence of B₄C particles on the solidification behavior of Al-20%Mg₂Si composite. The results revealed that the addition of B₄C particles up to 10 wt% reduced the nucleation temperature (T_N) and growth temperature (T_G) of the primary Mg₂Si phase. Moreover, the proper amount of B₄C added to Al-20%Mg₂Si composite has a significant effect on the microstructural alteration, mechanical, and tribological properties of the composite. The mean size of primary Mg₂Si in Al-Mg₂Si composite was 47 μm, in which with the addition of 5 wt% B₄C, the particle size decreased to 33 μm. The highest UTS (217 MPa) and El% (7%) was achieved in Al-20%Mg₂Si-5%B₄C hybrid composite. The cast Al-20%Mg₂Si composite revealed the brittle mode of fracture with some cleavage characterization, in which with the addition of 5%B₄C, the fracture mode altered to a more ductile fracture. The wear results revealed that the Al-20%Mg₂Si-5%B₄C hybrid composite has the highest wear resistance with the lowest wear rate (0.46 mm³/Km) and friction coefficient (µ = 0.52) under 20 N applied load compared to other fabricated composites with mild abrasion as the governed wear mechanism.

1. Introduction

In recent years, the focus of materials design has moved towards lightweight, low-cost, and environmentally sustainable. According to this trend, aluminum metal matrix composites (AMMCs) have received considerable attention due to their enhanced properties (low density, brilliant cast ability, excellent mechanical properties, and good wear resistance) and low production cost [1].

Magnesium silicide, Mg₂Si, displays high melting temperature (1085 °C), low density (1.99 × 10³ kgm⁻³), high elastic modulus (120 GPa), high hardness (4.5 GPa), and a low coefficient of thermal expansion (7.5 × 10⁻⁶ K⁻¹) [2]. As a result of the outstanding physical and mechanical properties of the Mg₂Si phase, Al-Mg₂Si or Mg-Mg₂Si composites are desirable candidate materials for aerospace, automobile, and other applications [2]. Nevertheless, owing to low temperature brittleness, machinability, and insufficient wear resistance, the application of Al-Mg₂Si composite is limited.

Among the various ceramic particles, boron carbide (B₄C) particles are one of the most promising reinforcement particles as it exhibits unique physical and mechanical properties such as light-weight armor plates for its ballistic properties, wear-resistant components because of its tribological properties, and as abrasive grit. In addition, boron carbide can also be used as a neutron absorber in nuclear reactors due to the nuclear characteristics associated with its high boron content [3,4]. It has bulk modulus of about 214.8–247 MPa [5,6], low thermal conductivity of 30–42 mK, low density (2.52 g cm⁻³), high hardness 2900–3580 kg/mm² and low CTE of 3.2·10⁻⁶ °C⁻¹ [7,8]. Good chemical stability and bonding characteristics with Al alloys are other reported features of B₄C particles [3]. Hence, the B₄C particles are often used to reinforce Al alloys to fabricate MMCs. Baradeswaran et al. [9] identified the influence of B₄C on the mechanical and tribological behavior of Al 7075 composites. They found increasing hardness of composites compared with the base alloy because of the presence of the increased ceramic phase. The wear resistance of the composites increased with increasing content of B₄C particles, and the wear rate was significantly less for the composite material compared to the matrix alloy. In another study, Lashgari et al. [10] studied the influence of heat treatment on tensile properties and wear behavior of A356-10%B₄C composite. They claimed that in Al-B₄C composite, T6 treatment was a dominant factor on the hardness improvement in comparison with hardness increasing due to the addition of B₄C hard particles. In addition, heat treatment process results in higher strength and wear properties of heat-treated metal matrix composites in comparison with an untreated state. Gudipudi et al. [11] investigated the effect of a novel ultrasonic assisted stir casting on mechanical properties of AA6061-B₄C composites. Based on their findings, individual B₄C distribution and the refinement of microstructure was achieved at 4 wt% B₄C. The improved specific ultimate and compressive strengths at 4 wt% B₄C were observed as 36.32% and 43.92%, whereas specific Vicker’s and Brinell hardness as 53.41% and 50.89% respectively. A hybrid composite contains two or more reinforcement materials; in which each of the reinforcements deliver superior properties in the composite. As a result, unique mechanical, physical, and tribological properties exist in the hybrid composites, which are not attainable in other materials. Stir casting is implemented to produce the ex-situ (hybrid) aluminium MMCs due to their simplicity and lower cost compared to other fabrication processes [12]. Moktar et al. [13] reported that with the addition of 10 wt% SiC particles to Al-20%Mg₂Si composite, the morphology of primary Mg₂Si was altered to polygonal and its size decreased to 30 µm. Similarly, Sukiman et al. [14] reported that when the Al-15%Mg₂Si composite was added with 6 wt% YSZ, the mean size of primary Mg₂Si reduced to 72.4 µm with subsequent improvement in tensile properties of the composite. Uvaraja et al. [15] examined the effect of SiC and B₄C particulates-reinforced composite on Al6061 matrix. It was observed that hybrid metal matrix composites (HMMCs) showed a high level of hardness as compared to alloy because silicon carbide and boron carbide particles were uniformly embedded in the Al6061 matrix. Aziz et al. [16] studied the heat treatment and wear properties of Al₂O₃ and TiC particulates in reinforced Al6063 hybrid composites. The hardness of the composites was improved after heat treatment due to the presence of reinforcements in the composite, which accelerated the aging kinetics and caused an increase in dislocation density at the surrounding area of the reinforcement particles. Benal et al. [17] studied the effects of reinforcement with SiC and E-glass fibers on the wear properties of Al6061 hybrid composites. Due to the presence of both hard reinforcements and coherent precipitates in the matrix, increased hardness was observed during aging treatment. Umanath et al. [18] studied the effect of stir casting processing method to prepare Al6061-SiC-Al₂O₃ hybrid composites. It was observed that by adding reinforcement particles to the alloy matrix, micro-hardness increases with increased reinforcement due to solid solution hardening of the matrix. Altinkok et al. [19] evaluated the tensile behavior and microstructure of Al₂O₃/SiCp-reinforced AMMCs fabricated by the stir casting technique. Strength of the hybrid composite was higher than the unreinforced alloy. The significant enhancement in tensile strength of the composites was attributed to the grain and particle size, reinforcement shape, and uniform particle distribution in the alloy matrix. Uvaraja et al. [20] focused on the tribological behavior of novel hybrid composite materials by varying SiC reinforcements and a fixed percentage of B₄C particles. It was observed that the increase in hardness of the composite was mainly owed to the existence of harder SiC and B₄C particles in the matrix alloy, which act as obstacles to the dislocation motion. Sharma et al. [21] examined the metallography and bulk hardness of artificially aged Al6061-B₄C-SiC stir cast hybrid composites. A significant improvement in hardness, tensile strength, and a marginal decrease in ductility of hybrid composites were observed compared to aluminum alloy. Uthayakumar et al. [22] studied the wear performance of Al1100-SiC-B₄C hybrid composites under dry sliding conditions. An increase in hardness and wear resistance was observed due to the existence of harder SiC and B₄C reinforcement particles in the matrix alloy.

Although there are several studies in the literature on the structure and properties of Al hybrid composites reinforced with B₄C and other particles such as SiC, Al₂O₃, etc.; however, information regarding the structure and properties of Al hybrid composite reinforced with B₄C and Mg₂Si (Al-Mg₂Si-B₄C) is rather limited. Therefore, the purpose of the present study is to elucidate the influence of the different content of B₄C particles on microstructure alteration, solidification behavior, mechanical properties, and wear properties of Al-20%Mg₂Si composite.

2. Materials and Methods

2.1. Materials Fabrication

The initial materials to prepare Al-Mg₂Si-B₄C hybrid composites were pure aluminium (99.9% purity), pure magnesium (99.9% purity), pure silicon (98.9% purity), and pure B₄C ceramic powder with a size of 10–30 μm. In order to fabricate the hybrid composites, Al ingot was melted at 800 °C using an induction furnace, and then Si was added to the molten metal at the same temperature. After dissolving the Si in Al, the melt temperature was reduced to 750 ± 10 °C and pure magnesium was then introduced to the melt. The temperature was reduced to prevent the oxidation of Mg. After homogenization and removal of the dross, the melt was poured into a steel mold to produce the composite ingot. To fabricate the hybrid composites, 2 kg of composite ingot was melted in a resistance furnace and then, B₄C particles (2.5 wt%) were added gradually into the Al-Mg-Si melt. The melt was stirred constantly at a speed of 500 rpm for 15 min at 750 °C to obtain a uniform dispersion of reinforcement particles in the molten metal [23,24]. In fact, due to high viscosity of the composite melt, long stirring time is needed to achieve good mixing of the reinforcement particles inside the melt. The hybrid composite melt was cast in the pre-heated (220 °C) cast iron die to solidify. The procedure was repeated to prepare hybrid composites with 0, 5, and 10 wt% B₄C. The fabricated samples were used for microstructure examination, mechanical tests, and wear tests.

2.2. Cooling Curve Thermal Analysis

Parallel to the casting of the composites, in order to examine the influence of B₄C particles on characteristic temperatures of primary Mg₂Si phase during solidification, thermal analysis was conducted, in which the molten metal was poured into a preheated cylindrical ceramic mold (800 °C for 20 min) which was embedded by a K-type thermocouple located in the middle, 15 mm above the bottom of the mold. The temperature-time data were achieved using EPAD-TH8-K and EPAD-BASE module connected to a computer with DEWESoft 7.0.5. The FlexPro 2019 data analysis software was used to extract the recorded data and smooth and plot the cooling curves, as well as the first and second derivative curves for extracting solidification parameters. For the fabricated composites, the slope of the cooling curve before nucleation temperature of primary Mg₂Si was used to calculate the cooling rate. The cooling curve calculated for all thermal analysis tests was about 0.7 °C/s.

2.3. Mechanical Tests

The uniaxial tensile test was carried out based on ASTM B557M-02a by a computerized universal testing machine (Instron 5982) equipped with a strain gauge extensometer at a constant cross-head speed of 1 mm/min in room temperature. To confirm repeatability of the results, the tensile test was conducted on at least three specimens for each test. Hardness of the specimens was measured by a Matsuzawa Vickers hardness testing machine with the diamond indenter having an angle of 136° between opposite faces at a load of 5 N with a dwell of about 30 s.

2.4. Sliding Wear Test

Pin-on-disc sliding wear testing machine (TR-20, DUCOM) was utilized to investigate the wear properties of the composites according to the ASTM G99-04a standard under applied loads of 20 and 40 N, at constant parameters of 60 mm track diameter, 200 rpm sliding speed and 2000 m sliding distance, against a hardened chromium steel disc (Rc 64). The pin samples were 25 mm in length and 6 mm in diameter. The height loss of the specimens was measured using a linear variable differential transformer (LVDT) to monitor the wear loss of the specimens in microns. The volumetric loss was calculated by multiplying the height loss with cross section area of the test pin. A frictional force sensor was used to measure the generated frictional force on the specimens in Newton. A set of three samples was tested for each experimental condition and the average values were used for further analysis.

2.5. Microstructure Characterization

For microstructure characterization, the specimens were cut from the middle portion of cast billets and then underwent the grinding and polishing procedures in accordance with the standard metallography routines. Microstructures were analyzed using scanning electron microscopy, SEM (Hitachi S3400N) equipped with EDX facility. The fracture surfaces of the tensile test as well as the worn surface of the wear test specimens were cleaned with acetone using ultrasonic cleanser and examined using SEM. An I-Solution image analyzer was used to calculate the quantitative characteristics of the microstructure density, and aspect ratio as

$$\mathrm{Mean}\mathrm{density}=\frac{1}{\mathrm{m}}{{\displaystyle \sum _{\mathrm{j}=1}^{\mathrm{m}}\left(\frac{1}{\mathrm{n}}{\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{D}}_{\mathrm{i}}}\right)}}_{\mathrm{i}}$$

(1)

$$\mathrm{Mean}\mathrm{aspect}\mathrm{ratio}=\frac{1}{\mathrm{m}}{{\displaystyle \sum _{\mathrm{j}=1}^{\mathrm{m}}\left(\frac{1}{\mathrm{n}}{\displaystyle \sum _{\mathrm{i}=1}^{\mathrm{n}}\frac{{\mathrm{a}}_{\mathrm{i}}}{{\mathrm{b}}_{\mathrm{i}}}}\right)}}_{\mathrm{i}}$$

(2)

where D_i, a_i, and b_i are the density, longest, and shortest dimensions of a single Mg₂Si particle respectively, n is the number of Mg₂Si particles measured in a single field (0.60126 mm²), and m is the number of the evaluated fields. Furthermore, X-ray diffraction (XRD) (PHILIPS binary diffractometer with Cu-kα radiation application) was utilized to analyze the phase constituents of the fabricated hybrid composite.

3. Results and Discussion

3.1. Microstructure Examination

The microstructural micrograph of Al-20%Mg₂Si-10% B₄C hybrid composite is shown in Figure 1a. As seen, the composite structure consists of α-Al as matrix and Mg₂Si and B₄C as reinforcement particles. The corresponding elemental mapping as well as the XRD patterns of Al-20%Mg₂Si-10% B₄C hybrid composite is illustrated in Figure 1b and Figure 2 respectively, which indicate the existence of the Al, Mg₂Si and B₄C in the HMMC.

Figure 3a–d exhibits the SEM micrographs of Al-20%Mg₂Si composite added with different B₄C concentrations. As seen in Figure 3a, in the composite without B₄C particles (base), primary Mg₂Si particles are large and irregular and often in polyhedral shape surrounded with eutectic Mg₂Si cells. With the addition of 2.5 wt% B₄C into the Al-Mg₂Si composite, the morphology of primary Mg₂Si is still in polyhedral shape, as shown in Figure 3b. However, when the composite was treated with 5 wt% B₄C, the morphology of primary Mg₂Si slightly altered to polygonal as depicted in Figure 3c. Moreover, B₄C particles are distributed homogeneously throughout the aluminium matrix. With further addition of B₄C particles until 10 wt%, the Mg₂Si particles retain the polygonal morphology; however, some agglomeration of B₄C particles are observed in the composite area as illustrated in Figure 3d. Similar behavior of produced agglomeration as a result of increased B₄C reinforcement content in Al composites was reported by other researchers [25,26,27]. This phenomenon can be attributed to the insignificant wettability of ceramic particles with aluminum melt as the particles float on top of the melt surface owing to the surface tension and high interfacial energy of the reinforcing particles [28]. Besides, the lower density of B₄C particles compared to the aluminum causes the B₄C particles to float and leads to the non-uniformed dispersion where the B₄C particles agglomerate and segregate at a particular place in the melt.

3.2. Quantitative Analysis of Microstructure

Figure 4 shows that the alterations in average particle size, aspect ratio, and particle formation per unit area (density) correspond to the various B₄C additions. The results match well with the microstructure observation in Figure 3, in which with the addition of 2.5 wt% B₄C, the average particle size decreased by 13% from 47 to 41μm, the aspect ratio decreased by 3.7% from 1.34 to 1.29, and the density also decreased by 16% from 135 to 113 particle/mm². With the addition of 5 wt% B₄C to Al-20%Mg₂Si composite, the particles size, aspect ratio, and density of primary Mg₂Si further decreased to 33 μm, 1.27 and 94 particle/mm². However, as observed in Figure 3, with a further addition of B₄C to 10 wt%, the mean size and aspect ratio of primary Mg₂Si increased to 38 µm and 1.28 respectively. Nevertheless, the density decreased to 63 particle/mm² (Figure 4). Therefore, according to the results in Figure 3 and Figure 4, it can be observed that Al-20%Mg₂Si-5%B₄C hybrid composite illustrates the best condition in terms of primary Mg₂Si refinement and B₄C particles distribution.

3.3. Influence of B₄C Addition on Cooling Curves

Figure 5 demonstrates the cooling curve of Al-20%Mg₂Si composite with the first and second derivative curves corresponds to different contents of B₄C. The peaks on the first and second derivative curves represent distinct phase transformations that occurred during solidification. According to the binary phase diagram of Al-20%Mg₂Si, the first phase is the primary Mg₂Si, while the second phase is the formation of eutectic Al-Mg₂Si. It is observed that the addition of B₄C altered the solidification behavior of the Al-20%Mg₂Si composite by changing the solidification temperatures (nucleation temperature (T_N), growth temperature (T_G)) of primary Mg₂Si phase that is correlated with the microstructures of the composites, as observed in Figure 3.

Figure 6 shows the nucleation temperature (T_N) and growth temperature (T_G) of the primary Mg₂Si phase which correspond to the various additions of B₄C. The plotted graph of T_N has the same pattern as T_G of primary Mg₂Si, representing the same influence of the B₄C on these temperatures. Based on the result, it can be observed that the T_N of primary Mg₂Si decreased when the B₄C addition was introduced from 2.5 to 10 wt%. The same trend of graph is also observed for the T_G of primary Mg₂Si phase. From the temperature of 659.2 °C for base Al-20%Mg₂Si composite, T_N of primary Mg₂Si decreased to 659 °C, 657 °C, and 655.8 °C after the addition of 2.5, 5, and 10 wt% B₄C respectively.

Figure 7 depicts the duration (Δt) of primary Mg₂Si particles formation. The t_GMg₂Si-t_NMg₂Si is used to indicate the time needed for the primary Mg₂Si to nucleate and grow until complete solidification before the beginning of the eutectic cells. In the base Al-20%Mg₂Si composite, the duration of primary Mg₂Si to grow is about 13.82 s. The duration is reduced to 12.59 and 10.32 s as the composite was added with 2.5 and 5 wt% B₄C, respectively. Nonetheless, the duration of primary Mg₂Si particles formation increased to 12.42 s when the composite is treated with 10 wt% B₄C. The difference between the duration is small, which is no more than 3.5 s. It is probable that B₄C addition plays a role in controlling the growth of Mg₂Si particles, which results in the changing particle size. The obtained results demonstrate that when Al-20%Mg₂Si composite is added with 5%B₄C particles, the duration of primary Mg₂Si formation is less than other B₄C additions, which is consistent with the results in Figure 3 and Figure 4, in which the addition of 5%B₄C to Al-20%Mg₂Si composite caused the most refinement effect on primary Mg₂Si particles.

3.4. Refinement Mechanism of B₄C Addition

The results obtained from microstructure and thermal analysis clearly demonstrate that addition of B₄C particles to Al-Mg₂Si composite caused partial refinement of the Mg₂Si particles. The appropriate refinement has been obtained with addition of 5 wt% B₄C. Figure 8 illustrates the SEM microgrpah of primary Mg₂Si particles including dark particle in its center with corresponding line scanning spectra analysis. As seen, the EDS analysis reavled that the dark color particle in the centre of primary Mg₂Si is B₄C. The existance of this particle in the center of Mg₂Si crystal propose that B₄C particle may provide favorable nucleation substrates for the Mg₂Si particles precipitation.

In order to find out whether the refining effect is related to the role of B₄C particles as heterogeneous nucleation sites for primary Mg₂Si, the crystal structures of Mg₂Si and B₄C were compared and the lattice misfit between these crystal structures was calculated. The crystal structures of B₄C and Mg₂Si are shown in Figure 9. It can be clearly observed that the atoms distributed on the B₄C and Mg₂Si planes are in a different manner. In addition, the calculated lattice misfit between the B₄C and Mg₂Si is 43.6%. According to Bramfitt’s two-dimensional lattice misfit theory, if the mismatch between two planes is below 15%, one phase can act as a heterogeneous nucleation substrate for another [29]. Thus, it is observed that B₄C particles is not likely to meet the character of heterogeneous nucleation substrates for primary Mg₂Si crystals.

In addition, Figure 6 depicts that with the addition of B₄C particles to Al-Mg₂Si composite, the nucleation temperature (T_N) of primary Mg₂Si was reduced. It has been reported that provision of heterogeneous nucleation substrates for primary Mg₂Si particles through addition of refiners occurred at higher nucleation temperatures which results in a refined microstructure of the composites [30,31]. Hence, it can be concluded that the refinement of Mg₂Si particles as a result of B₄C addition is related to the growth stage rather than nucleation. The possible reason for refinement of primary Mg₂Si particles is the presence of the B₄C particles that act as a physical barrier during the growth of the primary Mg₂Si phase during solidification. According to the particle pushing phenomenon, the B₄C particles are rejected by the solidification front and finally stuck in grain boundary areas [32].

It was expected that with the addition of higher content of B₄C particles (10 wt%) to Al-20%Mg₂Si composite, the size of the primary Mg₂Si decreases. However, as shown in Figure 3 and Figure 4, addition of 10 wt% B₄C results in a reverse trend as the mean size of primary Mg₂Si particles increased. In fact, in Al-20%Mg₂Si-10%B₄C hybrid composite, the B₄C particles are agglomerated in some areas in the matrix with non-uniform distribution throughout the composite matrix; therefore, they cannot effectively hinder the growth of primary Mg₂Si particles in the preferred Mg₂Si growth directions, which results in an increase in the particles size. Moreover, the average number of primary Mg₂Si particles per unit (mm²) (density) decreased with the increasing B₄C concentration up to 10 wt%, as illustrated in Figure 3. The drop in the density of primary Mg₂Si particles as a result of B₄C addition can be associated with the formation of some boron-containing compounds in the matrix of the composite. According to Shirvanimoghaddam et al. [25], and Li et al. [33], some B-containing compounds are detected in the various B₄C reinforced aluminium composites, in which some B elements are dissolved from B₄C particles into the liquid-solid interface. As observed in Figure 10, the SEM micrograph with corresponding EDX results depicts that the presence of these boride compounds consist of Al and Mg. The formation of Al-Mg-B consumes the existed Mg in the melt, and hence, the amount of Mg atoms to react with Si to produce Mg₂Si decreased. This findings are aligned with the results by Azarbarmas et al. [34] where 1%B was added to Al-15%Mg₂Si composite. In fact, partial dissolution of B₄C in Al leads to the formation of B-containing compounds. Viala et al. [35] have pointed out that as a result of the reaction between Al and B₄C at temperatures above 700 °C, AlB₂, Al₃BC, and Al₄BC are produced. The dissolution of B₄C immediately saturates the molten Al with carbon and boron since the solubility of these elements in Al is low [36]. However, as the amount of Al-Mg-B phase is small, it is difficult to detect them in the XRD patterns (Figure 2). The observation in the density of primary Mg₂Si particles as a result of increasing B₄C content is consistent with the decreasing trend of nucleation temperature (T_N) of primary Mg₂Si phase in Figure 6. In fact, the reduction of Mg in the melt as a result of the formation of Al-Mg-B compound leads to a shift in the composition of the Al-Mg₂Si composite to lower the Mg₂Si content (eutectic point); hence, the nucleation temperature of primary Mg₂Si phase decreased.

According to Azarbarmas et al. [34], in the Al-Mg₂Si composite treated with B element, beside the segregation of boron on the solid-liquid interface, some B atoms also adsorbed on the growth front and changed the surface energy of the Mg₂Si crystals. Nevertheless, in the present study, due to the interaction of B with the melt and formation of B-rich intermetallic compounds, more B atoms are consumed, in which the extra B atoms are inadequate in the melt to absorb the primary Mg₂Si growth front and poison its anisotropic growth and alter its morphology significantly. Therefore, as shown in Figure 3 and Figure 4, addition of B₄C particles into Al-20%Mg₂Si composite has a minor impact on the aspect ratio of primary Mg₂Si particles, in which the maximum alteration of the aspect ratio occurred after the addition of 5 wt% B₄C (5.22%) compared to the base composite. This result demonstrates that B₄C addition has no obvious changes on the structure of primary Mg₂Si particles.

3.5. Mechanical Properties

3.5.1. Hardness

The average Vickers hardness of the fabricated composites as a result of different B₄C additions is illustrated in Figure 11. It can be observed that with the addition of 2.5 and 5.0 wt% B₄C ceramic particles to Al-20%Mg₂Si composite, the hardness value increased to 79 and 100 HV, respectively in comparison with the base composite (75 HV). The enhancement in the hardness property is attributed to the refinement of primary Mg₂Si particles as well as increasing the content of B₄C reinforcements. As observed, the hardness value of Al-20%Mg₂Si-5%B₄C hybrid composite is greater than other fabricated composite, which is due to the uniform distribution of B₄C particles. The existence of hard B₄C and fine Mg₂Si particles increased the obstacles to the grain boundary sliding [37]. Another factor for the improvement of the hardness of the composite due to B₄C addition is increase in the dislocation density as a result of thermal expansion mismatch among the reinforcement and matrix at the particle-matrix interfaces [11]. These factors result in high restrictions to the localized deformation during indentation. However, with further addition of B₄C particles to 10 wt%, the hardness value decreased to 84 HV. The reason for the decreasing hardness value is due to the agglomeration of B₄C particles in the matrix of the composite (Figure 3d). In addition, as depicted in Figure 4, by increasing the B₄C concentration to 10 wt%, the size of primary Mg₂Si particles increased and the density of primary Mg₂Si particles in the composite decreased as well, hence, the number of the Mg₂Si particles to resist the applied load during the hardness test is reduced. Therefore, both B₄C agglomeration and low density of primary Mg₂Si particles cause a higher exposure of the soft aluminium matrix to the intender, and as a result, the average hardness of the composite is reduced.

3.5.2. Tensile Properties

The engineering stress-strain curves of Al-Mg₂Si composite added with various contents of B₄C ceramic particles is illustrated in Figure 12a. The ultimate tensile strength (UTS) and elongation percentage to failure (El%) values of the composites are demonstrated in Figure 12b,c respectively. As seen, the addition of 2.5 and 5 wt% B₄C to Al-20%Mg₂Si composite increased the tensile properties, in which the addition of 5 wt% B₄C results is the highest value as the ultimate tensile strength (UTS) and percentage elongation (El%) increased from 177 MPa and 4.6% in the base Al-20%Mg₂Si composite to 217 MPa and 7% in Al-20%Mg₂Si-5%B₄C hybrid composite. It can be observed that there is a consistency between these results and the microstructural observations in Figure 3 as Al-20%Mg₂Si-5%B₄C hybrid composite exhibits a uniform distribution of B₄C reinforcements as well as refined primary Mg₂Si particles. The high strength values of Al-20%Mg₂Si-5%B₄C hybrid composite is due to the existence of B₄C particles in the composite matrix with uniform distribution, which creates dislocation pile-up in their neighborhood. Thus, the strength of the composite enhanced as a result of increasing dislocation interactions and dislocation density in the matrix near interfaces of the matrix and reinforcement.

In addition, partial refinement of primary Mg₂Si particles has a significant impact on strength enhancement of the composite. As depicted in Figure 3a, in base Al-20%Mg₂Si, the primary Mg₂Si particles exist with polyhedral shape and sharp tips, in which these sharp areas are potential sites for stress concentration which cause a drop in tensile properties. Nevertheless, in Al-20%Mg₂Si-5%B₄C hybrid composite, the primary Mg₂Si particles are finer and their morphology is regular (Figure 3c) compared to the base material, which results in tensile improvement. The most probable explanation of this behavior could be a deformation stregthening of hybrid composites which are brittle in nature but demonstrate higher strength and elongation when added with B₄C particles due to Hall-Petch dependence on the Mg₂Si particles size. The effect of particles size on tensile strength can be discribed through dislocation theory. The formation of plastic deformation needs to overcome the ultimate stress, which can attribute to a mass of moving dislocation. However, grain boundary will hinder the dislocation glide, and even leads to the dislocation pile-up. When the content of dislocation is pile-up increases a certain value, it will become the driving force of dislocation glide [37]. Furthermore, the particle size becomes larger, the content of dislocation pile-up is more; and the driving force of dislocation glide is bigger. That is why alloy with small particle size exhibits high strength. At the same time, due to small particle size increasing the number of particle boundary, the ability of hindering the dislocation motion is high. Therefore, the UTS for the plastic deformation is strongly influenced by Mg₂Si particles size.

However, as shown in Figure 12b,c, when the concentration of B₄C increased to 10 wt%, the UTS and El% of the composite reduced to 150 MPa and 2.7%, which is lower than other fabricated composites. The reduction in the tensile properties of the composite from the addition of higher B₄C concentration (10 wt%) is due to the agglomeration of B₄C particles as well as their non-uniform distribution in the matrix (Figure 3d). In fact, in the agglomerated areas, the bonding strength between the Al matrix and B₄C particles is low and the load transferring from the matrix to B₄C particles is not appropriate. Such insufficiencies cause a crack or decohesion of nucleation sites, which lead to composite failure at low stress levels. Furthermore, the addition of 10 wt% B₄C to Al-20%Mg₂Si composite results in increment in the average size of primary Mg₂Si particles (Figure 3 and Figure 4), which leads to a decrease in the tensile properties. A bigger size of primary Mg₂Si provides more potential substrates for nucleation and propagation of a crack, resulting in the failure of the composite by particle fracture [38].

3.5.3. Fracture Surface

The SEM images of tensile fracture surfaces of the Al-20%Mg₂Si composite added with different percentages of B₄C are exhibited in Figure 13a–f. Primary Mg₂Si particle is intrinsically brittle and has more intrinsic defects, which cause cellular fracture mode in the fracture surface of base Al-20%Mg₂Si composite due to cleavage in fracture planes of coarse Mg₂Si particles that are indicated by arrows (Figure 3a). Furthermore, the sharp corners of the irregular primary Mg2Si particles in the base Al-Mg₂Si composite serve as stress concentration sites for crack initiation and propagation which results in fracture of these coarse particles (Figure 3b) [39].

However, after adding B₄C particles up to 5 wt% to the Al-20%Mg₂Si composite, the fracture mode changed from brittle in the base composite to ductile, as depicted in Figure 13c,d. An interesting result is achieved for Al-20%Mg₂Si-5%B₄C hybrid composite, in which the fracture surface contains decohered primary Mg₂Si particles and a more ductile area. The obtained result is because of the presence of partially refined primary Mg₂Si particles and also B₄C particles, which have uniform distribution and good bonding with the Al matrix. The matrix alloy can be reinforced with a uniform distribution of the particle due to the effective role of the particle-rich region in the prevention of crack propagation; however, a particle-poor region offers canals for the propagation of crack [40]. Due to the decreasing size and aspect ratio of Mg₂Si particles after the addition of 5 wt% B₄C particles (Figure 4), the Mg₂Si provides less potential areas for crack nucleation compared to the base composite, and hence crack initiation is hindered. Furthermore, as the bonding between B₄C particles with the matrix is good, fracturing almost occurs in the B₄C particles rather than in the particle-matrix interface. This is in accordance with the model proposed by Babout et al. [41] in simulation of the fracture behavior in MMCs. Based on their suggestion, regardless of particle density, the fracture of interface is strength controlled, and decohesion occurred once the tensile stress in the particles exceeded the interface strength as a critical value. Based on these results, it is concluded that the Al-B₄C interface is steady and stable, hence, the initiation and propagation of the cracks is from Mg₂Si particles and not the Al-B₄C interface. Moreover, as seen in Figure 13d, the fracture surface exhibits several dimples, which are indications of a ductile fracture. This result is aligned with high strength and elongation of composite, as shown in Figure 12.

Figure 13e,f depicts the fracture surface of Al-20%Mg₂Si-10%B₄C hybrid composite. As observed, B₄C particles agglomerate in some areas on the fracture surface. The particle agglomeration increases the local stresses and proffers substrates for nucleation of crack and low energy propagation canals across the linked brittle particles. Failure can be portrayed by higher stresses generated in these areas. The interface of B₄C/matrix where the B₄C particles agglomerate shows interfacial decohesion, in which the failure took place along this interface. If the bonding strength in the interface between particle and matrix is weak, debonding occurs between matrix and particles before a particle fracture [39]. As the B₄C particles debonded from the matrix in a brittle manner, this feature reveals a weak interface bonding which creates favorable substrates for the initiation of crack and drops the load bearing capacity of the composite. In this condition, the crack needs less energy to propagate through the interface which can be reconciled with low strength and elongation of composite, as depicted in Figure 12. The crack initiates at the interface between particle and matrix, then propagates through the matrix, and finally joins other cracks, resulting in composite failure.

Porousity and bifilm on fracture surface are clearly observed in Figure 13e. It has been reported in many investigations that with increasing volume fraction of the reinforcement particles, internal defects, such as porousity and oxide layer (bifilm) expand [42]. During composite fabrication via vortex system and pouring of the molten metal matrix composite in the mould cavity, the bifilms or folded-over films are entrained, leading to successive impairing of UTS and El% of the composite (Figure 12). Not only does the unbounded oxide-oxide interface of the bifilms introduce stress concentration, the films also act as pre-existing microcracks.

3.6. Wear Behavior

Figure 14 demonstrates that the wear rate of Al-20%Mg₂Si composites correspond to different B₄C additions under the applied loads of 20 and 40 N. As seen, with the addition of 5 wt% B₄C, the wear resistance increased when the wear rate of Al-20%Mg₂Si composites decreased from 0.85 mm³/km to 0.46 mm³/km under 20N applied load. This improvement was the result of the load bearing capacity of the composites by ceramic particles during sliding. This was similar to the results reported by [43] in Mg-TiO₂ magnesium matrix composite, where, the addition of 5 wt% TiO₂ into the Mg matrix improved the wear resistance of the composite. The enhancement in wear resistance could be associated with the increase in hardness, in which the hardness of the Al-20%Mg₂Si composites with the addition of 5 wt% B₄C increased to 100 HV (Figure 11). The connection between hardness and wear rate can be explained by Archard’s law, which describes an inverse proportionality between hardness and the wear rate of a material [44].

Another reason for the decrease in the wear rate of the composite is due to the partial refinement of primary Mg₂Si particles, as shown in Figure 3 and Figure 4. It was expected that during the wear test, the wear rate of the hybrid composite with a finer Mg₂Si size decreases because grain boundary strengthening occurs with smaller particle size, leading to strain hardening [45]. With the addition of 10 wt% B₄C to the Al-20%Mg₂Si composites, the wear resistant was supposed to increase; however, as seen in Figure 14, the wear rate increased to 0.73 mm³/km, causing the increase in material removal. The high wear rate of Al-20%Mg₂Si-10%B₄C hybrid composite was due to the agglomeration of the B₄C particles in the matrix (Figure 3d), which resulted in the ‘three-body’ wear during sliding and increment of the composite’s wear rate. Additionally, due to the fragile bonding between the matrix and B₄C in the agglomerated regions, the B₄C particles were unable to support the applied load efficiently. Therefore, the interface offered a site for crack nucleation and tended to pull out the particles from the wear surface, which led to the formation and propagation of crack in the subsurface wear region [46]. In addition, as mentioned before in Figure 4, increasing the content of B₄C to 10 wt%, decreased the density of primary Mg₂Si particles. As a result, the load bearing effect of the hybrid composite reduced and during the wear rate the soft Al matrix was subjected to the asperities of the steel disc which led to an increase in the wear rate. Figure 14 also illustrates the increased wear rate when the applied load is increased from 20N to 40N. This is expected as the number of contact asperities and the separation of surface asperities increased at the higher loads. Increasing the applied load also contributed to the accumulation of wear debris in the space between the pin and disc that led to a more serious abrasive wear, increasing the wear rate.

The coefficient of friction (COF) of the Al-20%Mg₂Si composites added with various content of B₄C for the applied loads of 20 and 40 N is depicted in Figure 15. As seen, similar to the trend of wear rate, under the applied load of 20N, the COF (µ) of the Al-20%Mg₂Si composite decreased from 0.46 in the base Al-20%Mg₂Si composite to 0.43 and 0.39 when the composite was treated with 2.5 and 5 wt% B₄C respectively. Hence, Al-20%Mg₂Si-5%B₄C hybrid composite has the lowest COF compared to other fabricated composites. The reason for the reduction of the COF in the composite added with 5% B₄C can be related to the increased hardness of the composite (Figure 11) as finer Mg₂Si particles can support more load, and decrease the contact area between the pin and the sliding disc. This indirectly prevents the pin surface from acquiring more scratches. In addition, Devaraju, et al. [47] reported that the segregation of boron which acted as a dry lubricant between the sliding surfaces, leads to a decreased friction coefficient.

However, with a further increase in the B₄C content to 10%, the COF increases to 0.45. Due to the agglomeration of B₄C particles in 10 wt% B₄C hybrid composite, the minority of the Mg₂Si particles appeared in coarse size. The fracture of the large and brittle Mg₂Si could initiate the crack propagation in the matrix, and small Mg₂Si particles are pulled out from the matrix, which further acts as the source for abrasive wear, causing the COF to increase. The increasing COF could also is due to the fragmentation of boron oxide layer. B₂O₃ layer occurred when the pulled out B₄C particles reacted with the atmosphere. Its delamination during dry sliding formed scratches and grooves on the pin surface that increased the friction between the pin and the sliding surface [48].

An increase in the applied load to 40 N increases the COF of the composites compared to the 20 N applied load; however, the Al-20%Mg₂Si composite added with 5 wt% B₄C owns the lowes COF (0.52) compared to other fabricated composites and addition of 10 wt% B₄C incresed the COF to 0.54, which is similar with the trend of COF of the composites under 20N.

Figure 16a–f demonstrates the worn surface of the fabricated composites added with 0, 5, and 10 wt% B₄C particles under the applied loads of 20 and 40 N. The worn surface of Al-Mg₂Si composite (Figure 16a) displays several continuous grooves parallel to the sliding direction. Moreover, the counterfaces between the pin and the sample surface increased; hence, more materials were detached from the sample’s surface. This detachment may be due to the adhesive friction that subsequently increased the wear rate (Figure 14). The principal wear mechanisms of the samples were abrasion and delamination wear. By increasing the load to 40 N, delamination and adhesion govern the wear mechanisms, in which extensive pits and large size of delamination flakes exist on the worn surface, indicating the alteration of wear mechanism to severe wear (Figure 16b). With the addition of 5 wt% B₄C particles to the Al-20%Mg₂Si composite, the delamination pit areas as well as the groove width decreased, illustrating a mild abrasive wear mechanism, as shown in Figure 16c. This is consistent with decreasing wear rate and coefficient of friction results depicted in Figure 14 and Figure 15, respectively.

On the other hand, as observed in Figure 16e, by adding 10 wt% B₄C to the Al-20%Mg₂Si composite, deep parallel grooves with large widths indicate a severe abrasion, which is regularly produced by rigid particles between the pin and disc that cut into the worn surface. As observed in Figure 3d, the introduction of a high percentage of B₄C particles (10 wt%) to the Al-20%Mg₂Si composite caused the B₄C particles to cluster in some regions in the matrix with a non-uniform distribution. Therefore, during the sliding wear test, the agglomerated B₄C particles were pulled out from the matrix, fragmented, and became entrapped between the pin and disc. As a result, abrasion took place, leading the material to be omitted from the abrasion groove Figure 16e. Hence, the predominant wear mechanisms were severe abrasion and delamination. In addition, by increasing the applied load to 40 N, the flake-like delamination pit areas as well as abrasion grooves become wider and deeper, as seen in Figure 16f. This extensive delamination is due to crack nucleation and propagation sourced from the fracture and detachment of B₄C particles.

4. Conclusions

The effects of various B₄C additions on the microstructural, mechanical, and tribological properties of Al-20%Mg₂Si composite were investigated. It can be concluded that:

• With the addition of 5 wt% B₄C, the mean size of primary Mg₂Si in Al-20%Mg₂Si composite decreased from 47 to 33 µm, and its morphology was altered from octahedral to polygonal shape. However, with further addition of B₄C to 10 wt%, the morphology of primary Mg₂Si remains in polygonal shape with an increase in its size to 38 µm.
• The tensile result showed that the best level of B₄C was 5 wt% in which the UTS and El.% of Al-20%Mg₂Si composite increased from 177 MPa and 4.6% to 217 MPa and 7% in the Al-20%Mg₂Si-5%B₄C hybrid composite, respectively. The mechanisms of strengthening of the composites were load transfer effects, dislocation strengthening, and grain refinement. In addition, the Vickers hardness value of Al-20%Mg₂Si composite increased from 75 Hv to 100 Hv after the addition of 5 wt% B₄C particles to the composite.
• Examination of the fracture surface revealed that the fracture of the Al-20%Mg₂Si composite was brittle (including the cleavages). With the addition of 5 wt% B₄C particles, the fracture surface owned more dimples, which represents the domination of the ductile fracture.
• The results of the sliding wear test showed that the addition of 5 wt% B₄C to the Al-20%Mg₂Si composite improved the wear resistance of the alloy by decreasing the wear rate by 46%. The enhancement in wear resistance was due to the self-lubrication function of B₄C, uniform distribution of B₄C particles and good bonding of B₄C with the matrix as well as the small size of primary Mg₂Si particles, which in turn decreased the friction coefficient of the composite.
• Examination on the worn surface revealed that the governed wear mechanisms of the Al-20%Mg₂Si composite were abrasion and delamination, which changed to mid abrasion in the Al-20%Mg₂Si-5%B₄C hybrid composite.