Processing And Properties Of Aluminum Matrix Composites Reinforced By Ceramic Whiskers Synthesized In-situ

Owing to the excellent properties including low density, size stability, highspecific strength, corrosion resistance and low expansion coefficient, aluminum matrixcomposite became the research focus in the field of metal matrix composites, and hasbright prospects of application in aerospace, transportation, and construction industry.Ceramic whisker material has been considered to be an ideal reinforcement by virtue ofits low defect density, high strength, and stabilization of chemical properties and hightemperature properties. However, there exist some inevitable problems, such as theagglomeration, poor interfacial reaction and dispersion, when using traditional methodslike powder metallurgy or casting for the preparation of whisker reinforced compositesdue to that the whiskers are added externally in the matrix by mechanical mixingmethod. Therefore, exploring a new technology that can make the whiskersreinforcement uniformly diapered and strongly bonded with the matrix, is very neededfor improving the performance and realization of industrialization application of thecomposites. In this paper, the dispersion of the whiskers in the Al matrix was improvedafter surface modification, and Al matrix composites reinforced by in-situ generatedaluminum salt whiskers were successfully prepared. Further, the physical andmechanical properties, microstructure and strengthening mechanism of the whiskerreinforced Al composites were discussed systematically.The Al₂O₃and Cu, Ni, Ag surface coating layer were obtained on the surface ofmagnesium borate （MgB₂B₂O₅） whiskers by electroless plating method, by usingAl（NO₃）₃, Cu（NO₃）₂NiSO₄and AgNO₃as raw materials, and the influence of differentmixing methods on the dispersion of the whiskers was studied. The Mg₂B₂O₅whiskerafter surface modification reinforced Al matrix composites were obtained, and themechanical properties of the composite were studied. The results showed that theuniformly mixed composite powders were obtained, as the surface coating of Al₂O₃orCu, Ni, Ag layer can significantly reduce the agglomeration caused by electrostaticforce. Between the whiskers and the matrix, a well two-phase transition layer wasobtained after surface modification on the whiskers, and hardness and wear resistanceof Al matrix composite are significantly increased, which proved that the well bindinginterface between the reinforcement and the matrix played a decisive role to the performance of the composites.By reaction synthesis processing, a variety of ceramic whiskers includingmagnesium borate（Mg₂B₂O₅）, aluminum borate （Al₄B₂O₉） and magnesium aluminate（MgAl₂O₄） whiskers were prepared by high energy ball-milling of the compositepowders of metal and boric acid. The influence of pretreatment methods, heatingtemperature and time on the growth of whiskers were studied, and the in-situ growthmechanism of aluminum salt whiskers in the Al matrix is also discussed. The resultsshowed that, compared with conventional method, high-energy ball-millingpretreatment can promote the growth process of whiskers and reduce the preparationtemperature of ceramic whisker, which provides a new way for in-situ generation ofwhiskers on the Al matrix. The diameter of whiskers was determined by the calcinationtemperature, with increasing the heating temperature, the average size and the diameterof the whiskers are obviously increased. After heating at800°C, the MgAl₂O₄whiskerswith1μm in diameter and more than10in aspect ratio were synthesized, and themorphology of the whiskers did not changed obviously after prolonging the calciningtime to more than30min. According to the comparison between the obtained productsand the raw materials with different ratio, and the analysis of DTA results duringcalcination process, the growth mechanism of these aluminum salt whiskers isspeculated to be SLS （solution-liquid-solid） growth mechanism.MgAl₂O₄whisker reinforced Al matrix composite was prepared by in-situsynthesis method and hot-extrusion treatment. The distribution and combination of thewhiskers were characterized, and the microstructure of the composite wassystematically studied, as well as physical properties, mechanical properties andstrengthening mechanism of the whisker reinforced Al matrix composite. The resultsshowed that, the MgAl₂O₄whisker mainly generated by the reaction between MgO andAl₂O₃in the raw material, which can eliminate adverse effect of Al₂O₃on thealuminum surface in calcination. As Al matrix is also involved in the reaction, the newgeneration of MgAl₂O₄whisker is generated to match with the semi-molten Al matrix,and a matching between whisker and Al matrix interface at atomic level could beachieved. The in-situ generated MgAl₂O₄whiskers have excellent distribution andinterfacial bonding with the matrix, so the hardness, strength, and high temperaturestability of the composite were improved effectively. The hardness, softeningtemperature and thermal expansion coefficient of the composite contained22.5%MgAl₂O₄whiskers can reach120HV,400°C and18.5×10^-6/°C, respectively. With theincreasing of the content of whisker, the strengthen efficiency of the MgAl₂O₄whisker on the Al matrix can increased linearly due to in-situ synthesized whiskers havingperfect distribution in the matrix. The composite with20%whiskers showed the bestperformances on strength, elastic modulus and high temperature stability, which areresult from the coordinated effects of several reinforcement mechanisms, includinghard properties of the ceramic, dispersion strengthening and framework formed bywhiskers.