Interfacial Microstructure And Mechanical Properties Of 6061Al Matrix Composite Reinforced By Al₁₈B₄O₃₃ Whiskers With Coating

ZnO and ZnAl₂O₄ were coated onto the surface of aluminum borate whiskers (Al₁₈B₄O₃₃w, denoted as ABOw) by sol–gel method in this paper, respectively. The initial mass ratio of ZnO to ABOw was selected as 1:10. Then 6061Al matrix composites reinforced with aluminum borate whiskers were fabricated by squeeze casting. The volume fraction of the whiskers in composites was 20%. According to coating, composites were abbreviated and marked as ABOw/6061Al (non-coating), ABOw/ZnAl₂O₄/6061Al (ZnAl₂O₄ coating) and ABOw/ZnO/6061Al (ZnO coating), respectively.The coating process of aluminum borate whiskers was studied by differential scanning calorimetry and thermogravimetry (DSC-TG) analysis and X-ray diffraction (XRD) analysis. The morphology and phase structure of aluminum borate whiskers were studied by scanning electronic microscope (SEM) and X-ray diffraction analysis. The microstructure and interfacial reaction of composites were studied by optical microscope (OM), scanning electronic microscope, X-ray diffraction analysis and transmission electron microscopy (TEM). Measure the specific area of uncoated and ZnAl₂O₄-coated aluminum borate whiskers with specific area apparatus. The effect of coatings on interface wettability was investigated by critical pressure infiltration test. The effect of coatings on interfacial reaction was investigated by age-hardening behavior test of composites. The effect of coatings on tensile properties of composites and the effect of aging on tensile properties of composite were investigated by tensile test of composites, and the tensile fractographs of composites were examined by scanning electronic microscope. The results show, by means of sol-gel coating process, the ZnO layer and ZnAl₂O₄ layer can be uniformly coated on the surface of aluminum borate whiskers. ZnO coating and ZnAl₂O₄ coating do not affect homogeneous distribution of aluminum borate whiskers in aluminum matrix. The chemical wettability of ZnO coating is better than the physical wettability of ZnAl₂O₄ coating.Serious interfacial reaction takes place during squeeze casting and thermal exposure in ABOw/6061Al composite. ZnAl₂O₄ coating does not participate in interfacial reaction, can also hinder the reaction between aluminum borate whiskers and magnesium in matrix alloy at the same time during squeeze casting and thermal exposure in ABOw/ZnAl₂O₄/6061Al composite effectively. In addition, there is obvious phase relation between Al and ZnAl₂O₄. The thermit reaction between ZnO coating of aluminum borate whiskers and Al takes place during squeeze casting in ABOw/ZnO/6061Al composite, and thermal exposure does not change the type and morphology of interfacial product.ZnAl₂O₄ coating and ZnO coating postpone peak-aging process of composites, vary aging kinetics of composites, and ZnO coating is more effective than ZnAl₂O₄ coating on postponing peak-aging process of composites.ZnO coating can improve the ultimate tensile strength of the as-cast composite greatly on condition that the elongation to fracture of the as-cast composite does not decline basically. ZnAl₂O₄ coating can improve the ultimate tensile strength and elongation to fracture of the as-cast composite at the same time, but the improvement level of the ultimate tensile strength is lower than the level improved by ZnO coating. ZnO coating makes the composite keep good thermal stability, ZnAl₂O₄ coating is inferior to ZnO coating in keeping good thermal stability of the composite. The peak-age can improve the tensile properties of the composite obviously.