**2. Mechanical and physical properties**

Metal matrix composites have been shown to exhibit significant improvements in certain physical and mechanical properties over their monolithic metallic counterpart, However, the mechanical properties are strongly dependent on micro structural parameters, in particular, size, shapes volume fraction and orientation of the particles and the composition of matrix.


Table 2. Properties of Al 6061 and Al 7075 with and without reinforcement

It is a general observation that the Vickers microhardness observed is greater than the matrix alloy. This is a exemplified by composites, 6061/SiC(p), 6013 SiC(p) and 7075/Al2O3(p) Figure 1 shows the effect of Vol.% of particulates (SiC) on the modulus of elasticity of Al 6061 / SiC, and Al 7075/ Al2O3 composites (Vaeeresh et al., 2010).

The development of metal matrix composites has been a major breakthrough in the last twenty years. The quantum leap in recent years has established their potential for weight critical application in engineering components and structures in aerospace.

It is shown that the tensile strength is increased with increasing volume fraction of SiC particulates. This applies to all metal matrix composites including discontinuously reinforce composite reinforced with SiC particulates or whiskers Figure 2 and 3. The effect of strength may be attributed to the generation of dislocations on cooling of the metal matrix

Continuous reinforcement uses monofilament fibers, wires or fibers such can carbon fibers. The reinforcement materials commonly used are graphite SiO2, SiC, TiC, Al2O3 and glasses.

0 2 4 6 % Filler

Fig. 1. Microhardness of Al6061-SiC and Al7075-Al2O3 composites (Vaeeresh et al., 2010)

Metal matrix composites have been shown to exhibit significant improvements in certain physical and mechanical properties over their monolithic metallic counterpart, However, the mechanical properties are strongly dependent on micro structural parameters, in particular, size, shapes volume fraction and orientation of the particles and the composition

Parameters Al 6061 Al 7075 SiC Al2O3 Flastic Modulus 70 – 80 70 – 80 410 300 Density 2.7 2.81 3.1 3.69 Poisson's Ratio 0.33 0.33 0.14 0.21 Hardness (HB – 500 ) 30 60 28 W 1175 Tensile Strength(MPA) 115 220 3900 2100

It is a general observation that the Vickers microhardness observed is greater than the matrix alloy. This is a exemplified by composites, 6061/SiC(p), 6013 SiC(p) and 7075/Al2O3(p) Figure 1 shows the effect of Vol.% of particulates (SiC) on the modulus of

The development of metal matrix composites has been a major breakthrough in the last twenty years. The quantum leap in recent years has established their potential for weight

It is shown that the tensile strength is increased with increasing volume fraction of SiC particulates. This applies to all metal matrix composites including discontinuously reinforce composite reinforced with SiC particulates or whiskers Figure 2 and 3. The effect of strength may be attributed to the generation of dislocations on cooling of the metal matrix

Table 2. Properties of Al 6061 and Al 7075 with and without reinforcement

critical application in engineering components and structures in aerospace.

elasticity of Al 6061 / SiC, and Al 7075/ Al2O3 composites (Vaeeresh et al., 2010).

120

100

Al6061-SiC Al7075-Al2O3

80

**2. Mechanical and physical properties** 

of matrix.

V H N

composites. Such dislocations have been observes by TEM. A high dislocation density was observed on Al 6013/SiC (p) interface.

In a TEM experiment, the generation of dislocations started only at 500 K (Vogelsang et al. 1986). It has also been suggested that dislocation were generated in Al – 6061/ 20 SiC MMC below 573 K.

Fig. 2. Effect of the size of the platelets (Massardier et al., 1993)

Fig. 3. Young's modulus vs volume percent of SiCw, SiCp and reinforcement (Zaki, 2001)

The elongation (%) of the MMC decreased with increased particulate contents as shown by Al 6061 / 20 SiC (p) – The mechanism of fracture toughness is not fully understood. The presences of large clusters of particles promote crack propagation whereas their uniform distribution retards crack propagation. The fracture toughness values of selected alloys are given in Table 3.

Corrosion Behavior of Aluminium Metal Matrix Composite 389

The above studies were conducted in accordance with ASTM designation G 31 – 72 (ASTM, 2004). The results of studies on Al6092 – T6, Al/*B*4C/20P, Al 6092 – T6 /2oSiC(p), and 6092 – T6 20vol%Al2O3 and monolithic 6061-T6 Al, immersed for 90 days in air exposed 0.5 Na2SO4 solution, 3.5 wt% NaCl, ASTM sea water and real sea water were recently described (Hongho et al. 2009). In alloy 6092 – T6 Al/B 4C/20P MMC specimen in ASTM Sea water bubbles were observed. The current over most of the area was found to be anodic. The solution at the anode site was found to be acidic (PH 6.4). Corrosion products were formed as observe after monitoring for three days and the area became more alkaline (PH 8.4). A similar phenomena occurred with alloy 6092 reinforced with 20 Vol. % SiC (p) and gradually the alkalinity increased because of its change of area from and anodic to cathodic. The corrosion rates of MMCS in sea water and ASTM sea water were lower than those in 0.5 M Na2SO4 and 3.5 wt % NaCl. The rates of monolithic 6061 – T6 Al in both real and ASTM sea water were significantly lower than those in 3.5 wt % NaCl. The surface morphology after the test showed similar general features, one major feature of the surface morphology was the presence of intermetallic precipitates on the surface. The EDS studies suggested these precipitates to contain Al, Mg, O, and C. Mg and HCO3 irons as the main species

The formation of precipitates is a greater concern in MMC, as localised corrosion is controlled by the formation of such precipitates. The role of precipitates would be discussed in the relevant section of the paper. In general the corrosion rate of Al MMC decreased with

If is generally accepted that MMC are in general more prone to corrosion than their monolithic counterparts (Berkely et al., 1998; Turnbull and Corros, 1992; Trzskoma, 1991). Conflicting views have been presented on the causes of the localised corrosion. The results of the studies showed that galvanic corrosion between the matrix and the reinforcement occurs. However, this is related to the machining conditions. Three different machining process; Wielding Electrical Discharge Machine (WEDM), Cemented Carbide Turning and Single Point Diamond Turning were employed for investigation. The test results for

 E Corr (mV) Epitl (mV) E Pil – E corr (mV) I Corr (Am-l) WED –761.4 – 633 v 128.4 3.80 TE – 4

Carbide Turning – 673.6 – 655 186 3.194 E – 2 Diamond Turning – 928.3 – 655 288.3 1.052 E – 3 Table 5. Electrochemical parameters for different machining conditions (Yue et al., 2002)

a. Immersion and long term exposure tests in sea water or 3.5 wt % NaCl.

b. Localized corrosion studies

d. Corrosion inhibition e. Corrosion mechanism

corrosion products.

time due to the formation of precipitates.

different process are shown in Table 5 (Yue et al., 2002).

**3.2 Localled corrosion of ALMMC's** 

c. Flow induced corrosion and Erosion corrosion

**3.1 Immersion & long term exposure studies** 


Table 3. Fracture toughness of selected MMCS

The strains to failure (%) for different Al2O3 reinforcement are shown in Table 4 Strain to failure decreases with increase of volume fraction of reinforcement.


Table 4. Strain to failure of Alloy Al 6061 with increasing volume fractions (Dehlan and Syed, 2006)

Al MMC are finding increasing applications as rotor material in automotive brake systems (Shorowords et al., 2004). Effect of Studies on the effect of sliding velocity on wear friction and tribochemistry of MMC reinforced with 13% SiC or B4C have shown that sliding velocity leads to lower wear rates and lowers friction coefficient for both MMCs.. Studies on interaction between MMC and phenolic brake pads showed that the transfer layer consisting of phenolic pad material acted as a protective layer and reduced wear rates and coefficient of friction. Honda has used aluminum metal matrix cylinder liners in some of their engines including B2lAl and H23A, F20 C and F22C.

The effect of cutting speed on tool wear has been investigated. The cutting tool wear increased with increased reinforcement ratios. At constant speed and feed rate, the lowest wear rate has been found in 5 Wt % SiC (p) and the highest wear with 15 Wt % SiCp increased cutting speed increased the tool wear rate.

From the above description, it may be concluded that the development of MMC has been a big breakthrough in search for stiff high strength materials for aerospace and automotive industry particularly. Whereas the mechanical properties of MMC have remained the focus of attention, the work on corrosion behavior of MMC did not proceed hand in hand with the mechanical and tribological properties. The work on corrosion was undertaken the last decade and a considerable progress has been made in the understanding of corrosion behaviour of metal matrix composites in recent years.
