The matrix for the present studies selected was Al2024 alloy and were procured from Fenfee Metallurgical, Bangalore, in the form of ingots. The chemical composition of Al2024 alloy was supplied commercially with the chemical composition (in weight percent) of 4.850% Cu, 1.310% Mg, 0.667% Mn, 0.254% Fe, 0.110% Si, 0.079% Zn, 0.033% Cr, 0.008% Ti and balance Al. The reinforcing materials selected were tungsten carbide (WC) of 60 μm particle size and the properties of the matrix and reinforcement materials used are presented in the Table 1.

The Al2024-WC composites were prepared by the vortex method [8] . The tungsten carbide contents used for the preparation of the composites were 0%, 1%, 2%, 3%, 4% and 5%. Addition of Tungsten Carbide into the molten aluminium alloy melt above its liquidus temperature of 500˚C was carried out by creating a vortex in the melt using a mechanical stainless steel stirrer coated with aluminite to prevent migration of ferrous ions from the stirrer material into the aluminium alloy melt. The melt was rotated at a speed of 500 rpm over a period of eight

^*Kg/mm².

minutes in order to create the necessary vortex. The WC particles were preheated to 400˚C and added to the melt through the vortex. The melt was subsequently degassed by passing hexa-chloro-ethane (C₂Cl₆) tablets. The molten metal was then poured into permanent moulds for casting. The cast specimen of size 25 × 300 mm² MMCs were obtained.

Carefully polished and mirror finished specimens were examined under Scanning Electron Microscope (SEM) to obtain microphotographs. Tensile tests were conducted at room temperature using a Universal Testing Machine (UTM) in accordance with ASTM Standard E 8-82. The tensile specimens of diameter 12.5 mm and gauge length 62.5 mm were machined from the cast composites with the gauge length of the specimens parallel to the longitudinal axis of the castings. For each composite, four tensile test specimens were tested and the average values of the UTS, Young’s modulus and ductility were measured.

The hardness tests were conducted in accordance with ASTM Standard E 10 using a Brinell hardness tester with a ball indenter of 5 mm diameter and a load of 250 kg. The load was applied for 30 sec. eight hardness readings were taken for each specimen at different locations to circumvent the possible effects of particle segregation. Compression tests were conducted on a UTM in accordance with ASTM Standard E 9 at room temperature. In this test the compression loads were gradually increased and the corresponding strain was measured until the specimen failed. Each result is an average of four readings.

Micrographs of Al2024 alloy and Al2024-WC composites are observed from Figure 1(a) & Figure 1(b). The soundness of prepared casted composites in terms of increased density, reduced porosity with uniform distribution of WC particulates in the matrix alloy was noticed with the help of SEM analysis [14] . SEM structures also revealed good bonding among matrix and reinforcement particles which yields better load transfer from matrix to reinforcement material Figures 1(c)-(f).

The effect of WC reinforcement on the hardness of casted Al2024-WC particulate composites is observed from Figure 2. Hardness test helps in understanding material resistance to surface penetration and maximum stress required to cause specific surface deformation. The hardness value of each test specimen is an average of six test measurements. It is observed from the test results that increase in the tungsten carbide content made increase in hardness value monotonously. In fact, as the WC content is increased from 0% to 3% the hardness increases by about 9%.

In metals, there is a relation between Ultimate Tensile Strength (UTS) and hardness, increase in UTS yields increase in hardness; same would be seen in processed composites [15] . The reason for improved hardness is due to hard WC dispersoid contributing positively to the hardness of composites. The hardness value increased to about 9% for 3 wt% of WC reinforcement, compared to as cast Al2024 alloy without reinforcement. There after a decrease in hardness was observed for 4 wt% of WC reinforcement, probably due to the voids which nucleate during the plastic straining of the reinforcement.

Figure 3 shows the graph of effect of reinforcement content on ultimate tensile strength (UTS) of processed

composites. Each value of UTS is an average of six measurements. From the graphs is noticed that improvement of tensile strength was achieved up to 3 wt% WC, however it decreased beyond 4 wt% WC. Better tensile strength was found at 3 wt% WC. As the WC content is increased from 0% to 3%, the UTS increased by about 57.5%. The decrease in tensile strength of the composites beyond 3 wt% WC is attributed to improper bonding between the matrix and reinforcement materials. Finer the grain size better is the hardness and strength of composites. The increase in UTS can be attributed to the presence of hard WC particulates that impart strength to the

matrix alloy, thereby providing enhanced resistance to tensile stresses.

Addition of WC particulates results in decreasing the interspatial distance between Al 2024 and also helps in pile up of dislocations. This in turn causes restriction in plastic flow due to uniform distribution of particulates in Al 2024 alloy thereby improving tensile strength of the processed composite. Similar observations were made by Ghosh and Ray [16] , who fabricated Al₂O₃ particulate reinforced aluminium alloy composites using the compo-casting method and by McCoy et al. [17] who produced TiB₂-particulate-reinforced aluminium alloy composites with particulate contents ranging from 10 to 25 vol%.

From Figure 4, the percentage variation in Young’s modulus provides a comparative insight in to the actual experimental results recorded for the Al2024-WC particulate reinforced composites. It is observed from the result that addition of WC content in to the Al2024 matrix improved young’s modulus of prepared composite monotonously by significant amount. In fact, as the WC content is increased from 0% to 3%, the Young’s modulus increases by about 47.6% and when increased from 3% to 4% the Young’s modulus decreases monotonically. The increase in young’s modulus is closely attributed to the prediction of rule of mixture. McDanels [18] had reported similar results for particle reinforced Aluminium composites.

Figure 5 shows the effect of the reinforcement content on the ductility of cast Al2024-WC particulate composites measured in terms of percentage elongation. The ductility of the composite material is less compared to as cast Al 2024 alloy. The brittleness also increased with increasing reinforcement content, since the matrix material suffered with ductility due to reinforcement. As the WC content increased from 0% to 5%, the ductility dropped by about 100%. similar results were reported by Beitz W. and Kuttner K. H. [19] & Zhu H. X. and Liu S. K [20] . The reduction in ductility is due to the hard ceramic phase due to localized crack initiation and em-brittlement owing to local stress concentration sites at the particle reinforcement interface. The reinforcing particles resist the passage of dislocations by creating stress fields. Past studies have also revealed, loss of ductility due to the voids nucleating during plastic straining of the reinforcement.

It is observed from Figure 6, that the compressive strength of the composites is higher than that of the base alloy due to the presence of hard WC particles. The WC particles have higher compressive strength than the matrix. Hence an increase in compressive strength is observed with increasing wt% of reinforcement. The compressive strength however decreased with 4 wt% of WC. Webster [21] and Towle et al. [22] also reported about the increase in compressive strength while working on whisker reinforced composites. The increase in compressive strength can also be attributed to decrease in inter particle spacing between WC particles, since WC is much harder than the matrix. The hard ceramic WC particles resist deformation stress whilst increasing composite

strength of the composite. Nevertheless, the addition of hard ceramic particulates has caused the MMCs to behave as brittle rather than ductile materials, as is evident from the above results.