Microstructure And Properties Of Al₂O₃Coated Al₁₈B₄O₃₃ Whisker Reinforced Aluminum Matrix Composite

AlO（OH） was coated on the surface of aluminum borate whiskers byhydrothermal synthesis method. After calcined, AlO（OH） turned to Al₂O₃andaluminum matrix composites was fabricated with the coated whiskers by Squeezecasting method. The matrix was2024and The volume fraction of the reinforcementwas20%. The coating morphology and phase which is affected by hydrothermaltime, hydrothermal temperature and the content of coating has been studied by useof scanning electron microscope （SEM） and X-ray diffraction （XRD）. Thedecomposition temperature of precursor was tested by differential scanningcalorimetry and thermogravimetry （DSC-TG）, while the microstructure morphologyof the composites which includes inside precipitates, interface morphology anddislocation has been observed by transmission electron microscopy （TEM）.Mechanical properties and thermal expansion properties of different compositeswere tested and the effect of hydrothermal process on properties was analyzed.The tensile fracture of composites was observed by SEM.SEM observation and XRD test result showed that AlO（OH） coating wasevenly coated on the aluminum borate whiskers by hydrothermal method. Within theparameters of the experiment, along with the increase of hydrothermal time andhydrothermal temperature, coating changed from disordered state to regular shapedsmall particles, the crystallinity increased while the dispersion of production turnedwell.DSC-TG test indicated that, in the heating process, AlO（OH） decomposed at450℃，therefore，sintering temperature of the perform was determined. The phase ofcoating after calcined was analyzed by XRD, which showed that after calcinedcoating product was γ-Al₂O₃.The result of TEM observation showed that, interfacial reaction was seriousand much MgAl₂O₄was generated in whisker-reinforced composites withouthydrothermal treatment. When γ-Al₂O₃was coated, interfacial reaction becameobviously weak and the amount of MgAl₂O₄decreased. The precipitates of as-castcomposites was θ, it was different in size and distributed unevenly. After aging, theprecipitates turned to be θ’ and distributed diffusely in matrix, while equilibriumphase θ was found on the interface.The result of room temperature tensile indicated that, after hydrothermalcoating, strength and plasticity of the composites improved. When the content ofcoating was30:1, mechanical properties of the composites turned to be the best. Strength increased along with the increase of coating temperature. The Strength ofcomposites grow to507MPa when the hydrothermal temperature is200℃.Theinvestigation of thermal expansion indicated that, thermal expansion coefficient andstrength of the composites kept correspondence, high strength materials had lowthermal expansion coefficient. Besides, compared with as-cast composites, thermalexpansion coefficient of quenched composites decreased, while thermal expansioncoefficient of annealed composites increased. When the composites were thermalcycled, thermal expansion coefficient improved while strength and plasticitydecreased. After the first thermal cycling treatment, thermal expansion coefficientbecame stable.