Comprehensive Study on the Properties of AZ91/x-Si3N4 Composites for Their Prospective Application

Abstract

Metal alloy matrix composites are generally lightweight structural materials with a high strength-to-weight ratio. They can be extensively used in various fields of modern engineering applications, such as aerospace and automotive components and biomedical engineering. This study focuses on the development and characterization of lightweight metal alloy matrix composites for industrial applications, with a particular emphasis on magnesium (Mg) alloys as a replacement for aluminum-based alloys. Mg alloys offer significant weight advantages, being 33% lighter than aluminum and 75% lighter than steel, making them highly desirable for use in various engineering fields. In the present study, Mg (AZ91) alloy reinforced with x-Si₃N₄ composites (x = 0, 1, 3, 5, 7, 9 wt.%) were fabricated using a liquid state process. The AZ91/x-Si₃N₄ composites were evaluated through physical, mechanical, wear, and microstructural characterization. The experimental results, supported by statistical analysis, demonstrated that the incorporation of Si₃N₄ particles amplified the mechanical properties, wear resistance, and porosity of the composites. However, the presence of the reinforced particles resulted in reduced forgeability and elongation, limiting certain deformation characteristics. The existence of the reinforced particles within the composites was confirmed through SEM analysis, providing visual evidence of their distribution and interaction within the Mg alloy matrix. Finally, it was concluded that the implication of the study could be sought for the light structural parts of aerospace, automotive, biomedical, and prosthetic applications.

1. Introduction

Recently, lightweight metallic composites have gained significant commercial use in various industries. These composites are being employed for the production of components used in electronic devices, nuclear reactors, biomedical applications, sports equipment, marine vessels, automotive parts, and aerospace technology. The unique combination of lightweight properties and desirable mechanical characteristics makes these composites highly versatile and suitable for a wide range of applications across different sectors. The use of lightweight metallic composites is a promising trend in modern engineering, offering opportunities for enhanced performance and efficiency in diverse industries. Magnesium and its alloys are lightweight metals having density 1.7–1.9 g/cm³, lesser than aluminum (Al)’s metal density (2.67 g/cm³). The common metals present in the Mg alloys are Al, Zn, Mn, Si, Zr, Cu, and rare earth metals [1,2,3]. The numerous Mg alloys available are casting alloys such as AZ63, AZ81, AZ91, AM50, AM60, ZK51, ZK61, ZE41, ZC63, etc., and wrought alloys like AZ31, AZ61, AZ80, ZK60, ZE41, ZC71, etc. These magnesium alloys are mostly used to fabricate the magnesium metal matrix composites (MMMCs). The main alloying elements in magnesium alloys are depicted in Table 1. These elements have different functions and are added for various purposes like enhancements in strength, hardness, ductility, fluidity for casting, wear, corrosion resistance, etc. Amongst these alloys, AZ91 is widely used for the purposes of electronic packaging, computer housings, clutch and brake brackets, rotor fittings for helicopters, textile machines, wheelchairs, and other medical equipment [4,5,6].

There are several reinforcements which can be used to reinforce the magnesium alloys for fabricating the MMMCs such as TiC, TiO₂, TaC, TiN, TiB₂, AlN, SiC, SiO₂, ZrC, ZrO₂, ZrB₂, ZnO, Al₂O₃, MgO, Si₃N₄, CaB₆, WC, B₄C, BN, Cr₂O₃, C (graphite), etc., and a few of them have been repeatedly used in previous research works [7,8,9]. Amid these, Silicon Nitride (Si₃N₄) ceramic particles are more competent, hard, have a high melting temperature, elevated thermal stability, and a low cost as compared to other ceramic particles. Si₃N₄ has a high melting temperature (1900 °C) and low density (3.17 g/cm³). Si₃N₄ ceramic-reinforced metal matrix composites are extensively used in aerospace, medical, electronic, and automobile components. They are also known as high-temperature materials.

Many research studies on MMMCs have been conducted by researchers and scientists. Balaji, E. et al. [10] used friction stir processing (FSP) to manufacture ZE43 magnesium surface composites with different Silicon Nitride (Si₃N₄) percentages (6%, 12%, and 18%). The surface hardness and wear resistance of the composites were enhanced by the inclusion of reinforcing particles. Compared to other produced composites, the surface composite containing 18% Si₃N₄ had a superior hardness of 122VHN and a lower wear rate. Wang et al. [11] fabricated AZ91/12 Si₃N₄ and AZ91/25Si₃N₄ composites by an infiltration method and found that there was an improvement in the strength and hardness of the composites. Srivastava et al. [12] investigated the structure and performance of Ni-based Al₂O₃, SiC, and Si₃N₄ composites and disclosed that the reinforcements were uniformly distributed in the matrix. The vibration and damping characteristics of Si₃N₄-reinforced MMMCs were studied by Dhinakarraj et al. [13]. The results showed the influence of Si₃N₄ on the performance of damping characteristics. Xia et al. [14] assessed porous Si₃N₄ with a porosity of 51% and flexural strength of 77 MPa. Li et al. [6] fabricated an AZ91/CNT composite by a powder metallurgy technique and described an improvement in the strength and elongation. A Mg (AZ91) alloy reinforced with a graphene nanopowder composite showed an enhancement in mechanical properties [5]. Ramanujan et al. [15] and Junliu Ye et al. [16] studied a AZ31 alloy reinforced with eggshell particulates and Ti particles and discussed various mechanical properties such as the Young’s modulus, impact strength, elongation, wear behavior, etc. According to Pasha et al. [17], Mg/Si₃N₄ nanocomposites containing 1.0 vol% Si₃N₄ nanoparticles have 1.4 times increase in tensile strength and 2 times increase in ductility while maintaining an elastic modulus of the composite that is within the range of the local cortical bone elastic modulus. Moreover, the authors immersed the nanocomposites in simulated physiological fluids to assess in vitro degradation and bioactivity and found 2.2 times reduction in degradation rate in comparison to pure Mg, and thus provided the basis for their use as biodegradable implant materials for osteosynthesis.

Matta et al. [1] studied the various methods used to fabricate AZ91 alloy-based composites and finally reported that stir casting and liquid settling techniques were the best methods for the preparation of AZ91 composites because of commercial use. Bommala et al. [2] discussed the mechanical properties of an AZ91D Mg alloy including density (1.81 g/cm³), ultimate tensile strength (240–250 MPa), and Young’s modulus (45 GPa). In the past decade, numerous studies have been conducted on MMMCs, with a focus primarily on powder metallurgy routes, infiltration techniques, and other methods for fabricating these composites [18,19,20,21,22,23,24,25,26,27,28,29]. However, the stir casting method was found to be less investigated; furthermore, there are gaps in the existing literature regarding AZ91/Si₃N₄ composites fabricated through a stir casting technique that need to be addressed. Therefore, in the present study, AZ91/x-Si₃N₄ composites at different wt.% are fabricated using a stir casting technique. This study includes an experimental analysis of the physical properties, mechanical properties, and wear characteristics, as well as a microstructural analysis of the composites.

2. Materials and Methods

The specimens for the tensile test were produced by employing casting techniques, utilizing the mold depicted in Figure 1. Additionally, it is worth noting that the mold was prepared specifically for this purpose.

Magnesium alloy, AZ91, having a density of 1.8 g/cm³ was taken as a matrix while Silicon Nitride (Si₃N₄) was used as a reinforcement. Both AZ91 alloy (ingots) and Si₃N₄ (powders) materials were obtained from a market for the fabrication process. The average particle size of Si₃N₄ was 40–50 µm with 99% purity having a density of 2.2–3.5 g/cm³. The chemical composition of AZ91 alloy ingots is illustrated in Table 2. The reinforcement quantity was varied as 0, 1, 3, 5, 7, and 9 by wt.%.

2.1. Fabrication of AZ91/x-Si₃N₄ Composites

The fabrication process involved several steps. Firstly, the AZ91 alloy ingot was melted in an electric furnace at 650 °C, which required about 30 min. Simultaneously, the Si₃N₄ reinforcement was pre-heated at about 600 °C in a separate heating chamber. Once the ingot was fully melted, the slags were detached after melting the ingot, and then Si₃N₄ particles were gradually added while manual stirring was carried out for approximately 2 min to prevent cavitation and ensure uniform distribution. After this initial stirring, an electric motor stirrer was used for an additional 10 min at a stirring speed of approximately 400 rpm to further enhance the homogeneity of the mixture. The molten composite blend was then poured into a permanent mold at approximately 650 °C, as shown in Figure 2. Once solidified, the sample was removed from the mold and prepared according to the required dimensions for various tests. Throughout the stirring process, efforts were made to maintain a closed environment at a temperature of 650 °C. These procedures were repeated for all combinations of samples as per the fabrication process parameters mentioned in Table 3. Figure 3 provides a block diagram illustrating the setup for the stir casting process, while Figure 4 depicts the major steps involved in the study.

2.2. Testing of AZ91/x-Si₃N₄ Composites

In the present study, a comprehensive set of tests was performed to analyze the fabricated AZ91/x-Si₃N₄ composites. The testing included physical characterization to determine properties such as density and porosity, as well as mechanical (hardness, compressive, tensile, flexural, impact strength, Young’s modulus, and forgeability), and metallurgical (microstructure) properties.

2.2.1. Physical Properties

In the analysis of the fabricated composites, the physical properties of density and porosity were examined. The theoretical density was determined using the mixture rule, as expressed in Equation (1). The experimental density, on the other hand, was measured using Archimedes’ principle. The porosity of the composites, expressed as a percentage, was estimated using the formula given in Equation (2).

(1)$\mathrm{Density}\mathrm{of}\mathrm{composite}{\rho }_c=\frac{\left({\rho }_m\right)\left({\rho }_f\right)}{\left\{\left({\rho }_m\right)\left(\frac{w_f}{w_c}\right)\right\}+\left\{\left({\rho }_f\right)\left(\frac{w_m}{w_c}\right)\right\}}$

where ρ = density;

w = weight;

Suffix m, f, c = matrix, filler, composite.

(2)$\mathrm{Porosity}\mathrm{of}\mathrm{composite}=\frac{volumeofthepores}{volumeofthespecimen}$

2.2.2. Mechanical Properties

Hardness is a material property that measures resistance to indentation or scratches. Determining the hardness of a material is significant because it is associated with other properties such as strength. In this study, the hardness test as per ASTM E10 standard method [30] was performed using a Brinell hardness testing machine, and five indentations were made on each sample. The average of all five readings was considered for the analysis, with the hardness value reported as the Brinell Hardness Number (BHN).

Strength is a crucial property of structural materials and plays a significant role in understanding the behavior of composite materials. In this study, the strength of the fabricated composites was investigated using a universal testing machine. Tensile strength according to ASTM E8 [31] as shown in Figure 5a, compressive strength as per ASTM E9 method [32] as shown in Figure 5b, and elongation were measured. For both tensile and compressive tests, three samples were taken for each combination. The samples used for the tensile test before and after the test are depicted in Figure 6 and Figure 7, respectively, while Figure 8 illustrates the samples used for the compressive test as well as the samples after the test. These tests provide valuable insights into the mechanical properties and performance of the fabricated composite materials.

The Izod impact strength test was conducted on the metallic material specimens according to ASTM E-23 standards [33]. The specimens were prepared accordingly. The test was performed using an impact testing machine equipped with a pendulum hammer weighing 300 N. Three readings were taken for each specimen, and the average of these readings was used for the analysis. Table 4 provides detailed information about the Izod test, including specific parameters and measurements.

The flexural strength, also known as the bending strength, was determined using a three-point flexural test. This test method is illustrated in a block diagram, shown in Figure 9. The test specimens used had a rectangular cross-section. Three samples of each combination were tested, and the average of the readings obtained was considered for further analysis and examination of the flexural strength properties. The flexural strength was calculated using the general formula expressed in Equation (3).

(3)$\mathrm{Flexural}\mathrm{strength},\mathsf{\sigma }=\frac{3FL}{2b{d}^2}$

where F = applied force, L = length of support, b = width of the specimen, and d = thickness of the specimen.

Forgeability, which measures the change in height compared to the original height during compression, was evaluated in this study. The forgeability test was performed using a hydraulic pallet press machine. Three samples were tested for each combination of composites, and the average value was calculated for analysis purposes.

2.2.3. Metallurgical Properties

In order to observe the structural changes in the fabricated materials, metallurgical analysis was conducted using scanning electron microscopy (SEM) JEOL model JSM 6510-LV, Akishima, Japan. This technique was employed to examine the microstructure of the materials and gain insights into any changes or features at a microscopic level. SEM analysis is valuable for understanding the morphology, grain size, and distribution of phases within the fabricated materials.

3. Results and Discussion

3.1. Density and Porosity

Figure 10 illustrates the physical properties of stir-cast AZ91/x-S₃N₄ composites, specifically the density and porosity. It can be observed from the chart that theoretical (ρ_th) and experimental (ρ_ex) density are increased with the addition of Si₃N₄-reinforced particles. This increase in density is attributed to the higher density of Si₃N₄ particles (3.17 g/cm³) compared to the density of the AZ91 alloy (1.8 g/cm³). The addition of Si₃N₄ reinforcement leads to an overall increase in the density of the composite material. Indeed, it is observed that the theoretical density is higher than the experimental density in the AZ91/x-S₃N₄ composites. On the other hand, the increase in experimental density provides clear evidence of the presence of reinforcement particles within the AZ91 alloy matrix. A higher experimental density indicates a greater amount of Si₃N₄ particles, while a lower experimental density suggests a lower quantity of Si₃N₄ particles. This correlation strongly supports the conclusion that Si₃N₄ particles are indeed present in the fabricated materials.

The results depicted in Figure 6 demonstrate that there is a slight increase in the porosity of the composites. Several factors could contribute to this, including the presence of air bubbles during the vortex induction process and impurities at the micro level. However, the observed increase in porosity is only 1.33%, which is relatively small and may not be of significant concern for many applications. Furthermore, this level of porosity is generally acceptable for various applications, including those in aerospace, automotive, and other industries.

3.2. Hardness and Elongation

Figure 11 illustrates how the presence of Si₃N₄ affects the hardness property of the composite material that was produced. The hardness of the AZ91 alloy was measured to be 34 BHN, but when 9 wt.% of Si₃N₄ particles were added, the hardness increased to 76 BHN. Additionally, it was observed that increasing the amount of Si₃N₄ reinforcement resulted in a corresponding increase in hardness. This is because Si₃N₄ particles are inherently hard, and when they are incorporated into the softer matrix, they contribute to the overall hardness of the composite. The greatest increase in hardness was observed at a reinforcement content of 7 wt.%. Furthermore, it can be inferred that the greater the hardness and quantity of reinforcement, the higher the hardness of the composites, up to a certain extent. Guan et al. [22] reported, in their review, 72–84 and 44–65 hardness (HV) for AZ91/CNT and SiC/Mg composites, respectively. Further, Dudek et al. [34] found the hardness result to be 80 HBN for Mg recycled waste composites, and similar results have been reported by other researchers [35,36,37,38]. It is evident that the hardness results in the present study reflect the previously reported results.

Figure 11 also presents the changes in elongation percentage, which is shown on the secondary axis. According to the graph, the elongation is predicted to be 5.2% at 0 wt.% Si₃N₄ and decreases to 2.1% for the AZ91/9%Si₃N₄ composite. This indicates an increase in brittleness as the reinforcement particles are incorporated. The high brittleness of the Si₃N₄ particles, when combined with the ductile AZ91 matrix, leads to the strengthening of the composites. In other words, the hardness and elongation are related to the strength of the composites. The percentage elongation in this study is consistent with previous publications [5,6].

3.3. Tensile and Compressive Strength

Strength is the most important parameter for judging the mechanical properties of the composite materials. Figure 12 illustrates the variation in strength with the weight fraction of Si₃N₄ particles in the composite materials. The base alloy matrix exhibits distinct ultimate tensile and compressive strengths of 154 MPa and 302 MPa, respectively. However, these strengths increase to 231 MPa and 425 MPa, respectively, when 9 wt.% of reinforcement is added. It is noteworthy that materials with higher strength also tend to have higher hardness. The strength values obtained in this study are comparable to earlier findings [5,6]. The increase in strength of the composites can be attributed to the presence of hard reinforcement particles. The reinforced particles introduce residual stress, which can be either compressive or tensile in nature. This stress hinders the movement of dislocations, thereby strengthening the material. Additionally, the presence of reinforcements in the matrix leads to grain refinement, resulting in a reduction in grain size. Furthermore, the strength of the material can be explained by the Hall–Petch equation represented in Equation (4).

(4)$\sigma ={\sigma }_0+\frac{K}{\sqrt{d}}$

where σ = final yield strength;

σ₀ = initial/original yield strength;

d = average grain diameter of particle;

k = strengthening constant, which depends on the material. Figure 12

Influence of Si₃N₄ on tensile and compressive strength of AZ91/x-Si₃N₄ composites.

[Figure omitted. See PDF]

3.4. Impact Strength

The impact strength is another important mechanical property that characterizes the suitability of the fabricated composites for various applications. This property is typically assessed by subjecting the material to a sudden load, often applied through the gravitational force of a heavy hammer block. Figure 13 illustrates the variation in impact strength with the addition of Si₃N₄ reinforcement. The graph shows that the impact strength increases as Si₃N₄ is added to the composite. This can be attributed to the presence of the hard Si₃N₄ ceramic particles within the AZ91 matrix. Similar trends have been observed in previous research studies [15,16].

3.5. Flexural Strength and Young’s Modulus

Figure 14 illustrates the influence of Si₃N₄ reinforcement on the flexural strength (in MPa) and Young’s modulus (in GPa) of the fabricated composites. The graph demonstrates that an increase in the amount of reinforcement leads to higher values of both flexural strength and Young’s modulus, reaching peak values of 125 MPa and 98 GPa, respectively. These findings align with the observations made by researchers in [39]. In their study, it was noted that the addition of graphite to the Mg alloy resulted in a decrease in flexural strength.

3.6. Forgeability

The influence of load and Si₃N₄ on the forgeability property is described in Figure 15. The graph demonstrates the relationship between forgeability, load, and Si₃N₄ reinforcement. It shows that as the amount of Si₃N₄ reinforcement increases, the forgeability of the composites tends to improve. This improvement can be attributed to the presence of hard ceramic particles, such as Si₃N₄, which can enhance the strength and deformation resistance of the composite. The reinforcing particles act as barriers to dislocation movement, thus improving the material’s ability to withstand deformation without fracturing. On the other hand, the graph also illustrates that increasing the applied load during forging has a negative effect on forgeability. Higher loads impose greater stress on the material, leading to increased deformation and strain. This results in a larger change in height (deformation) relative to the initial height, which ultimately reduces the forgeability value. In other words, excessive loads can lead to increased susceptibility to cracking or failure during forging.

3.7. Wear Analysis

Dry-sliding wear analysis is a method used to assess the wear behavior of materials under conditions of sliding contact without any lubrication. In Figure 16, the dry-sliding wear behavior of the fabricated composites is evaluated at different sliding distances (300 m, 600 m, and 900 m) using a constant sliding speed of 2 m/s and a constant applied load of 50 N. This test aims to analyze how the sliding distance affects the wear properties of the composites. The graph illustrates that the wear loss increases as the sliding distance increases. This can be attributed to the increased rubbing between the surfaces over a longer sliding distance, resulting in more material removal and higher wear loss. In other words, the more the surfaces rub against each other, the greater the wear loss experienced by the material. Additionally, the wear loss is observed to decrease with a higher concentration of reinforcement in the composites. This reduction in wear loss is due to the presence of the reinforcement particles between the pin material (sample) and the disc. These reinforcement particles act as solid lubricants when they are removed from the pin material during the initial stages of wear. This solid lubrication effect reduces the friction and wear between the surfaces, resulting in a lower wear loss. However, as the sliding distance increases, the wear loss also increases. This is because the longer duration of rubbing between the surfaces leads to more material removal and wear, outweighing the beneficial effect of the solid lubrication provided by the reinforcement particles. Comparing these findings to past research, it is observed that the addition of graphite to the composites results in a decrease in wear resistance. Graphite tends to have a lower wear resistance compared to other reinforcement materials. Therefore, the inclusion of graphite in the composites can lead to increased wear loss [39].

3.8. Morphology

The JSM-6510 Series scanning electron microscope (SEM) (JEOL, Akishima, Japan), shown in Figure 17, is specifically employed for examining and analyzing the morphology and metallurgical characteristics in the present study. Morphology refers to the metallurgical characteristics that provide insights into the structure and formation of materials.

Figure 18 illustrates SEM micrographs (at different ×) of the AZ91 Mg alloy, reinforcement (Si₃N₄), and the AZ91/x-Si₃N₄ composites which are presented to describe various features including the reinforced particles, fracture surfaces from tensile testing, and worn surfaces. Figure 18c,d show the presence of the reinforced Si₃N₄ particles in the composites as compared to Figure 18a,b, which show the AZ91 Mg alloy and reinforcement (Si₃N₄), respectively. These particles can be observed within the matrix material, confirming their incorporation into the composite structure. The presence of these reinforced particles is important as they contribute to the enhanced mechanical properties of the composites. Figure 18e,f reveal the fractured surfaces of the tensile tested samples. The presence of dimple shapes on these surfaces indicates a ductile fracture behavior. Dimple-shaped features are characteristic of materials that undergo plastic deformation before fracture, indicating that the composites exhibit ductile behavior during the tensile test. Further, the micrographs (as shown in Figure 18g,h) interpret the worn surfaces of the composites. These surfaces provide insights into the wear mechanisms experienced by the material during testing. Various wear mechanisms, including delamination, adhesive, abrasive, and fatigue wear, can be observed. The sharp particles present in the composites are seen to cause cuts on the surface, indicating abrasive wear. Additionally, grooves and debris particles can be perceived on the worn surfaces, which is highlighted in the images. These debris particles act as solid lubricants during wear and contribute to reducing wear in the composites. The presence of similar debris particles and their roles as solid lubricants have been reported by researchers [40,41,42,43,44,45]. Also, the highlighted zones in Figure 18 demonstrate the aforementioned various features of micrographs.

4. Conclusions

Magnesium alloy composites (AZ91/x-Si₃N₄) were fabricated by stir casting using predefined casting parameters. The physical, mechanical, wear, and metallurgical properties of the fabricated composites were investigated. The following conclusions can be drawn from the study:

• Physical properties such as density (experimental and theoretical) and porosity were increased due to the presence of Si₃N₄ particles. The maximum porosity was depicted to be 2.01%, which is considerable for use in applications.

• Mechanical properties like hardness and strength (compressive, tensile, impact, flexural, and Young’s modulus) were enhanced and found to be 76 HBN (231 MPa, 425 MPa, 29 J/cm², 125 MPa, and 98 GPa) consecutively at 9 wt.% of Si₃N₄, whereas elongation decreased. Further, the forge ability was augmented when increasing the load, while it declined on addition of the reinforcement.

• Wear characteristic evaluation showed a decrease in wear loss due to the presence of Si₃N₄ particles, which act as a solid lubricant when first separated from the parent alloy. This property makes the material suitable for biomedical implants having relative motion, as after initial wear it becomes self-lubricating. Mg being biocompatible and biodegradable aids in human bone development; this quality of the AZ91 composite makes it a strong contestant for biomedical and prosthetic applications. Moreover, magnesium and its alloys are regarded as some of the most important materials for energy storage because they have the ability to store hydrogen gas as magnesium hydride. Among the available materials for renewable energy storage, magnesium has the highest theoretical storage capacity at 7.6 weight percent and an outstanding thermodynamic reversibility.

• Morphological study verified the existence of the reinforced particles in the parent alloy. SEM of the fractured sample showed that the ductile fracture is more dominant over the brittle fracture. Further, various wear mechanisms were predicted on the worn surfaces.

Author Contributions

Conceptualization, M.T.A.; methodology, M.T.A., D.H. and S.A.; data curation, A.M.A. and A.A.; writing—original draft preparation, M.T.A.; writing—review and editing, T.A., A.M.A. and A.A.; funding acquisition, T.A. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. (a) Die and (b) C-clamp for casting tensile test samples.

Figure 2. Stir casting set-up for composite casting.

Figure 3. A stir casting set-up diagram.

Figure 4. Different steps of the composite casting, preparation, and testing.

Figure 5. (a) Tensile testing (ASTM E8) and (b) compression and forgeability testing (ASTM E9).

Figure 6. (a–f) Tensile test samples (before test).

Figure 7. Tensile test samples (after test).

Figure 8. Compression test samples (a) before testing and (b) after testing.

Figure 9. Three points flexural strength test of AZ91/x-Si3N4 composites.

Figure 10. Density and porosity variation in AZ91/x- Si3N4 composites.

Figure 11. Influence of Si3N4 on hardness and elongation of AZ91/x- Si3N4 composites.

Figure 13. Effect of Si3N4 on impact strength of AZ91/x-Si3N4 composites.

Figure 14. Flexural strength and Young’s modulus of AZ91/x-Si3N4 composites.

Figure 15. Influence of Si3N4 on forgeability of AZ91/x-Si3N4 composites at a different load.

Figure 16. Effect of Si3N4 on wear of AZ91/x-Si3N4 composites at 50 N load, 2 m/s sliding speed, and different sliding distances.

Figure 17. Scanning electron microscopy (JSM-6510 Series).

Figure 18. SEM micrographs (at different ×) of AZ91 alloy and Si3N4 (a,b at 3000×), AZ91/x-Si3N4 composites samples highlighting Si3N4 (c,d at 1000×), tensile fractured samples showing fracture mechanism (e,f at 100× and 3000×, respectively) and worn surface at 100 N load, sliding speed 2 m/s, and 1000 m sliding distance (g,h at 200×).

Figure 18. SEM micrographs (at different ×) of AZ91 alloy and Si3N4 (a,b at 3000×), AZ91/x-Si3N4 composites samples highlighting Si3N4 (c,d at 1000×), tensile fractured samples showing fracture mechanism (e,f at 100× and 3000×, respectively) and worn surface at 100 N load, sliding speed 2 m/s, and 1000 m sliding distance (g,h at 200×).

Table 1

Main alloying elements with ASTM designation of magnesium alloys.

Alloying Element	Aluminum–Manganese	Aluminum–Zinc–Manganese	Aluminum–Silicon–Manganese	Rare Earth Metal–Zirconium	Zinc–Copper–Manganese	Zinc–Zirconium–Rare Earth Metal	Zinc–Zirconium	Manganese	Zirconium
ASTM Designation	AM	AZ	AS	EZ	ZC	ZE	ZK	M	K

Table 2

Element composition of AZ91 alloy (by weight %).

Major Elements	Aluminum	Zinc	Manganese	Silicon	Copper	Magnesium
Weight %	8.3–9.0	0.7–1.0	0.20–0.25	0.02 max	0.10 max	Remainder

Table 3

Process parameters for fabrication of AZ91/x-Si₃N₄ composites.

Parameters	Melting Temp(°C)	Si3N4 Preheating(°C)	Pouring Temp(°C)	Stirring Speed(rpm)	Stirring Time(min)
Quantity	650	600	650	400	10

Table 4

Izod impact strength test details.

S.No	Izod Impact Strength Test
1	Materials tested	AZ91/x-Si3N4 composites
2	Position of specimen	Vertical
3	Notch type	V-shape (45°)
4	Notch face direction	In front of striker
5	Type of striker	Farming hammer
6	Striking point	Upper tip of specimen
7	Specimen size	75 mm × 10 mm × 10 mm

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