| In the present work, aluminum, silica and carbon powders were used as starting materials to produce the alumina, silicon carbide and silicon as reinforcements for Al matrix. In order to fabricate the in situ Al based composites in both Al-Si O2 and Al-Si O2-C systems, ball milling and reaction sintering routes were used. the starting powders were ball milled with low energy, then sintered in vacuum furnace to synthesize new reinforcements. In case of Al-Si O2 system, three in situ formed(Al2O3-Si)/Al composites with a different volume fraction of 10, 20 and 30 vol.% were fabricated using low energy ball milling and reaction hot pressing. The effect of volume fraction of reinforcement on microstructure was studied. When the volume fraction of reinforcement were 10 and 20 vol.%, micro-structural observation showed that the in situ synthesized Al2O3 and Si were dispersed uniformly and had fine sizes less than 2 μm. whereas, when the volume fraction was 30 vol.%, a massive primary Si(~130 μm) along with an increase of Al2O3(~2 μm) was observed.Differential thermal analysis(DTA) was used to investigate the reaction mechanisms in both the Al-Si O2 and Al-Si O2-C systems. Heating at 900oC for 1hr was sufficient to complete the reaction between Al and Si O2 forming Al2O3 and Si new products.Whereas, adding the carbon in Al-Si O2 system had led to the formation of Al4C3 and Si C besides Al2O3 and Si. The in situ synthesized Al2O3, Si C, Al4C3, and Si were dispersed uniformly and had fine sizes less than 2 μm. The optimum sintering parameters in case of Al-Si O2-C system were determined as 1050°C for 1hr. When the Si O2/C/Al molar ratio was(6/3/9), more Al2O3 and Si were produced along with the complete prevention of Al4C3 in the Al-Si O2-C system. The prevention of Al4C3 from the Al-Si O2-C system can be attributed to the presence of an excess of Si around Al4C3, which leads to a decreases in the diffusion length between Si and Al4C3. Therefore, more free Si which was produced from the aluminothermic reaction between Al and Si O2 can react with Al4C3 to form Si C.Mechanical properties including brinell hardness and room tensile strength of the as sintered and as extruded composites were tested and analyzed. Effective comparisons between the fabricated composites and unreinforced Al matrix were down as well. The as sintered(Al2O3-Si)/Al composites with different volume fractions depict notable increments in yield and tensile strength(YS, UTS) coupled with a drop in ductility as the volume fraction increased from 10 to 30 vol.%. When the volume fraction increases from 10 to 20 vol.%, YS and UTS were increased from 59 MPa, 121 MPa to 94 MPa, 171 MPa, respectively, with a decrease of ductility from 12.4 % to 6.7 %. In the case of high volume fraction, despite the employed reinforcement fraction of 30 vol.%, a further decrease of UTS(107MPa) and ductility(0.14%) with a slight increase of yield strength(98MPa) was observed. This was attributed to many factors including:(i) the large content of the embedded Al2O3 in massive Si blokes,(ii) high pores that produced when a high volume fraction was used, resulting in the degradation of its mechanical properties.hen the Si O2/Al/C molar ratio was(6/3/9), YS and UTS are significantly higher than those of composites fabricated with the Si O2/C/Al of(3/0/9) and(3/3/9), respectively. This improvement is attributed to the uniform dispersion of high content of reinforcements in situ formed in Al matrix, as well as a clean interface between these fine reinforcements and the Al matrix. Mechanisms governing the tensile fracture process are discussed. The fracture mechanisms of the in situ(Al2O3-Si)/Al composite have changed from a ductile fracture to a brittle fracture with increasing the volume fraction of reinforcement. The mechanisms governing the tensile fracture of the in situ(Al2O3-Si)/Al composites can be explained by the void nucleation and growth followed by crack initiation from the predominantly brittle Si phase at the interface with Al matrix. Therefore, the damage of the composite is mainly attributed to the silicon Si content and morphologies of its brittle phase. In case of adding the carbon with different molar ratios, the fracture mode changed from ductile to brittle with the variation of Si O2/C/Al molar ratio from(3/0/9),(3/3/9) to(6/3/9), respectively. This transition is attributed to the presence of large content of Al2O3 and Si besides Si C.The effect of extrusion on the microstructures and tensile properties of the in situ(Al2O3-Si)/Al composites was studied.The hot extrusion played a key role in improving the mechanical properties of the in situ(Al2O3-Si)/Al composites, All as extrude(Al2O3-Si)/Al composites with different volume fractions of 10, 20 an 30 vol.% showed an improvement in YS, UTS and elongation when compared to their as sintered composites. The Ys, UTS and brinnel hardness of the as extruded composites with volume fractions of 10, 20 an 30 vol.% were(133MPa, 201 MPa and 18.3%),(173MPa, 258 MPa and 14.8%) and(191MPa, 213 MPa and 1.35%), respectively. Despite the as extruded composites with 30 vol.% showed better tensile properties when compared to the as sintered state, but its properties are still relatively lower than that of the as extruded composite with 20 vol.%. This was attributed the presence of the massive primary Si in the composite that didn’t affect much by the employed ratio during the extrusion process. This suggests the employment of high ratio to refine these Si blocks thereby improving its ductility.The dry sliding wear and friction behavior of the in situ Al based composites fabricated in Al-Si O2 and Al-Si O2-C system were investigated. These include the influences of the normal load, sliding velocity, sliding distance, volume fraction of reinforcements and Si O2/C/Al molar ratios on wear properties, worn surfaces, and friction coefficient. The increment in volume fraction of reinforcement from 10 to 20 vol.% improved significantly the wear resistance of the in situ(Al2O3-Si)/Al composite. However, when the volume fraction was 30 vol.%, large massive Si along with the embedded Al2O3 are formed, as well as the porosity increased, resulted in a further decrease of the wear resistance. The mechanism of wear is a combination of adhesion and abrasion by micro-cutting, plowing at low volume fraction and the severity of wear increased where deep craters, large grooves and cracks are formed. High amounts of Fe on the worn surface that contains 30 vol.% suggests an adhesive wear beside an abrasive and delimitative wear by fracture and micro-cutting. As the load increased, the wear loss of the in situ(Al2O3-Si)/Al composites increased. Also, as the sliding velocity increased, the wear loss decreased systematically, and the Al2O3 oxides were formed resulting in the reduction of the friction coefficient. When the Si O2/C/Al molar ratio was(6/3/9), a significant improvement of wear resistance was obtained. As result, to produce such composites coupling between the in situ synthesized Si and other reinforcements in both the Al-Si O2 and Al-Si O2-C systems by reaction sintering, the careful control of the produced amount of Si crucial in achieving a good wear properties as well as good mechanical properties. |
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