【作者】 李志军；

【导师】 费维栋；

【作者基本信息】 哈尔滨工业大学 ， 材料物理与化学， 2007， 博士

【摘要】 本文采用化学沉淀方法对硼酸铝晶须表面进行Bi₂O₃涂覆处理,利用挤压铸造法制备了晶须体积分数为20%的硼酸铝晶须增强铝基复合材料（Bi₂O₃和晶须的质量比分别为0、1:10、1:20、1:30和1:40,简写为ABOw/Al、10、20、30和40ABOw/BO/Al复合材料）。利用扫描电镜、透射电镜、X射线衍射和差热扫描分析仪等方法研究了复合材料微观组织结构并确定了界面反应;分析了Bi₂O₃含量对复合材料挤压铸造过程中的渗透压力、拉伸性能和热膨胀性能的影响规律;研究了Bi₂O₃含量对复合材料高温压缩变形和热挤压变形行为的影响机制,并对挤压态复合材料的力学性能进行了测定。差热分析和透射电镜观察均证实,晶须表面涂覆的Bi₂O₃和液态铝在复合材料的挤压铸造过程中发生了反应,在硼酸铝晶须和基体的界面上生成了低熔点的金属铋。界面反应改善了晶须和液态铝的润湿性,一方面,降低了复合材料在挤压铸造过程中渗透压力,提高了复合材料的成品率;另一方面,减少了复合材料中微孔洞的数量,提高了复合材料的拉伸性能,特别是断裂延伸率。但Bi₂O₃含量不易超过0.55mass%（40ABOw/BO/Al复合材料）,否则由于低强度涂层含量的增加反而恶化了复合材料的拉伸性能。另外,结合金属铋在凝固时体积膨胀及熔化时能松弛晶须周围应力的现象,阐述了含Bi₂O₃的复合材料物理热膨胀系数随温度变化幅度较大的原因。复合材料高温压缩变形的研究结果表明:由于在晶须和基体的界面上引入低熔点的金属铋,可以松弛高温变形过程中界面应力,使晶须和基体的界面容易滑动,降低了晶须周围的位错密度,其结果是,改善了ABOw/BO/Al复合材料的高温塑性,降低了复合材料最大压缩流变应力;并且,减轻了晶须折断程度,弱化了ABOw/BO/Al复合材料的应变软化现象。另外,两种复合材料的最大流变应力均随温度的升高或应变速率的降低而逐渐减小。由于挤压温度高于界面上金属铋的熔点,那么挤压过程中界面上液相铋有助于改善复合材料的热挤压成型能力。随着Bi₂O₃含量的增加,复合材料的挤压力越来越小。而且,含有Bi₂O₃的挤压态复合材料表面质量不受挤压温度的影响,表面光滑无裂纹,远好于挤压态ABOw/Al复合材料的表面质量。更为重要的是,挤压后复合材料晶须的长度也随着Bi₂O₃含量的增加而增加。挤压态复合材料拉伸性能和硬度由晶须的长度、挤压温度和脆性Bi₂O₃含量共同决定,挤压态40ABOw/BO/Al复合材料的拉伸性能和硬度在挤压温度400℃达到最大值。更多还原

【Abstract】 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℃.更多还原