| In the present work hybrid, aluminium oxide (Al2O3) and zirconium tri-aluminide (Al3Zr) reinforced2024A1aluminium matrix composites (AMCs) with a tailored reinforcement distribution were successfully fabricated. In order to fabricate the in situ composite, techniques such as ball milling and powder metallurgy (PM) was used. The reinforcements were in situ synthesized using reaction hot pressing of the aluminium and ZrO2system. Al3Zr as a reinforcement has good properties such as low density, good oxidation resistance, high melting point and good thermal stability, which makes Al3Zr a potential candidate for high temperature structural materials. However, brittleness at low temperatures has limited its application. In order to overcome this, Al3Zr was dispersed in a ductile phase material-2024Al. Moreover, a tailored distribution of Al3Zr and Al2O3in the form of a network microstructure resulted in better properties compared to a similar composite with a microstructure consisting of discretely dispersed phases embedded in an otherwise homogeneous matrix. Network structured composites with different volume fractions of reinforcement,3,5and10vol.%were produced. The effects of subsequent hot extrusion and heat treatment on the microstructure and properties of the network structured composite were studied. Mechanical properties such as hardness, compressive strength, room and high temperature tensile strength for the as sintered, extruded and heat treated network structured compoiste was characterised. Effective comparisons were drawn between properties of the unreinforced2024A1alloy, composite with a homogeneous distribution and the network structured composite. The strengthening mechanism present in the composite along with fracture studies was carried out on the network structured composite.The reaction system between2024A1and ZrO2was studied using differential thermal analysis (DTA). Low energy ball milling at150rpm and2hours was used to disperse the fine ZrO2powders onto the surface of the spherical2024A1powders, thereby retaining the roundness of the aluminium powders. The reaction between Al and ZrO2commenced at730℃and proceeded towards completion as the energy of the system was raised. Sintering parameters-temperature, time and pressure-had a profound effect on the microstructure of the network structured composite. In situ Al2O3nano-particles and Al3Zr with varying morphology and sizes were produced under different sintering conditions. Al3Zr with varying morphologies such as particle, plate and flakes were observed. The sintering parameters optimized to produce the (Al3Zr+Al2O3)/2024Al composite, was840℃with a sintering time of lhour and a pressure of25MPa. Microstructural analysis revealed that the interface between Al3Zr and the matrix was clean and free from contaminants.Compared with the unreinforced alloy the10vol.%network structured (Al3Zr+Al2O3)/2024Al depicts a notable increase in yield and tensile strength (YS, UTS). The network structured composite displayed a66.7and18.6%increase in YS and UTS when compared to the unreinforced alloy. It is worth noting that the elastic modulus of the composite (86.4GPa) was significantly higher than that of the unreinforced alloy (74.2GPa). Tensile tests revealed that the as sintered (Al3Zr+Al2O3)/2024Al with a network distribution of reinforcements, had a higher strength and ductility when compared to the composite with a homogeneous distribution. Furthermore, the composite with a network distribution shows a4.1and12.5%increase in YS and UTS when compared to the homogeneous composite. Most interestingly, the network structured composite displays a76.9%increase in elongation when compared to the composite with a homogeneous distribution prepared in the same way. The hardness of the network structured (Al3Zr+Al2O3)/2024Al was higher than the unreinforced alloy and the homogeneous composite. The superior strength was attributed to the strengthening effect provided by the quasi-continuous network reinforcement distribution over that of the homogeneous distribution. The enhancement in ductility was attributed to the large and interpenetrating nature of the aluminium alloy matrix through the particle network. The effect of volume fraction on the mechanical properties of the network structured composite was investigated. As the reinforcement volume fraction in the composite increased from3to10%, an increase in strength of the composites coupled with a drop in ductility was observed.Hot extrusion and heat treatment played a key role in influencing the micro structure and mechanical properties of the network structured (Al3Zr+Al2O3)/2024Al. Hot extrusion significantly enhanced the strength and ductility of the network structured composites along the extrusion direction. Upon extrusion, Al3Zr along with other intermetallic particles were broken up into smaller particles. The aging characteristics of the network structured composites before and after extrusion was studied. The peak aged composites shown significant enhancements in strength after heat treatment. This was attributed to the presence of transition precipitates formed in the2024A1matrix. The extruded and peak aged10vol.%(Al3Zr+Al2O3)/2024Al with a network structure showed a UTS of510MPa and an elongation of5.7%which is significantly higher than that of the as sintered composite. This effective strengthening was attributed to a number of factors namely; the refinement in matrix grain size, the reduction in volume fraction along the extruded network, breaking up of Al3Zr into smaller particles, an increase in relative density and matrix strengthening due to the precipitates formed while heat treatment.The results of high temperature tensile tests indicated that the network of hybrid reinforcements effectively strengthened the aluminium matrix upto400℃. Fracture charcaterstics indicated, that upto400℃the Al3Zr particles was effective in load bearing which further confirmed the thermal stability of the reinforcements. This was confirmed by particle clevage as revealed in the fracture surfaces. However, at400℃, interfacial debonding was observed. The analysis of fractographs of the network structured composite at room and high temperature indicate that the crack propagation prefers to propagate along the network boundary region indicating that the network structure contributes effectively to the strengthening of the composite. The large2024A1matrix contributes to the ductility of the composites.Theoretical and numerical analysis of the (Al3Zr+Al2O3)/2024Al network structured composite was attempted using micro mechanical modeling. The self-consistent method was used to model the micro structure of the as sintered network structured composite. The finite element results of the elastic modulus were in close proximity to the experimentally measured values. Thus the self consistent embedded cell model can be used effectively for material design. |
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