Properties of ceramic-reinforced aluminium matrix composites - a review
© Das et al.; licensee Springer 2014
Received: 3 July 2014
Accepted: 29 July 2014
Published: 20 August 2014
A review of various properties of ceramic-reinforced aluminium matrix composites is presented in this paper. The properties discussed include microstructural, optical, physical and mechanical behaviour of ceramic-reinforced aluminium matrix composites and effects of reinforcement fraction, particle size, heat treatment and extrusion process on these properties. The results obtained by many researchers indicated the uniform distribution of reinforced particles with localized agglomeration at some places, when the metal matrix composite was processed through stir casting method. The density, hardness, compressive strength and toughness increased with increasing reinforcement fraction; however, these properties may reduce in the presence of porosity in the composite material. The particle size of reinforcements affected the hardness adversely. Tensile strength and flexural strength were observed to be increased up to a certain reinforcement fraction in the composites, beyond which these were reduced. The mechanical properties of the composite materials were improved by either thermal treatment or extrusion process. Initiation and growth of fine microcracks leading to macroscopic failure, ductile failure of the aluminium matrix, combination of particle fracture and particle pull-out, overload failure under tension and brittle fracture were the failure mode and mechanisms, as observed by previous researchers, during fractography analysis of tensile specimens of ceramic-reinforced aluminium matrix composites.
KeywordsAluminium matrix composites Optical Physical Mechanical Properties
Ceramic-reinforced aluminium matrix composite materials are well known for their high strength-to-weight ratio, superior tribological properties and corrosion resistance behaviour, for which they are replacing their monolithic alloys in the field of automobile, marine and aviation engineering. Since the last three decades, researchers have shown their interest in these materials and are trying to improve their property to make them suitable for use in complex areas.
The strength of composite materials depends upon composition, grain size, microstructure and the fabrication process. The objective of this paper is to review the effect of the fabrication process on particle distribution and the effect of reinforcement fraction, particle size, heat treatment and extrusion process on physical and mechanical properties of ceramic-reinforced Al matrix composites, as experienced by previous researchers.
Optical and physical properties
A uniform distribution of SiC particles was observed in AA 7075/SiC composite, fabricated using stir casting method, at a stirring speed of 650 rpm and stirring time of 10 min (Bhushan and Kumar 2011). Vanarotti et al. (2012) observed a homogeneous distribution of SiC particles in the cast Al 356/SiC (5 and 10 wt.%) composites, fabricated by stir casting technique, under a metallurgical microscope. The particles showed a strong tendency to accumulate in the colonies which froze in the last stage of solidification and contained eutectic phases, and the SiC particles were also observed to be accommodated on the grain boundaries. During microstructural investigation of SiC-reinforced Al 6063 matrix composites using a metallurgical microscope, Alaneme and Aluko (2012) observed that the volume percent of SiC did not influence its pattern of distribution either in the as-cast condition or in the heat-treated (solution treatment followed by age hardening) condition. Microstructural features of bamboo leaf ash (BLA)- and SiC-reinforced Al-Mg-Si alloy hybrid composites, fabricated by a two-step stir casting process, revealed good distribution of the reinforcing particles in the matrix with minimal particle clusters (Alaneme et al. 2013). Boopathi et al. (2013) observed non-uniformity in the distribution of reinforced particles in the case of Al-SiC and Al-fly ash composites; however, their uniform distributions were observed in the micrographs of Al-SiC-fly ash hybrid composite, fabricated by stir casting technique. Umanath et al. (2013) observed a uniform distribution of ceramic reinforcements in Al 6061/SiC/Al2O3-T6 heat-treated hybrid metal matrix composites, processed by stir casting method.
Density and porosity of SiC-reinforced AA 1050 matrix composites (Manoharan and Gupta 1999 )
Density (g cm−3)
Al-6 wt.% SiC
2.71 ± 0.03
Al-8 wt.% SiC
2.72 ± 0.02
Al-17 wt.% SiC
2.75 ± 0.07
Veeresh Kumar et al. (2012) observed that the density of Al 7075-SiC composites increased with SiC contents and was in line with the values obtained by the rule of mixtures. Alaneme et al. (2013) evaluated the percent porosity of BLA- and SiC-reinforced hybrid Al composites by comparing their theoretical and experimental densities. The experimental density was determined by dividing the measured weight of the test sample with its volume, while the theoretical density was evaluated by using the rule of mixtures. The density of the cast composites was observed to be reduced with the increase in BLA content; however, the percent porosity did not show any significant trend with the increase in BLA content. For all the cast composites, the percent porosity was within the acceptable limiting value of 4%. Boopathi et al. (2013) reported that in the presence of silicon carbide and fly ash in aluminium, the density of hybrid composites decreased. Umanath et al. (2013) observed more porosity around Al2O3 particle reinforcement as compared to the location around SiC particle reinforcement in Al 6061/SiC/Al2O3-T6 heat-treated hybrid metal matrix composites. It was also reported that the porosity of the specimens increased with increasing volume fractions of the particulate reinforcement.
Vanarotti et al. (2012) observed that the Brinell hardness number of Al 356/SiC composite increased with the increasing weight fraction of SiC reinforcement in the matrix alloy. The BHN was observed to be 70 and 78 for 5 and 10 wt.% of SiC reinforcement, respectively. Alaneme et al. (2013) reported that the hardness of SiC- and bamboo leaf ash-reinforced Al alloy hybrid composites decreased with the increase in BLA content. Boopathi et al. (2013) evaluated the Brinell harness number of Al-SiC, Al-fly ash and Al-SiC-fly ash metal matrix composites and reported that aluminium in the presence of 10% of SiC and 10% of fly ash was the hardest instead of Al-SiC and Al-fly ash composites.
Tensile strength and ductility
Room temperature mechanical properties of the extruded SiC/AA 1050 composite samples (Manoharan and Gupta 1999 )
Experimental fracture strain
Calculated fracture strain
Al-6 wt.% SiC
93.8 ± 6.2
104.4 ± 5.1
0.17 ± 0.08
Al-8 wt.% SiC
97.4 ± 3.6
113.3 ± 2.8
0.16 ± 0.02
Al-17 wt.% SiC
80.5 ± 2.3
120.3 ± 7.9
0.10 ± 0.01
Alaneme et al. (2013) reported that ultimate tensile strength, yield strength, specific strength and percent elongation of SiC- and BLA-reinforced Al-Mg-Si alloy hybrid composites decreased with the increase in BLA content, and this trend was due to the presence of silica in the BLA, which has lower hardness and strength values in comparison with SiC. Boopathi et al. (2013) observed that the presence of SiC and fly ash in Al 2024 alloy improved its tensile strength and yield strength, however reduced its ductility.
Compressive strength, flexural strength and toughness
Izod test results of SiC-fly ash-reinforced hybrid Al 6061 matrix composites (Ravesh and Garg 2012 )
Fly ash (wt.%)
Izod test result
Alaneme et al. (2013) reported that the fracture toughness of SiC- and BLA-reinforced hybrid Al-Mg-Si alloy composites improved with the increase in BLA content, which may be attributed to the increased presence of silica which is a softer ceramic in comparison with SiC. The fracture toughness of the hybrid composite was observed to be superior to that of the single reinforced Al‐10 wt.% SiC composite.
Specimen preparation is an important aspect for microstructural examinations of any metal, alloy or composite. The first step of specimen preparation is metallographic polishing, using emery cloths, ranging from coarse to very fine grades, and then by using diamond paste to get a mirror finish on the surface. For a detailed study of the microstructures and grain boundaries of the matrix or reinforcement, the polished sample is to be etched using some suitable etching agent. Kellor's reagent is used by most of the researchers (Cocen and Onel 2002, Rao et al. 2010 and many more) for etching; however, equal proportions of HNO3 and HCl were used as etching agent by Alaneme et al. (2013).
Some researchers have observed only the uniformity in distribution of reinforced particles or whiskers in the matrix phase, whereas others have investigated thoroughly for the metallurgical aspects of metal matrix composites through a high-resolution microscope. It is easy to attain uniformity in distribution of reinforced particles in the matrix phase, when the MMC is developed through solid-state processing. However, solid-state processing is not economical and also not suitable for mass production, as compared to the stir casting method of processing of MMCs. One of the major challenges in composite fabrication is the uniformity in distribution of reinforced particles, which affects directly the properties and quality of the composite material (Singla et al. 2009). Some researchers claim the uniform distribution of reinforced particles with localized agglomeration at some places, when the MMC is processed through liquid metallurgy or stir casting method.
Most of the researchers have determined the density of MMCs using the Archimedes principle; however, some have also determined it by dividing the measured weight of the test samples with their volume. Theoretical density has been calculated using the rule of mixtures and percentage of porosity by comparing the experimental density with the theoretical density. In most of the cases, measured (experimental) density was found to be increased with reinforcement fraction (Manoharan and Gupta 1999; Demir and Altinkok 2004; Veeresh Kumar et al. 2012); however, Sahin and Murphy (1996), Purohit et al. (2012) and Alaneme et al. (2013) observed it to be reduced with the increase in reinforcement. Increased porosity in the composites was claimed as the basic reason for reduction of density with the increase in reinforcement content.
Researchers have determined the hardness of ceramic-reinforced aluminium matrix composites in various units, such as HV, HB, HRB and HRC. Most of the researchers observed the hardness to be improved with the increase in reinforcement fraction (Sahin and Murphy 1996; Veeresh Kumar et al. 2010; Rao et al. 2010; Bhushan and Kumar 2011; Purohit et al. 2012; Uvaraja and Natarajan 2012; Ravesh and Garg 2012; and many more); however, Suresha and Sridhara (2012) and Alaneme et al. (2013) observed the hardness to be reduced with the increase in reinforcement content in the composite, and the presence of porosity may be the reason for the reduction of hardness. The particle size of reinforcement had an adverse effect on hardness (Deshmanya and Purohit 2012). The hardness of ceramic-reinforced composites improved by heat treatment (Rao et al. 2010), ageing temperature (Song et al. 1995) and ageing time (Kalkanli and Yilmaz 2008).
From the open literature, it was observed that Young's modulus, yield stress, ultimate tensile stress and breaking (fracture) stress of ceramic-reinforced aluminium matrix composites were higher than those of their monolithic alloys and increased with the reinforcement fraction of ceramic materials; however, the ductility (percent elongation) of the composites reduced. Manoharan and Gupta (1999) observed that the yield strength first improved and then reduced with the increase in SiC content in the aluminium matrix composite. The particle size of reinforcing materials affected the yield strength and tensile strength of the composite adversely (Ravi Kumar and Dwarakadasa 2000). The tensile strength of the composite can be improved by thermal treatment (Xu et al. 1997; Kalkanli and Yilmaz 2008) or extrusion (Manoharan and Gupta 1999; Cocen and Onel 2002). However, ductility increased greatly and the yield stress reduced drastically by HIP treatment of SiCp/Al 359 matrix composite (Xu et al. 1997).
The mechanism and mode of failure during tensile testing of aluminium matrix composites has been reported in various ways by different authors. Srivatsan and Prakash (1995) reported that initiation and growth of fine microcracks lead to macroscopic failure of the composite; however, Lu et al. (1999) observed that the failure of the composite was controlled by ductile failure of the aluminium matrix, and it was due to the nucleation, growth and coalescence of voids. Combination of particle fracture and particle pull-out was reported by Ravi Kumar and Dwarakadasa (2000) to be the fracture mechanism of the AMC. Vanarotti et al. (2012) reported that overload failure under tension was the fracture mechanism of the Al 356 matrix alloy and 5 wt.% SiC-reinforced Al 356 matrix composite; however, brittle fracture was observed for a higher weight fraction (10%) of SiC-reinforced Al 356 matrix composites.
Compressive strength was found to be increased with the increase in reinforcement fraction in the aluminium matrix composites (Ravi Kumar and Dwarakadasa 2000; Purohit et al. 2012) and with increasing strain rate during compression (Lu et al. 1999).
The flexural strength (bending strength) of ceramic-reinforced aluminium composites increased with increasing reinforcement content up to 10 wt.% (Kalkanli and Yilmaz 2008) and up to 13 vol.% (Demir and Altinkok 2004), beyond which it reduced.
The toughness (impact strength) of ceramic-reinforced aluminium matrix composites increased with the increase in reinforcement fraction (Ravesh and Garg 2012; Alaneme and Aluko 2012) or by ageing treatment (Alaneme et al. 2013).
It was difficult to attain a perfectly homogeneous distribution of reinforced particles in the matrix phase, when the aluminium matrix composites were processed through liquid metallurgy or stir casting method.
Density in the aluminium matrix composite was found to be increased with reinforcement fraction; however, increased porosity levels in the composite caused reduction in density.
It was observed that the hardness of aluminium matrix composites can be improved with the increase in reinforcement fraction or by reducing the particle size of reinforcement; however, the presence of porosity affects hardness adversely. The hardness of ceramic-reinforced composites can also be improved by heat treatment, ageing temperature and ageing time.
Young's modulus, yield stress, ultimate tensile stress and breaking (fracture) stress of ceramic-reinforced aluminium matrix composites were higher than those of their monolithic alloys and increased with the reinforcement fraction of ceramic materials; however, the ductility (percent elongation) of the composites reduced.
Fractography studies revealed that the mechanism and mode of failure during tensile testing of aluminium matrix composites may be due to initiation and growth of fine microcracks leading to macroscopic failure, ductile failure of the aluminium matrix, combination of particle fracture and particle pull-out, overload failure under tension and brittle fracture.
The compressive strength of ceramic-reinforced aluminium matrix composites was found to be increased with the increase in reinforcement fraction in the aluminium matrix composites and with increasing strain rate during compression.
The flexural strength (bending strength) of ceramic-reinforced aluminium matrix composites increased up to a certain percentage of reinforcement, beyond which it reduced.
The toughness (impact strength) of ceramic-reinforced aluminium matrix composites increased with reinforcement fraction or by ageing treatment.
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