| In this study, exothermic dispersion method was applied to fabricate in-situ (α-Al2O3+ZrB2) particle reinforced aluminum matrix composites from Al-ZrO2-B2O3 powder mixtures. The reaction outcome and microstructures of composites were analysed by modern means such XRD, SEM, TEM, and EDS. And the product particle size and organization distribution were also investigated.Combining the thermodynamic calculation with DSC curves and XRD patterns, synthetic mechanism of the Al-ZrO2-B2O3 system were analysed. When the B2O3/ZrO2 mole ratio was 1, B2O3 would firstly react with Al to make the formation of B and amorphous Al2O3. Then the most part of B combined Al to form AIB2, and a small part of the B laid on the surface of ZrO2 to form Zr (O,B)2 solid solution. With the increase of temperature, more energy was introduced to the system, resulting in the reaction of Al and Zr (O,B)2 to produce amorphous Al2O3 and ZrB2. Finally, the reactions among Al, ZrO2 and AlB2 could take place and make the appearance of (α-Al2O3 and ZrB2, followed by the transform of amorphous Al2O3 to stable α-Al2O3. The transitional phases were identified to be AlB2, Zr(O,B)2 and amorphous Al2O3.The microstructural features of the composites were observed. The particles have a uniform distribution in aluminum matrix, and the α-Al2O3 particles have a size ranging from several ten nanometers to several micrometers. The morphology of the particles increased with the increase of milling time and holding time. Particles with nearly rounded shape and white contrast were identified as α-Al2O3 by EDS analysis and particles having gray contrast are ZrB2, some of them appear with a hexagon shape. There are small nanoparticles that were identitied as ZrB2, embedded inside some alumina particles, which is similar to the intragranular structure observed in composite ceramics. The formation mechanism of such intragranular structure were studied. During the heating stage, when amorphous Al2O3 experienced a series of phase transitions and eventually transformed to α-Al2O3, the surface area of the Al2O3 would reduce and a tiny fraction of ZrB2 be wrapped. Also in the process of heat preservation, due to the different wettability between reinforced phase and matrix, a small amount of intragranular particle also could generate.It is found that the density as well as the hardness of Al-ZrO2-B2O3 composites increases with the increase of milling time. The tensile strength and elongation of the composites with the reinforcement volume fraction of 20% and 30%were 188MPa,2.38% and 166MPa and 1.60%,respectively.The tensile strength of the composites was higher than that of the industrial pure aluminum, and fracture morphology analysis showed that the main factor which restricted the tensile properties of the composites was hole.The wear test results show that the wear mass loss of Al-ZrO2-B2O3 composites increased along with the load, however the friction coefficient firstly increased and then reduced. With the increase of sliding speed, the wear mass loss of the composites increase until the load was 80N. And the friction coefficient decreased all the time. The SEM photos and EDS analysis of the wear surface showed that the wear mechanism of 20% system was mainly adhesive wear in the condition of the sliding speed was 0.8m/s and the load was 40N, while the 30% system was the main abrasive grains wear. Above all, when the reinforcement volume fraction reached 30%, the wear property of the composites was better than that when the reinforcement volume fraction was 20%, which indicates that α-Al2O3 and ZrB2 particles played a important role in improving the wear resistance. |
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