Microstructure Analysis of Aluminium and Hybrid Reinforced Metal Matrix Al6061/B4C/ZrO2/SiC Composites

Introduction

Table 1. Mechanical Properties of Matrix Alloy AA-6061.

Figure 1. SEM image of AA6061.

Figure 1. SEM image of AA6061.

Table 2. AA6061 chemical properties.

Reinforcement Materials

Boron Carbide (B₄C)

Boron Carbide is a high hardness material. The use of boron carbide (B₄C) as a reinforcing material is predicted to improve mechanical rigidity, toughness, abrasion resistance, and elasticity. The combination should be impeccably clean, resistant to degradation, and sufficient to bear high temperatures. Due to the first beneficial features of the B₄C, hybrid metal matrix compounds have been postulated to use in automobile industry. The addition of B₄C is to improve the tribological behaviour of Metal Matrix Composites (MMS). Boron Carbide (B₄C) is a very highly abrasion substance, third in toughness only to diamonds and cubic boron nitride. It simply outperformed conventional abrasives due to their superior and economical effectiveness, including SiC and Al2O3. Manufacture of machinery and computer components, finishing and polishing of random products, High frequency trenches, Neutron integration in nuclear reactors, Antioxidant in super alloys, soldering for production of hard-faced anode and cathode, Booster propulsion systems and heavy-duty armored panels (including all-metal) are examples of processes. Compared to silicon carbide, the formation of the oxide film of silicon dioxide delays the high oxidation, which is a deficiency of boron carbide, which begins to oxidize at temperatures above 500 C.

Figure 2. SEM image of B₄C.

Figure 2. SEM image of B₄C.

Table 3. B₄C chemical properties.

Table 3. B₄C chemical properties.

Chemical	Percentage
C	19.7
B	79
Fe	1
Si	0.5
Ca	0.3
F	0.03

Table 4. Physical Properties of B₄C.

Table 4. Physical Properties of B₄C.

Material	Compressive strength	Youngs modulus	Bending strength	Melting Point
B4C	1960-3922MPa	4500 kg/mm2	30-50 kg/mm2	24550 C

Zirconium Dioxide (ZrO₂)

Zirconium Dioxide is also called zirconia and white crystalline oxide of zirconium. Baddeleyite is the most common form of mineral that has a monoclinic structural phase. Spherical zirconium, a stimulant concentrated cube shaped zirconia, is produced in various shades to be used as a crystalline material in stone and crystalline substances. The three known phases are monoclinic 11700 C, for tetragonal up to 23700 C and for cubic 23700 C.

Figure 3. SEM image of ZrO_2.

Figure 3. SEM image of ZrO_2.

Table 5. Properties of ZrO_2.

Table 5. Properties of ZrO_2.

Properties	Value
Density (g/cm3)	5.68
Melting point (0C)	2715
Appearance	Powder form white
Molar mass	123.218 g/mol

Silicon Carbide (SiC)

Silicon Carbide is a hard chemical compound enhanced with silicon and carbon. To improve the mechanical strength ratio of the compound, most researchers used SiC particles as a reinforcing material. Compared to other conventional alloys, SiC has higher rigidity, strength and wear resistance and good thermal conductivity. Silicon carbide was first used in technical circuits such as light-emitting diodes (LEDs) and sensors in earlier transmitters. SiC is an n-type transistor used in the semiconductor of electronic smart devices at high temperatures and high voltages.

Figure 4. SEM image of SiC.

Figure 4. SEM image of SiC.

Table 6. Chemical properties of SiC.

Table 6. Chemical properties of SiC.

Chemical	Wt. % Percentage
c	0.3
Al	0.1
Fe	0.08
SiO2	0.5
Si	0.3
SiC	98.5

Table 7. Thermal and Mechanical properties of SiC.

Table 7. Thermal and Mechanical properties of SiC.

Properties	Value
Melting Point	2730°C
Density	3.22 g/cm3
Poisson’s Ratio	0.35
Tensile strength	0.1379 GPa
Yield strength	21 GPa
Elongation	6%
Thermal Conductivity	120 W/m-K
Modulus of Elasticity	90 GPa

Research Methodology:

Microstructure Analysis

In this investigation, aluminium alloy 6061 was preferred as matrix material and B₄C, ZrO₂ and SiC powder particles as reinforcing materials. Figure 5, Figure 6, Figure 7 and Figure 8 illustrates the schematic SEM image of matrix and reinforced materials used in this study. For micro-structural study, the stir casting samples of various composites were divided into small sections (15 mm x 20 mm x 10 mm). The investigations reveal that the Al6061 matrix powder particle consisted of irregularly shaped spheres with an average grain size of less than 100 µm. Also, the B₄C powders were found to have a complex and rounded structure with a grain size distribution ranging from 2-8 μm. The SiC particles had a complex shape and ranged from 1 to 8 μm in size. After the stir casting, particle distribution in AA6061 matrix and effect of reinforcement was revealed by Optical Electron Microscope (OEM). As can be seen in Figure 9c–e, no defects were observed in the structure of Al6061/B₄C/ZrO₂ /SiC in the optical photograph. This occurred as a result of high densification of the reinforcing materials simultaneously with permanent deformation throughout the stir casting process. This analysis found that the B₄C/ZrO₂/SiC particles in the Al6061/2%B₄C/6% ZrO₂ /12%SiC composites generally have a uniform dispersion in the matrices for the optical image of reinforced with ZrO₂ /B₄C/SiC composites, as shown in Figure 9e. In the microstructures of AA6061+2% B₄C and AA6061/2%B₄C/4% ZrO₂ /8%SiC composites, it was shown that the production of agglomerations, microcracks are observed with the increase in ZrO₂ content. Even though there were smaller pores generated in the microstructure of 12% SiC hybrid composites, it showed a higher amount of agglomeration. During the stir casting process, it was found that particle breakage occurred at some interfaces as a result of particle interaction. The research revealed a homogenous composition of SiC particles in 12% SiC compositions in the functional structure in the AA6061 matrix. The structural study shows that the was equally scattered within the hybrid matrix material and therefore, observed negligible agglomerations of SiC and B₄C particles.

Figure 5. SEM image of particle distribution for AA6061 alloy.

Figure 5. SEM image of particle distribution for AA6061 alloy.

Figure 6. SEM image of particle distribution for B₄C alloy.

Figure 6. SEM image of particle distribution for B₄C alloy.

Figure 7. SEM image of particle distribution for SiC alloy.

Figure 7. SEM image of particle distribution for SiC alloy.

Figure 8. SEM image of particle distribution for ZrO₂ alloy.

Figure 8. SEM image of particle distribution for ZrO₂ alloy.

Figure 9. Optical Electron Microscope (OEM) of (a) AA 6061 (b) 2%B₄C (c) 2%B₄C / 2%ZrO₂/4%SiC (d) 2%B₄C /4%ZrO₂/8%SiC (e) 2%B₄C /8%ZrO₂/12%SiC.

Figure 9. Optical Electron Microscope (OEM) of (a) AA 6061 (b) 2%B₄C (c) 2%B₄C / 2%ZrO₂/4%SiC (d) 2%B₄C /4%ZrO₂/8%SiC (e) 2%B₄C /8%ZrO₂/12%SiC.

EDS Analysis

After the stir casting process, EDS analysis was carried out for both pure aluminium alloy and their reinforced composites. When examining SEM micrographs of the pure aluminum alloy, the researchers found that the microstructure rarely contained porosity (Figure 9). By using elemental composition analysis, alloy composition percentages in the structure were also estimated. Different zones of the hybrid material were subjected to elemental (EDS) analyses. The presence of B₄C particles can be shown in Figure 12, where the EDS analysis of spectrum 4 shows a significant boron content and high carbon ratio. It is clear from the EDS results from spectrum 4 that this particle is SiC due to its high silicon and carbon composition [16].

From the EDS in Figure 12, examination of the SEM image in Figure 9c shows that cavities have formed around the B₄C particles. These are the result of poor wetting properties of the matrix phase and B₄C particles. The wettability of aluminum with B₄C reinforcement has been shown to be problematic in most of the research.

Figure 13. EDS analysis of AA6061+2%B₄C+4ZrO₂+8%SiC alloy.

Figure 13. EDS analysis of AA6061+2%B₄C+4ZrO₂+8%SiC alloy.

It is clear from the EDS tests from spectrum 4 that this particle is SiC due to its high silicon and carbon content, as shown in Figure 14. The increases of 12% SiC particle on AA6061 matrix leads to form the clustering of particles and agglomeration, which was influenced mechanical properties of hybrid composites. According to the transient contact bond between the matrix AA 6061 and the 12% SiC reinforcement, these defects are detected by the crystallization process.

Fractography Analysis

The tensile test was carried out by Universal Testing Machine (UTM) at room temperature. During tensile testing, microstructural studies were carried out to investigate the fracture behaviors and the bonding of the interfaces between the matrices and the reinforcing elements. Samples were taken from complete sections of the fractured surfaces and mounted in SEM to examine the morphologies of the fractured surfaces. SEM observations of the fractured surfaces of Pure Al6061, B₄C (2%), ZrO₂ (2%, 4%, and 6%) and SiC (4%, 8%, and 12%) hybrid composites were obtained at different magnifications. The superior ductility in the production of pure aluminum matrix preferred hybrid composites leads to micro-perforation on every surface of the fractured material.

Higher quality dimple formation was observed on the fractured surface due to the composition of pure Al6061 material exhibiting higher ductility than the hybrid reinforced composite materials.

Figure 15. After the tensile test SEM image of the fractured surfaces.

Figure 15. After the tensile test SEM image of the fractured surfaces.

Dimple generation in the fractured regions is increasing by evaluating the compositions of the particles associated with the growth of the boron carbide ratio. This developed from the growth of B₄C reinforcement, which makes composite materials more brittle. From the test results, more dimples formation was observed in Al6061-2%B₄C-6%ZrO₂-12%SiC hybrid composites when compared to pure Al6061 alloys and Al6061+2%B₄C reinforcement. As a result of tensile test, 6% of ZrO₂ and 12% of SiC reinforcement shows greater tensile strength, indicating high ductility of hybrid composite materials.

Fractured surfaces are separated from the matrix by addition of B₄C reinforcement particles. Due to insufficient moisture content, the matrix phase and B₄C particles in B₄C-reinforced composites have a weak bond. The B₄C particles migrate more easily from the matrix and remain on both sides of the fracture surfaces when the material breaks. Furthermore, in SiC-reinforced hybrid composites, it is observed that the SiC particles are incorporated into the matrix phase during tensile due to better adhesion at the interface.

Figure 16 shows the feature of unbroken base alloy interfaces with strong matrix bonding. Micrograph observation demonstrates more ductile fracture in the A16061 alloy, which is mainly composed of SiC particles. Compared to the Al6061 composite, the fractured surface exhibits a decrease in the size of the uniformly distributed particles. The activity of dimples during neck formation in the Al 6061 matrix is visible in the samples prepared at each percentage of hybrids. In hybrid composites including B₄C, ZrO₂ and SiC, diffusion of reinforcement, residual fragments and tensile zones are clearly observed on these fractured surfaces by Scanning Electron Microscope (SEM).

Figure 17 and Figure 18 shows that no deformation occurs during significant stress transfer between the reinforcement and the matrix. There are some elongated dimples and deformed fragments in the matrix phase, which demonstrates the ductility nature of the hybrid composites. It can be shown through the fracture evaluation and the tensile strength that the addition of SiC particulates has effects over influencing particles. This particular hybrid composites showed indications of dimples, predicted ductile failure and had no fracture after transfer of load, and contained particles that were maintained with dimples. Also, the ductility of the (Al6061/B₄C/ZrO₂/SiC) hybrids is reduced as a result of the performance of the SiC and ZrO₂ particles as a stress concentration element and a propagating crack reducer during load transfer [17].

Conclusion

• Using a stir casting approach to examine the Al6061 microstructure and found that there were no pores in the hybrid composites (AA6061+B₄C/ZrO₂/SiC). The study discovered that the microstructures of the B₄C/SiC-reinforced composites typically showed a homogeneous dispersion of ZrO₂/SiC particles inside the matrix of 6%ZrO₂+12%SiC composites.
• Agglomeration increased simultaneously with increase in B₄C in 2%B₄C and pure AA6061 compositions. Less porosity was observed in the 12%SiC hybrids, although they exhibited a higher degree of aggregation than the micro-structure when compared to the other composites.
• The Scherrer equation was used to determine the crystal size. As a result, AA6061 had a larger crystal size of 42.58 nm as opposed to the hybrid composites of AA6061/2%B4C/8%ZrO2/12%SiC, which had a crystal size of 28.6 nm.

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