Interfacial Reaction And Hot Deformation Behaviours Of Bi₂O₃-Coated Aluminum Borate Whisker-Reinforced Aluminum Matrix Composite

Bi₂O₃ was coated onto the surface of aluminum borate whisker using chemical precipitation method. All of Al18B4O33 whisker reinforced aluminum matrix composites were fabricated using squeeze casting method, and the volume of whisker was 20%. The mass ratios between Bi₂O₃ and ABOw were 0, 1:10, 1:20, 1:30 and 1:40,and the corresponding abbreviations of composites are ABOw/Al, 10, 20, 30 and 40ABOw/BO/Al composites, respectively. The microstructure and interfacial reaction of composites were studied by transmission electronic microscope (TEM), scanning electronic microscope (SEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD),and so on. The effects of Bi₂O₃ coating content on the infiltration pressure, tensile properties and thermal expansion behaviors were analyzed. The hot extrusion and compression behaviors of composites with different Bi₂O₃ coating contents were also investigated, and the mechanical properties of the extruded composites were performed.Both DSC and TEM observation of the composite indicated that during the squeeze casting process, Bi₂O₃ coating could react with aluminum matrix in the composite, which produced the low melting-point Bi on the interface between whisker and matrix. The interfacial reaction improved the whisker/liquid aluminum wettability, which not only decreased the infiltration pressure of composite and enhanced the fabricated quality of composite, but also reduced the quantity of microhole and increased the tensile properties of composite, especially elongation to fracture. But if Bi₂O₃ coating content was above 0.55mass%(40ABOw/BO/Al composite), lots of brittle coating deteriorated the tensile properties of composite. The thermal expansion analysis of the composites showed that the changing amplitude of the thermal expansion coefficient of ABOw/BO/Al composite with temperature was much larger than that of ABOw/Al composite. The difference of the thermal expansion curves between ABOw/BO/Al and ABOw/Al composites was represented by the abrupt volume expansion as the solidification of Bi and the relaxing stress concentration of the interface as the melt of Bi.The investigation of the hot compressive deformation of composites showed that the low melt-point Bi on the whisker/matrix interface might relax the interfacial stress during deformation, which lead to decreasing the dislocation density near the whisker. So, the coating might improve the thermal plasticity of composite and decrease its maximum compressive flow stress. On other hand, the strain softening effect of ABOw/BO/Al composite was also smaller than that of ABOw/Al composite because the coating decreased the degree of whisker fracture. In addition, the maximum compressive flow stresses of two composites decreased with the temperature increase or the strain rate decrease.Because the extruded temperatures were above the melting point of Bi, the liquid phase on the interface must influence the hot extruded formability of composites. On one hand, the steady-state extruded load of the extrude composites decreased gradually with the Bi₂O₃ coating content increasing. On the other hand, many fir-tree cracks could be seen on the surface of the extruded ABOw/Al composite rods at all extrusion temperatures, especially at 450℃, but no cracks could be found on the surface of the extruded ABOw/BO/Al composite rods. Therefore, the addition of Bi₂O₃ coating improved the hot extruded formability of composites. In addition, the whisker length of the extruded composites also increased gradually with the Bi₂O₃ coating content increasing. The whisker length, extruded temperature and brittle coating content influenced the tensile properties and hardness of the extruded composites together, and then the tensile properties and hardness of the extruded 40ABOw/BO/Al composite attained the maximum values at the extruded temperature 400℃.