Microstructure Evaluation, Quantitative Phase Analysis, Strengthening Mechanism and Inﬂuence of Hybrid Reinforcements ( β -SiCp, Bi and Sb) on the Collective Mechanical Properties of the AZ91 Magnesium Matrix

: Gravitational melt-stir casting produced hybrid nano-reinforcements ( β -SiCp) and micro-reinforcements (Bi and Sb) of AZ91 composites. SiCp-diffused discontinuous β -Mg 17 Al 12 precipitation with a vital factor of SiC was exhibited at the grain boundary region, formulated Mg 3 Si throughout the composite and changed the present Mg 0.97 Zn 0.03 phases. The creation of Mg 2 Si (cu-bic) and SiC (rhombohedral axes) enhanced the microhardness by 18.60% in a 0.5 wt.% SiCp/AZ91 matrix. The microhardness of 1 wt.% SiCp/AZ91 was slightly reduced after Mg 0.97 Zn 0.03 (hexagonal) reduction. The best ultimate tensile value obtained was about 169.33 MPa (increased by 40.10%) in a 0.5 wt.% SiCp/AZ91 matrix. Microelements Bi and Sb developed Mg 3 Bi 2 , Mg 3 Sb 2 and monoclinic C 60 phases. The best peak yield strength of 82.75 MPa (increased by 19.85%) was obtained with the addition of 0.5 wt.% SiCp/1 wt.% Bi/0.4 wt.% Sb. The mismatch of the coefﬁcient of thermal expansion of segregated particles and the AZ91 matrix, the shear transfer effect and the Orowan effect, combined with the quantitative value of phase existence, improved the compressive strengths of the composites with 0.5 wt.% β -SiCp, 1 wt.% β -SiCp and 0.5 wt.% SiCp/1 wt.% Bi/0.4 wt.% Sb by 2.68%, 6.23% and 8.38%, respectively. Notably, the Charpy impact strengths of 0.5 wt.% and 1 wt.% β -SiCp-added AZ91 composites were enhanced by 236% (2.89 J) and 192% (2.35 J), respectively. of Bi and Sb with SiCp resulted in the formation of a massive phase of brittle Al 6 Mn. Al–Mn-based phases (developed huge voids and cavities) remarkably reduced impact values by 80% (0.98 J). The discussion covers the quantitative analyses of X-ray diffraction, optical microscopy and scanning electron microscopy results and fracture surfaces.


Introduction
The engineering sector (commercial and military aerospace vehicles, automotive industry and shipbuilding industry) has recently shown broad interest in magnesium alloys [1][2][3][4] because of their light weight, which reduces fuel consumption and green gas emission [5]. Magnesium alloys and their composites have low weight (density ρ = 1.74 g/cm 3 ), high specific strength (158 KN m/kg) and superior mechanical strength [6]. Nevertheless, the applications, quality and quantity of magnesium alloys are limited [7] because of the constrained slip arrangement of the hexagonal closed-packed (HCP) crystal lattice [8]; thus, their tensile, compressive and impact properties must be improved. The Federal Aviation Association (FAA) lately reconsidered the practicability of using WE43 and Elektron 21 magnesium alloys because of the increasing demand for novel magnesium alloys. The FAA removed the prohibition against the use of magnesium alloy in seat frame compartments [9]. 2 of 21 Thus, magnesium alloys with different nano-and microparticles need to be investigated through different mechanical tests.
Several studies have investigated magnesium metal matrix composites (MMCs) to enhance their mechanical properties, such as tensile [10], compression [11] and impact properties [12]. Many factors influence the strength of materials, such as the method of fabrication (ultrasonic stirring method) [13], extrusion of composite and extrusion speed [14], variation in extrusion temperature [13], ageing behaviour [15], variation in ageing temperature [16], annealing behaviour of composites [6] and size of reinforcements [17,18] (microelements and nanoparticles). However, the tensile, compression and impact properties of the hybrid β-SiCp nanoparticle and Bi/Sb microelement reinforcements of AZ91 have not been studied by many. Moreover, the scattering of reinforcements is vital to amend a composite's mechanical strength at a low cost [19,20].
The present paper investigated the hybrid reinforcements of AZ91 (β-SiCp, Bi and Sb) in the as-cast condition by the gravitational melt-stir casting method. The aim of adding SiCp was to settle in the grain boundary and suppress the discontinuous β-Mg 17 Al 12 precipitation of the AZ91 matrix [21]. In addition, the addition of Bi and Sb formed Mg 3 Bi 2 and Mg 3 Sb 2 phases, which acted as a straddle in the AZ91 matrix and improved the composite's mechanical properties [22,23]. The mechanical properties of the AZ91 composites with β-SiCp, Bi and Sb were studied by tensile, compression and Charpy impact tests.
The microstructure and phase changes in the composites were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron-dispersive X-ray spectroscopy (EDS). The qualitative and quantitative phases in the matrix and strengthening mechanisms, such as the CTE, the shear transfer effect, the Orowan effect and fracture surfaces, of all tested samples were studied in detail.

Experiment Materials
The present study was conducted on AZ91 magnesium matrix composites reinforced with β-SiCp with different weight percentages (50 nm average particle size), bismuth (50 µm) and antimony (150 µm). The nominal chemical composition of the mother alloy that conforms to AZ91 is shown in Table 1. The same patch production material was used in this study [17,24]. Guangyu Technology Co., Ltd., (Shenzhen, China) and Jiehan Technology Corporation (Taichung) from Taiwan supplied commercial AZ91 alloy and β-SiCp nanoparticles (>99.99% high purity), respectively. Johnson & Annie Co., Ltd., New Taipei, Taiwan, provided bismuth and antimony (>99.99% high-purity) reinforcements.

Composite Preparation
The different composites shown in Table 2 were produced by gravitational melt-stir casting with suitable flux-covered protective atmospheres in a mild steel crucible, as shown in Figure 1. Initially, the melts were rapidly heated at 760 °C and the temperature was maintained for 20 min. Then, these melts were stirred for 10 min at 1000 r/min to ensure uniformly dispersed reinforcements in the magnesium composites [12]. After each composite was fully melted, the melts were poured into closed moulds. During the process, the chamber was set up with mixed atmosphere gases of SF6, CO2 (1 vol.% and balance) and Ar at 400 °C and 700 °C to terminate oxidation and burning [22].

Microstructure
According to the American Society for Testing and Materials (ASTM) E3-11 standard of metallographic procedures [25], cubic specimens (10 mm × 10 mm × 10 mm) were sectioned and cut from the centre of the castings using an abrasive cut-off machine. Subsequently, these samples were mechanically ground with silicon carbide polishing papers of various grit sizes (400, 1000, 2000 and 4000 CW) and polished with diamond paste paper (0.3 µm) using water as a lubricant. Before the microstructural investigation, the polished samples were etched for 50 s with an etching solution (75 mL of ethanol, 25 mL of deionized H2O, 1 mL of nitric and acetic acids). The microstructure and microscopic morphology were analysed by optical microscopy (OM), SEM (JSM-6390L, JEOL, Tokyo, Japan) and EDS. Phase investigation was carried out for all alloy codes by EDS and XRD (Bruker D2 phaser, Billerica, MA, USA) with CuKα radiation at 45 Kv and a current strength of 0.8 mA for composition determination. Measurements were conducted from 20° to 80° at a scan rate of 5°/min with a 12 min scanning span. Initially, the melts were rapidly heated at 760 • C and the temperature was maintained for 20 min. Then, these melts were stirred for 10 min at 1000 r/min to ensure uniformly dispersed reinforcements in the magnesium composites [12]. After each composite was fully melted, the melts were poured into closed moulds. During the process, the chamber was set up with mixed atmosphere gases of SF 6 , CO 2 (1 vol.% and balance) and Ar at 400 • C and 700 • C to terminate oxidation and burning [22].

Microstructure
According to the American Society for Testing and Materials (ASTM) E3-11 standard of metallographic procedures [25], cubic specimens (10 mm × 10 mm × 10 mm) were sectioned and cut from the centre of the castings using an abrasive cut-off machine. Subsequently, these samples were mechanically ground with silicon carbide polishing papers of various grit sizes (400, 1000, 2000 and 4000 CW) and polished with diamond paste paper (0.3 µm) using water as a lubricant. Before the microstructural investigation, the polished samples were etched for 50 s with an etching solution (75 mL of ethanol, 25 mL of deionized H 2 O, 1 mL of nitric and acetic acids). The microstructure and microscopic morphology were analysed by optical microscopy (OM), SEM (JSM-6390L, JEOL, Tokyo, Japan) and EDS. Phase investigation was carried out for all alloy codes by EDS and XRD (Bruker D2 phaser, Billerica, MA, USA) with CuKα radiation at 45 Kv and a current strength of 0.8 mA for composition determination. Measurements were conducted from 20 • to 80 • at a scan rate of 5 • /min with a 12 min scanning span.

Harness Test
Vickers microhardness was measured in different regions of each specimen using an Akashi MVK-H1 Vickers hardness tester (Mitutoyo, Kawasaki, Japan) with diamond indentation (2.5 mm diameter, 300 gmf applying a load, 10 s dwell time). The microhardness value was calculated based on the average of 10 indentations with two samples.

Tensile Test
The dimensions and location of the dog bone cross-section of the tensile nanocomposite sample are shown in Figure 2. The samples were cut by a water jet cutting machine as per the ASTM E8 standard [26]. Tensile tests were performed on an 810 kN MTS Insight universal testing machine at a constant speed of 0.5 mm/min at room temperature. The alloy composite's mechanical properties (yield strength, ultimate tensile strength (UTS) and elongation) were evaluated by the average of at least three samples to ensure the precision of the final results.
The dimensions and location of the dog bone cross-section of the tensile nanocomposite sample are shown in Figure 2. The samples were cut by a water jet cutting machine as per the ASTM E8 standard [26]. Tensile tests were performed on an 810 kN MTS Insight universal testing machine at a constant speed of 0.5 mm/min at room temperature. The alloy composite's mechanical properties (yield strength, ultimate tensile strength (UTS) and elongation) were evaluated by the average of at least three samples to ensure the precision of the final results.

Compression Test
Three to five cylindrical compression samples (constant length-to-diameter ratio of 1:1) were obtained from the same composites at different locations, as shown in Figure 2, to evaluate the compressive mechanical properties of each composite code as per ASTM E9-19 [27]. The universal testing machine (MTS, 810 kN) (MTS, Eden Prairie, MN, USA)

Compression Test
Three to five cylindrical compression samples (constant length-to-diameter ratio of 1:1) were obtained from the same composites at different locations, as shown in Figure 2, to evaluate the compressive mechanical properties of each composite code as per ASTM E9-19 [27]. The universal testing machine (MTS, 810 kN) (MTS, Eden Prairie, MN, USA) was used at a strain rate of 0.5 mm/min for the compressive tests to evaluate the composites' compressive behaviour at room temperature.
Flow stress (σ) and true strain (ε) values were estimated [17,28] based on the given input load (F), the radius of the cylindrical specimen (R) and the original height of the sample (l 0 ) in the testing machine according to Equations (1) and (2):

Impact Test
Charpy impact test specimens (55 mm × 10 mm × 10 mm) were cut and evicted from different regions in each composite by a water jet cutting machine. Moreover, a CNC CHMER electrical discharge machining (CW-43CS, CHMER, Taichung, Taiwan) tool was used to make a V notch, as shown in Figure 2. Three samples were measured to evaluate the precise impact values. The samples were studied with a universal impact tester (model 74) as per ASTM E-23 [29]. All the testing specimens were polished well before testing to terminate uneven alignment, which could lead to improper results. SEM was employed to analyse all fracture surfaces after completing the impact test.

Microhardness
Vickers microhardness values of the hybrid nano-and micro-reinforced Mg MMCs are given in Figure 3 and Table 3. The microhardness of alloy codes 2 and 3 increased by 18.60% and 14.66% compared with the as-cast alloy AZ91, respectively, but decreased with increasing β-SiCp percentage from 0.5 to 1 wt.%. The 3 microhardness of the AZ91 alloy added with 1 wt.% β-SiCp was slightly reduced; thus, the addition of β-SiCp limited the microhardness of AZ91. The microhardness of alloy code 4 (AZ91 + 0.5 wt.% SiCp + 1 wt.% Bi + 0.4 wt.% Sb) increased by 17.49% compared with that of the pure as-cast AZ91 alloy, as shown in Figure 3 and Table 3. However, the microhardness decreased slightly for alloy code 4 (AZ91 + 0.5 wt.% SiCp + 1 wt.% Bi + 0.4 wt.% Sb) compared with alloy code 2.
Charpy impact test specimens (55 mm × 10 mm × 10 mm) were cut and evicted from different regions in each composite by a water jet cutting machine. Moreover, a CNC CHMER electrical discharge machining (CW-43CS, CHMER, Taichung, Taiwan) tool was used to make a V notch, as shown in Figure 2. Three samples were measured to evaluate the precise impact values. The samples were studied with a universal impact tester (model 74) as per ASTM E-23 [29]. All the testing specimens were polished well before testing to terminate uneven alignment, which could lead to improper results. SEM was employed to analyse all fracture surfaces after completing the impact test.

Microhardness
Vickers microhardness values of the hybrid nano-and micro-reinforced Mg MMCs are given in Figure 3 and Table 3. The microhardness of alloy codes 2 and 3 increased by 18.60% and 14.66% compared with the as-cast alloy AZ91, respectively, but decreased with increasing β-SiCp percentage from 0.5 to 1 wt.%. The 3′ microhardness of the AZ91 alloy added with 1 wt.% β-SiCp was slightly reduced; thus, the addition of β-SiCp limited the microhardness of AZ91. The microhardness of alloy code 4 (AZ91 + 0.5 wt.% SiCp + 1 wt.% Bi + 0.4 wt.% Sb) increased by 17.49% compared with that of the pure as-cast AZ91 alloy, as shown in Figure 3 and Table 3. However, the microhardness decreased slightly for alloy code 4 (AZ91 + 0.5 wt.% SiCp + 1 wt.% Bi + 0.4 wt.% Sb) compared with alloy code 2.

Tensile Properties
The room-temperature tensile curves and mechanical tensile properties of pure and hybrid nano-micro-reinforced (β-SiCp, Bi and Sb) as-cast composites are depicted in Figure 4. The comparison of the present study's yield strength, UTS, Young's modulus and elongation with those of other particles reinforced by MMCs in other studies is shown in Table 4. The engineering tensile properties of nanocomposites in as-cast conditions improved remarkably compared with those of the pure AZ91 counterpart.

Tensile Properties
The room-temperature tensile curves and mechanical tensile properties of pure and hybrid nano-micro-reinforced (β-SiCp, Bi and Sb) as-cast composites are depicted in Figure 4. The comparison of the present study's yield strength, UTS, Young's modulus and elongation with those of other particles reinforced by MMCs in other studies is shown in Table 4. The engineering tensile properties of nanocomposites in as-cast conditions improved remarkably compared with those of the pure AZ91 counterpart.    The variations in the mechanical properties of pure and reinforced AZ91 composites are depicted in Table 4. Tensile strength (169.33 MPa, increased by 40.10%), yield strength (72.82 MPa, increased by 5.47%) and elongation (21.6%, increased by 3.6 times) dramatically increased with an increase in β-SiCp. The increase in elongation reflects that 0.5 wt.% β-SiCp reinforcement increases the formability of the AZ91 matrix alloy substantially. When SiCp content is 0.5 wt.%, the grain refinement effect and the constructively refined Mg17Al12 phase of granulated morphology compounds cause the increase in elongation. Tensile strength (118.07 MPa, decreased by 2.31%) and yield strength (59.38 MPa, decreased by 14.01%) decreased with the addition of 1 wt.% β-SiCp, whereas elongation was preserved (10.6%, increased by 1.77 times). Furthermore, the addition of bismuth and antimony with β-SiCp in alloy code 4 increased the peak yield strength (82.75 MPa, increased by 19.85%) compared with pure AZ91 and the other alloys. In addition, UTS and elongation in alloy code 2 (159.60 MPa, increased by 32.04% compared with pure AZ91) and alloy code 3 (11.2%, increased by 1.87 times) increased compared with pure AZ91. Moreover, the present nanocomposite clearly shows a superior tensile value (UTS, enhanced by 20.25% [17]) compared with other magnesium MMCs [17,30] (such as micro-WS 2 and nano-SiCp) or large-sized particles in the as-cast condition, as depicted in Table 4. Figure 5 shows the room-temperature engineering compressive stress-strain curves and properties of pure AZ91 and AZ91/β-SiCp/Bi/Sb composites in the as-cast condition. Compressive strength and ductility developed instantaneously with the addition of β-SiCp/Bi/Sb reinforcements, as depicted in Table 5. The compressive properties of alloy codes 2, 3 and 4 improved by 2.68%, 6.23% and 8.38%, respectively, compared with pure AZ91 (345.14 MPa). The present study achieved a higher compressive strength compared with other micro-SiCp-reinforced composites in similar previous studies [31].

Impact Properties
The variations in the absorbed Charpy impact energy of pure AZ91 and reinforced (β-SiCp/Bi/Sb) nanocomposites are depicted in Figure 6. Table 6 shows that the absorbed energy expressively improved by 236% (2.89 J) and 192% (2.35 J) after the addition of β-SiCp reinforcements in alloy codes 2 and 3, respectively, compared with pure AZ91 (1.23 J). However, the addition of microelements (Bi and Sb) with the hybrid nanoelement (β-SiCp) in alloy code 4 decreased the impact energy by 80% (0.98 J).

Impact Properties
The variations in the absorbed Charpy impact energy of pure AZ91 and reinforced (β-SiCp/Bi/Sb) nanocomposites are depicted in Figure 6. Table 6 shows that the absorbed energy expressively improved by 236% (2.89 J) and 192% (2.35 J) after the addition of β-SiCp reinforcements in alloy codes 2 and 3, respectively, compared with pure AZ91 (1.23 J). However, the addition of microelements (Bi and Sb) with the hybrid nanoelement (β-SiCp) in alloy code 4 decreased the impact energy by 80% (0.98 J).
The variations in the absorbed Charpy impact energy of pure AZ91 and reinforced (β-SiCp/Bi/Sb) nanocomposites are depicted in Figure 6. Table 6 shows that the absorbed energy expressively improved by 236% (2.89 J) and 192% (2.35 J) after the addition of β-SiCp reinforcements in alloy codes 2 and 3, respectively, compared with pure AZ91 (1.23 J). However, the addition of microelements (Bi and Sb) with the hybrid nanoelement (β-SiCp) in alloy code 4 decreased the impact energy by 80% (0.98 J). Figure 6. Charpy impact test and absorbed impact energy of pure and modified AZ91 alloys. Figure 6. Charpy impact test and absorbed impact energy of pure and modified AZ91 alloys.   Table 7 show the presence of the different composition phases in hybrid nanocomposites (alloy codes 1 to 4) in the as-cast condition, which are confirmed by the XRD test results. The total weight of the phases must be equal to 1 as per Equation (3). The reference intensity ratio (RIR) method was used to estimate the quantitative value of the substantial phases according to Equation (4) [12]. RIRs are the ratios of the most substantial peak of the unknown phase to the peaks of standard phases.
where I i∝ and I ik are the strongest intensities of the i-th ∝ and k unknown phases, respectively, and RIR ∝c and RIR kc are the strongest intensity ratios of the unknown ∝ or k phase and corundum c.
where ∝ and are the strongest intensities of the i-th ∝ and unknown phases, respectively, and RIR ∝ and RIR are the strongest intensity ratios of the unknown ∝ or k phase and corundum c.   The diffraction peaks of solid solution of α-phase (Mg) and β-phase (Mg 17 Al 12 ) were perceived in all the AZ91 matrix composites. The diffraction peaks of SiC, Mg 2 Si and Mg 0.97 Zn 0.03 were found in alloy code 2. Additionally, the counts of SiC and Mg 2 Si were progressively enhanced with the addition of 1 wt.% β-SiCp in alloy code 3. In other words, the formation of Mg 2 Si substantially dissolved the brittle phase of Mg 17 Al 12 in alloy code 2 [19,21]. The Mg 2 Si phase was segregated throughout the composite based on the XRD pattern [32]. Moreover, the appearance of Mg 0.97 Zn 0.03 , SiC and Mg 2 Si in magnesium composites has also been observed in previous studies [12,30,[33][34][35]. Figure 7 portrays the phases of SiC, Mg 2 Si, Mg 3 Bi 2 , Mg 3 Sb 2 , C 60 , Mg 0.97 Zn 0.03 and Al 6 Mn detected in alloy code 4, and these morphologies of precipitates have been confirmed by earlier studies [12,22,[36][37][38]. A massive amount of Al-Mn-based intermetallic phases and brittle Al 6 Mn phases were dispersed throughout the casting. Furthermore, the EDS study confirmed the phases identified in the XRD patterns.

Microstructural Analysis
The OM and SEM images of the microstructures of pure AZ91 and reinforced AZ91 composites in the as-cast condition are exhibited in Figures 8-11. EDS was used to examine the presence of phases in the MMCs.       Alloy code 1 contains an α-Mg matrix and enormous β-Mg17Al12 intermetallic phases. Based on the EDS examination, alloy code 1 contains Mg, Al and a small quantity of Zn. Discontinuous β-Mg17Al12 precipitates arise when eutectic supersaturated Al and Mg cool down and at the eutectic Al supersaturated solution state. New Mg2Si phases developed at the boundary of alloy codes 2 to 4, in addition to the massive β-Mg17Al12 phases. The XRD patterns in Figure 7 indicate the presence of α-Mg, β-Mg17Al12 and Mg2Si. The rough grain structure of alloy code 1 (Figures 8a and 9a) was refined remarkably compared with that of alloy code 2 (Figures 8b and 9b) [39]. On the other hand, the grain sizes of alloy codes 1-4 were calculated by ImageJ software (1.53j, U.S. National Institutes of Health, Bethesda, MD, USA) and the linear intercept method. The grain sizes were estimated from the three images of each alloy code. The average grain sizes of alloy codes 1-4 were recorded as 87, 61, 47 and 81 µm, respectively. The grain refinement increased in alloy codes 2 and 3 remarkably.
In addition, illustrative SEM images (alloy code 2; Figure 11b) show the relatively good scattering of SiC nanoparticles in alloy code 2. The solid solution of discontinuous Mg 17 Al 12 , which was scattered in a sizeable free region in alloy code 1, as shown in Figure 9a, was dramatically dissolved into alloy code 2, as shown in Figure 9b.
Nevertheless, the addition of 1 wt.% β-SiCp to AZ91 (alloy code 3) can produce many Si nanoparticles and has the potential to reproduce and formulate Mg 2 Si (dendritic shape structure of the brittle phase), as shown in Figure 8c. Figure Table 7. Similarly, Figure 10d shows an enormous solid phase of Mg (52.19 wt.%) and Al (47.76 wt.%). This phase is viewed as a massive discontinuous Mg 17 Al 12 phase when the β-SiCp amount is increased to 1 wt.%. Moreover, XRD studies also confirmed the existence of these phases. The

The Strengthening Mechanism of Microhardness
Research analysis by different research groups [41,42] hasconfirmed that the content of rigid reinforcements in the ductile material matrix enhances the hardness of the composite. The present research also affirmed the same by comparing the microhardness of alloy codes 1 to 4. Each alloy had a different weight percentage and various combinations of reinforcements compared with the as-cast condition of AZ91. For alloy code 2 (0.5 wt.% SiCp added), the microhardness was enhanced by 12.69 HV because of the creation of the brittle texture of Mg 3 Bi 2 , Mg 2 Si (1.92%, cubic) and SiC (2.52%, rhombohedral axes (rhombo.h.axes)). SEM images (Figure 9b,c) reveal that alloy codes 2 and 3 had grain refinement and particle strengthening, which improved the hardness of the composites [43]. Similarly, the microhardness of alloy code 3 (1 wt.% SiCp added) increased by 10 HV compared with alloy code 1. However, the microhardness of alloy code 3 decreased by 2.69 HV, from 90.38% to 86.99%, compared with alloy code 2 because of the formation of Mg 0.97 Zn 0.03 (hexagonal) [12]. The values of the quantitative phases were estimated based on the XRD pattern results, as shown in Table 7. Furthermore, the microhardness of alloy code 4 (0.5 wt.% SiCp + 1 wt.% Bi + 0.4 wt.% Sb) increased by 11.93 HV, even with the additional brittle reinforcements Bi and Sb. Here, the microhardness was reduced because of the non-uniform distribution of different reinforcements and their high porosity (Figure 12d) [17]. The presence of reinforcements can be ascribed to the restriction of localized matrix deformation during indentation. The existence of growing agglomeration enhances the porosity concerning the high surface tension and poor wetting properties among the particles and molten melts. The mechanism of the higher viscosity of molten metals and developing propensity of particles to clump increases the agglomeration [44]. on the XRD pattern results, as shown in Table 7. Furthermore, the microhardness of alloy code 4 (0.5 wt.% SiCp + 1 wt.% Bi + 0.4 wt.% Sb) increased by 11.93 HV, even with the additional brittle reinforcements Bi and Sb. Here, the microhardness was reduced because of the non-uniform distribution of different reinforcements and their high porosity ( Figure  12d) [17]. The presence of reinforcements can be ascribed to the restriction of localized matrix deformation during indentation. The existence of growing agglomeration enhances the porosity concerning the high surface tension and poor wetting properties among the particles and molten melts. The mechanism of the higher viscosity of molten metals and developing propensity of particles to clump increases the agglomeration [44].

Strengthening Mechanism of the Mechanical Properties
As shown in Figures 4-6, the addition of nano-micro-reinforcements to the AZ91 matrix remarkably improved the tensile, compression and impact values compared with the as-cast pure AZ91. The strengthening factors included the mismatch in the CTE, the shear transfer effect of the load, Orowan strengthening development and the quantitative phase present in the composites. These effects contributed to the improvement of mechanical properties, which have also been confirmed by previous studies [17,21,30,33,45,46].
The dislocation effect and induced residual stresses developed because of the CTE mismatch between the AZ91 matrix and reinforcements. The plastic deformation arose in the SiCp vicinity to the Mg alloy because of thermal stresses. The increase in the mechanical properties caused by the CTE can be estimated by Equation (5) [21,45]:

Strengthening Mechanism of the Mechanical Properties
As shown in Figures 4-6, the addition of nano-micro-reinforcements to the AZ91 matrix remarkably improved the tensile, compression and impact values compared with the as-cast pure AZ91. The strengthening factors included the mismatch in the CTE, the shear transfer effect of the load, Orowan strengthening development and the quantitative phase present in the composites. These effects contributed to the improvement of mechanical properties, which have also been confirmed by previous studies [17,21,30,33,45,46].
The dislocation effect and induced residual stresses developed because of the CTE mismatch between the AZ91 matrix and reinforcements. The plastic deformation arose in the SiCp vicinity to the Mg alloy because of thermal stresses. The increase in the mechanical properties caused by the CTE can be estimated by Equation (5) [21,45]: where β is 1.25 (strengthening coefficient), µ m is 2.64 GPa (elastic modulus of the matrix; µ m can be calculated by where E m and ν are the Young's modulus and Poisson's ratio of the matrix AZ91, respectively), b = 0.32 nm (Burgers vector) [45], f is 0.0058 (the volume fraction of particles), d is the diameter of the particles from Table 2, ∆T is 300 K (test temperature and processing temperature difference) and ∆α is 25.5 × 10 −6 (CTE difference between matrix and reinforcement particles) [47]. Particle size and volume fraction are the critical factors for CTE values, as shown in Equation (5). Thus, ∆σ CTE increased when the particle size of the reinforcements decreased. The influence on strengthening was due to the ∆σ CTE between pure AZ91 and reinforced AZ91, as shown in Table 8. Shear load transfers from hard reinforcement particles to the rigid composites during the mechanical testing when sound structural integrity exists between the two phases in the matrix. The load transfer effect is represented by Equation (6) [17,21,33]: where σ m is the tensile yield strength and f = 0.00283 is the volume fraction of the reinforcement particle. The calculated load transfer values are mentioned in Table 8. Load transfer is a critical factor that improves yield strength [21,46]. Increasing load transfer consequently expands the volume of fractions, as shown in Table 8.
As per the Orowan mechanism, finer reinforcements (less than 100 nm) [48] with highly dispersed particles are more practical to enhance the mechanical properties of the composite [49]. The Orowan mechanism developed by the particles is shown in Figure 13 The Orowan mechanism is large and energetically not favourable for the dislocation to cut through in the composites. However, the Orowan mechanism is not vital in micro-sized reinforcements in magnesium MMCs because of the coarse texture and the large spacing of interparticles [48]. As shown in Table 8, the Orowan mechanism contributed about 21.05% against the CTE effect. The collaboration of the non-clipping particles remarkably increased the composite's strength as per the effect of the Orowan strengthening mechanism. The ∆σ Orowan caused by the second-phase dispersion [50] that acts as uniformly distributed spherical reinforcements in the matrix can be estimated by Equations (7) and (8) [30,46,47]: where G m = 2.64 GPa (shear modulus of the matrix), b = 0.32 nm [45,46], λ = 230.56 nm (the effective planar interparticle spacing), f = 0.00283 is the volume fraction of reinforcements calculated from the weight fraction and d p is the average diameter of the refinement particles from Table 2. The calculated Orowan strengthening mechanism is listed in Table 8.
where = 2.64 GPa (shear modulus of the matrix), b = 0.32 nm [45,46], = 230.56 nm (the effective planar interparticle spacing), = 0.00283 is the volume fraction of reinforcements calculated from the weight fraction and is the average diameter of the refinement particles from Table 2. The calculated Orowan strengthening mechanism is listed in Table 8. The CTE, the shear transfer effect of the load and Orowan strengthening development in the composites are presented in Table 8. All the effect values (increasing trend of total cumulative) show that the compression test delays crack propagation, dramatically strengthens the interface of the alloys and increases the compression properties (increasing trend), as shown in Table 5 [17].

Quantitative Phases Influences on Mechanical Properties
An outstanding amount of secondary phases was scattered throughout alloy code 2, and the existence of SiCp increased its microhardness [12]. A refined grain structure means that β-SiCp can restrict grain growth and maintain excellent dispersion ( Figure  11b); thus, SiCp is vital for accomplishing excellent mechanical properties, such as tensile and impact properties [33,34].
However, the tensile and impact properties of alloy codes 3 and 4 were reduced in the presence of SiCp, Mg2Si and C60, which can cause microcracks that coalescence into large cracks [12], as shown in Figure12c.
Nevertheless, the addition of 1 wt.% β-SiCp (alloy code-3) substantially decreased the mechanical strength (the yield and ultimate properties) because of the sinking and floating comportment of SiCp compared with AZ91 alloy because of its superior density and surface tension. The main prerequisite for these mechanical properties changed because of the presence of new intermetallic Al and Mn phases and massive Mg17Al12 (Figure  10c-d) phases. Figure 10c specifies the EDS results for alloy code-3. Alloy code 3 has several combinations of Al (48.78 wt.%) and Mn (47.26 wt.%), which are considered intermediate brittle phases, such as Al6Mn and others presented in Tables 7 and 9 in the massive Mg6Al12 phase. Similarly, Figure 10d shows an enormous solid phase of Mg (52.19 wt.%) and Al (47.76 wt.%). A massive discontinuous Mg6Al12 phase appeared when the β-SiCp amount increased to 1 wt.%. Moreover, XRD studies also confirmed the existence of these phases, which dominated the mechanical strength of alloy code 3. In addition, previous The CTE, the shear transfer effect of the load and Orowan strengthening development in the composites are presented in Table 8. All the effect values (increasing trend of total cumulative) show that the compression test delays crack propagation, dramatically strengthens the interface of the alloys and increases the compression properties (increasing trend), as shown in Table 5 [17].

Quantitative Phases Influences on Mechanical Properties
An outstanding amount of secondary phases was scattered throughout alloy code 2, and the existence of SiCp increased its microhardness [12]. A refined grain structure means that β-SiCp can restrict grain growth and maintain excellent dispersion ( Figure 11b); thus, SiCp is vital for accomplishing excellent mechanical properties, such as tensile and impact properties [33,34].
However, the tensile and impact properties of alloy codes 3 and 4 were reduced in the presence of SiCp, Mg 2 Si and C 60 , which can cause microcracks that coalescence into large cracks [12], as shown in Figure 12c.
Nevertheless, the addition of 1 wt.% β-SiCp (alloy code-3) substantially decreased the mechanical strength (the yield and ultimate properties) because of the sinking and floating comportment of SiCp compared with AZ91 alloy because of its superior density and surface tension. The main prerequisite for these mechanical properties changed because of the presence of new intermetallic Al and Mn phases and massive Mg 17 Al 12 (Figure 10c-d) phases. Figure 10c Tables 7 and 9 in the massive Mg 6 Al 12 phase. Similarly, Figure 10d shows an enormous solid phase of Mg (52.19 wt.%) and Al (47.76 wt.%). A massive discontinuous Mg 6 Al 12 phase appeared when the β-SiCp amount increased to 1 wt.%. Moreover, XRD studies also confirmed the existence of these phases, which dominated the mechanical strength of alloy code 3. In addition, previous similar studies have demonstrated that Al-Mn, β-Mg 17 Al 12 and Mg 2 Si diminish mechanical properties [12,34].  Tables 7 and 9, based on the XRD results. The mechanical properties were remarkably reduced because of these Al-Mn-based materials [51]. The results of the tensile test of alloy code 4 displayed in Figure 12d show that these elements can produce substantial amounts of brittle heterogeneous phases of C 60 , SiCp and Mg 2 Si, which can reduce mechanical properties slightly compared to those of alloy code 2 because of the formation of small microcracks (Figure 14d).  [12,34].
In the impact test, alloy code 4 had multiple Al-Mn-constructed brittle phases, as shown in Tables 7 and 9, based on the XRD results. The mechanical properties were remarkably reduced because of these Al-Mn-based materials [51]. The results of the tensile test of alloy code 4 displayed in Figure 12d show that these elements can produce substantial amounts of brittle heterogeneous phases of C60, SiCp and Mg2Si, which can reduce mechanical properties slightly compared to those of alloy code 2 because of the formation of small microcracks (Figure 14d).

Fracture Surface Analysis
As depicted in Figures 13-15, the fracture morphology of pure AZ91 and reinforced AZ91 composites (codes 1 to 4) was studied by SEM after tensile, compression and impact tests. The studies reflect the occurrence of a combination of ductile and brittle fractures. β-Mg 17 Al 12 obstructed the commencement of cracks and deformation in the AZ91 matrix [52].

Fracture Surface Analysis
As depicted in Figures 13-15, the fracture morphology of pure AZ91 and reinforced AZ91 composites (codes 1 to 4) was studied by SEM after tensile, compression and impact tests. The studies reflect the occurrence of a combination of ductile and brittle fractures. β-Mg17Al12 obstructed the commencement of cracks and deformation in the AZ91 matrix [52]. The bunting cracks, microcracks and cavities detected in the 1 wt.% SiCp-reinforcement-added composite alloy (code 3) shown in Figure 12c were more than those in alloy codes 1 and 2 in Figure 12a,b, respectively. Voids are inevitable in the stir casting method with environs but can be governable. These voids exhaust the gas induced in the contiguous particles and augment viscosity. The creation of voids and a secondary phase formulates the stress concentration between particles and matrix. Furthermore, the large number of particles leads to a higher concentration owing to the higher dislocation density [17]. When the load increases during tensile tests, the cleavage facet transfers and breaks the intrinsically brittle structure in the transverse direction of the plane [52]. The load can be moved from the α-Mg matrix to the reinforcement particles; at this moment, an enormous crack is initiated by the reinforcement particles [11]. The tensile values of alloy code 4 decrease slightly because of the reinforcement increments, which can formulate a large number of microcracks. In addition, the presence of Al-Mn-based components (Table 9) can substantially reduce mechanical strength [51].
The number of shear bands (shear tongues) that exist in the fractured surface increased due to the cumulative nano-and micro-reinforcements [11] in alloy codes 2 to 4 The bunting cracks, microcracks and cavities detected in the 1 wt.% SiCp-reinforcementadded composite alloy (code 3) shown in Figure 12c were more than those in alloy codes 1 and 2 in Figure 12a,b, respectively. Voids are inevitable in the stir casting method with environs but can be governable. These voids exhaust the gas induced in the contiguous particles and augment viscosity. The creation of voids and a secondary phase formulates the stress concentration between particles and matrix. Furthermore, the large number of particles leads to a higher concentration owing to the higher dislocation density [17]. When the load increases during tensile tests, the cleavage facet transfers and breaks the intrinsically brittle structure in the transverse direction of the plane [52]. The load can be moved from the α-Mg matrix to the reinforcement particles; at this moment, an enormous crack is initiated by the reinforcement particles [11]. The tensile values of alloy code 4 decrease slightly because of the reinforcement increments, which can formulate a large number of microcracks. In addition, the presence of Al-Mn-based components (Table 9) can substantially reduce mechanical strength [51].
The number of shear bands (shear tongues) that exist in the fractured surface increased due to the cumulative nano-and micro-reinforcements [11] in alloy codes 2 to 4 compared with alloy code 1, as shown in Figure 14. The cavities and dimples in the fracture surface decreased from alloy codes 2 to 4. In addition, the value of the strengthening mechanism had an increasing trend, as shown in Table 8. The cohesion force between matrix and refinement increased and dimples reduced because of grain refinement. In addition, the interface between plastic flow was due to the rigid reinforcements in the AZ91 matrix [17].
In alloy code 2, β-Mg 17 Al 12 diffused along the grain boundary increased the impact property dramatically. The presence of Mg 2 Si in alloy code 3 can initiate the formation of micro-and nano-cracks [12]. However, the impact value of alloy code 4 (AZ91 + 0.5 wt.% SiCp + 1 wt.% Bi + 0.4 wt.% Sb) was lower than that of as-cast AZ91 because of the number of reinforcements whose crystal structures existed in the matrix.
The twin boundary fracture, microcracks and surface cracks in the impact test fracture morphology are shown in Figure 15. Alloy code 4 had huge dimples, microcracks and surface cracks compared with the other alloy codes, as shown in Figure 15d. The assortment of C 60 , SiCp and Mg 2 Si heterogeneous phases can reduce the impact strength of the materials, and the presence of huge Al 6 Mn phases also plays a vital role in reducing the strength of the impact values [51]. The HCP structures of twin boundary cracks were observed in alloy code 4, and these major boundary cracks, which lead to the reduction of energy values, are shown in Figure 15d.

Conclusions
The different amalgamations of the hybrid nanoelement (β-SiCp) and microelements (Bi and Sb) of AZ91 (alloy codes 1 to 4) MMCs were investigated. Microstructure evaluation, phase quantitative analysis, the strengthening mechanism (CTE), the shear transfer effect and the Orowan effect and their impact on mechanical properties (tensile, compression and impact) were studied in detail. The following conclusions are derived from the present study:

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The addition of β-SiCp (0.5 and 1 wt.%) nanoparticles resulted in grain refinement, particle strengthening and creation of quantitative phases Mg 2 Si (cubic) and SiC (rhombo.h.axes), which improved the microhardness of the AZ91 matrix by 18