【作者】 吴春萍；

【导师】 易丹青；

【作者基本信息】 中南大学 ， 材料学， 2009， 博士

【摘要】 电触头是电器设备的关键元件之一,其功能是接通和切断电流。Ag-CdO触头材料由于具有优良的抗电弧侵蚀性、抗熔焊性和较低的接触电阻,广泛用于几伏到上千伏的多种低压电器中,曾被称为万能触点。但是Cd金属蒸气有毒,污染环境,对人体有害。因此,各国都在开展代镉材料的研究。Ag-SnO₂触头材料具有优良的抗电弧侵蚀性和抗熔焊性,是众多无镉触头材料中最有希望替代Ag-CdO的一种。国内外对Ag-SnO₂材料的加工工艺、微量添加剂的应用、电弧作用下材料的组织演变等进行了较广泛的研究。但是,关于Ag-Sn合金的内氧化机理以及如何解决Ag-SnO₂材料加工性能差的研究却比较少。因此,研究Ag-Sn合金内氧化机理以及Ag-SnO₂材料高温塑性变形行为具有重要的理论意义和实用价值。本文通过对Ag-Sn、Ag-Sn-Cu-Bi-Ni合金和Ag-Sn-M（M=Sb,La,In）合金粉末氧化动力学和氧化组织的研究,掌握了氧化温度、氧分压以及合金成分对合金氧化行为的影响。通过热压缩实验,研究了Ag-SnO₂-Sb₂O₃材料的高温塑性变形行为并绘制了其热加工图。首次采用内氧化+粉末热挤压工艺制备Ag-SnO₂-Sb₂O₃触头材料,并与内氧化+热锻工艺制备的Ag-SnO₂-Sb₂O₃触头材料进行比较,分析了热加工工艺对材料组织与性能的影响。得到了以下一些结论:（1）通过对Ag-2.99at.%Sn合金内氧化动力学的研究,首次推导出了Wagner氧化理论方程中,氧在Ag中溶解度（N_O^S）与氧分压(P_O2)及温度（T）间的关系式:此外,不同氧分压下,Ag-2.99at.%Sn合金的氧化速率（k）与温度的关系为:不同温度下,Ag-2.99at.%Sn合金的氧化速率与氧分压的关系为:（2）Ag-2.99at.%Sn合金内氧化后,氧化前沿（氧化区和未氧化区间的界面）平直,生成的氧化物颗粒弥散分布在Ag基体上;低温氧化（600-700℃）时,氧化物主要在晶界析出,高温氧化（800-850℃）时,氧化物主要在晶粒内部析出;氧分压越高,氧化物粒子尺寸越细。Ag-4.76at.%Sn合金氧化后,氧化前沿呈波浪形;在空气中氧化时,氧化物主要在晶界析出,在0.3MPa和0.9MPa氧压下氧化时,氧化层内形成了很多氧化带。随着氧化层往合金内部推进,氧化层内分布的氧化物颗粒尺寸和颗粒间距变大。（3）Ag-6.46Sn-1.3Cu-0.27Bi-0.17Ni（wt.%）合金内氧化是一个反应扩散控制过程。主要包括:氧在合金表面的吸附和分解;氧化物颗粒的形核和氧化带的形成;Cu的扩散和CuO颗粒的形核。合金冷轧后直接进行内氧化与合金冷轧经再结晶退火后再内氧化不同,前者氧化前沿比较平直;氧化层内形成了很多内氧化带;氧化层中氧化物颗粒尺寸和硬度随氧化层厚度的增加而增大。而后者氧化前沿呈不规则锯齿状,氧化层中氧化物颗粒尺寸、硬度和氧化物颗粒断裂应力均大于前者。由于冷轧提高了合金中的位错密度,增加了氧在合金中的扩散通道,所以前者的氧化速率(5.69×10^-9cm²/sec)要大于后者的氧化速率(2.39×10^-9cm²/sec)。（4）元素La、In比Sb更能改善Ag-Sn合金粉末的氧化特性。Ag-Sn-Sb合金粉末的氧化过程是氧原子向合金粉末内部扩散,同时有部分溶质元素（Sn和Sb）向外扩散的扩散反应过程。Ag-Sn-In合金粉末的氧化过程是氧原子向合金粉末内部扩散,而溶质元素（Sn和In）原位生成氧化物的扩散反应过程。元素La在Ag-Sn合金中都是原位形核生成氧化物。当La为0.44wt.%时,Sn在浓度梯度的驱动下向外扩散并与向内扩散的氧发生反应形成氧化物;当La含量为1.28wt.%和3.4wt.%时,Sn则原位氧化生成氧化物。（5）Ag-SnO₂-Sb₂O₃材料热压缩时,随着应变速率的增加,流变应力不断增加;低应变速率的流变曲线比较光滑,而高应变速率的流变曲线为小锯齿状。根据Ag-SnO₂-Sb₂O₃材料高温压缩实验求出的相关材料常数,建立了峰值屈服应力与应变速率以及温度之间的本构方程:（6）基于热加工图的基本原理,采用高温压缩实验数据分别绘制了Ag-6.92SnO₂-3.69Sb₂O₃和Ag-9.12SnO₂-1.46Sb₂O₃材料在应变为0.03、0.2、0.4、0.6和0.9时的热加工图,并在部分失稳条件下的样品中观察到了绝热剪切带、局部流变失稳带、楔形裂纹、颗粒断裂以及颗粒与基体界面形成空隙等失稳现象。通过加工图和相应显微组织的分析,确定了Ag-6.92SnO₂-3.69Sb₂O₃材料可加工的温度范围为: 720℃-840℃,应变速率范围为0.01s^-1-0.1s^-1;Ag-9.12SnO₂-1.46Sb₂O₃材料可加工的温度范围为:790℃-845℃,应变速率范围为0.01s^-1-0.18s^-1。（7）用内氧化+粉末热挤压工艺制备的Ag-6.32SnO₂-3.69Sb₂O₃和Ag-9.12SnO₂-1.46Sb₂O₃材料的相对密度分别是99.55%和99.89%,硬度分别是93HB和85HB,电导率分别是71IACS%和69IACS%。而用内氧化+热锻工艺制备的Ag-6.32SnO₂-3.69Sb₂O₃和Ag-9.12SnO₂-1.46Sb₂O₃材料的相对密度分别是98.89%和99.35%,硬度分别是84.3HB和74.8HB,电导率分别是61IACS%和66IACS%。由于粉末热锻保留了材料烧结坯中粉末颗粒的完整形态,而粉末热挤压既能破坏烧结坯中粉末颗粒的完整形态,又能获得流线型组织。因此,用内氧化+粉末热挤压工艺制备的Ag-SnO₂-Sb₂O₃触头材料组织和性能都要优于内氧化+热锻工艺。更多还原

【Abstract】 Electrical contact is one of the key parts of the electric apparatus, which function is making and breaking current. Ag-CdO contact materials have excellent resistance of electrical arc and welding and low electric contact resistance, which are widely used in many types of low-voltage apparatus from several to thousands of voltages and once considered as universal contacts. However, the steam of Cd metal is poisonous, which pollutes the environment and damages the human health. Consequently, many countries have developing the materials to substitute for the Ag-CdO materials. Ag-SnO₂ contact materials have excellent electric arc resistance and good welding resistance. Nowadays Ag-SnO₂ electrical contact materials are the optimum candidate for replacing the toxic Ag-CdO electrical contact materials. Lots of work has been done on the processing technic, application of micro-additive and microstructure variation of Ag-SnO₂ during the action of electric arc. However, there is a little report on the oxidation mechanism of Ag-Sn alloy and how to resolve the problem on bad process ability of Ag-SnO₂ materials. So the investigation on the oxidation mechanism of Ag-Sn alloy and high temperature plastic deformation behaviour of Ag-SnO₂ materials has an important theoretical significance and use value.Oxidation kinetic and microstructure of Ag-Sn alloy, Ag-Sn-Cu-Bi -Ni alloy and Ag-Sn-M （M=Sb, La, In） alloy powders were investigated and the effect of oxidation temperature, oxygen partial pressure and alloy composition on the oxidation behavior of these alloy were discussed in the present paper. High temperature deformation behaviors of Ag-SnO₂-Sb₂O₃ materials were studied through hot pressing experiment and the processing maps were plotted. In addition, Ag-SnO₂-Sb₂O₃ contact materials were prepared firstly using by internal oxidation+powders hot extrusion technology, which is compared with the Ag-SnO₂-Sb₂O₃ contact materials fabricated by internal oxidation + hot forging technology. The effect of processing technic on microstructure and properties of materials is discussed.The results obtained are as follows:（1） According to the investigation on the oxidation kinetics of Ag-2.99at.%Sn alloy, the relationship between the solubility of oxygen in silver （ N_O^S ） and the oxygen partial pressure ( P_O2 ) , temperature （T） is firstly deduced, which is as follow: In different temperature and oxygen partial pressure, the solubility of oxygen in silver can be easily calculated according to the above equation. In addition, at different oxygen pressure, the relationship between the oxidation rate and temperature is as follow:At different temperature, the relationship between the oxidation rate and oxygen partial pressure is as follow:（2） The oxidation front （interface between the oxidation zone and the oxidation-free zone） is flat like a line and the oxide particles uniformly and dispersedly distribute on the silver matrix after the Ag-2.99at.%Sn alloy internal oxidation. At the low temperature （600-700℃） oxidation case, the oxide particles mainly precipitate on the grain-boundaries; at the high temperature （800-850℃） oxidation case, the oxide particles mainly precipitate inside the grain. The higher oxygen partial pressure, the finer oxide particle size. While the oxidation front is undulation after the Ag-4.76at.%Sn alloy oxidation. The oxide particles mainly precipitate on the grain-boundaries when the Ag-4.76at.%Sn alloy is oxidized in air. When the oxygen partial pressure is 0.3MPa and 0.9MPa, many oxide bands are observed in the internal oxidation layer. The oxide particles size and inter-particle spacing become larger with the distance from surface.（3） The internal oxidation of Ag-6.46Sn-1.3Cu-0.27Bi-0.17Ni （wt.%） alloy is a process controlled by reaction diffusion. It mainly include: oxygen adsorption and decomposition on the alloy surface; nucleation of oxide particle and formation of oxide bands; diffusion of copper and nucleation of CuO. The internal oxidation of the cold rolled alloy and the recrystallization annealing alloy is different. The former oxidation front is flat and there are many oxide bands in the internal oxidation front. The oxide particle size and hardness become larger with the oxidation depth increasing. But the latter oxidation front is zigzag and the oxide particle size, hardness and the fracture stress of oxide particles are larger than that of the former. Because cold rolling increase the density of dislocation in alloy and the diffused channel of oxygen, the former oxidation rate （5.69×cm²/sec） is larger than the later oxidation rate （2.39×cm²/sec） .（4） Element La and In is more effective than element Sb for improving the oxidation character of Ag-Sn alloy powders. Oxygen atoms diffuse from surface to inner and part of solute atoms （Sn and Sb） diffuse from inner to surface during the oxidation of Ag-Sn-Sb alloy powders. Oxygen atoms diffuse from surface to inner and solute atoms （Sn and In） form oxides in situ during the oxidation of Ag-Sn-In alloy powders. La₂O₃ forms in situ during the oxidation of Ag-Sn-La alloy powders. When the La content is 0.44wt.% in the Ag-Sn-La alloy powders, Sn atoms will diffuse from inner to surface and react with the oxygen and form oxides. When the La content is 1.28wt.% and 3.4wt.%, all the Sn atoms will form oxide in situ.（5） With the increasing of strain rate, the stresses of Ag-SnO₂-Sb₂O₃ materials compressed in high temperature are increased. In low strain rate (0.01s^-1and0.1s^-1), the flow curves are smooth, but in high strain rate(1s^-1 and 10s^-1), those are fluctuate. According to the relative material parameters obtained from the compression experiment, the deformation constitutive equations of Ag-SnO₂-Sb₂O₃ materials describing the relationship of yield stress peak value, strain rate and temperature are given as follows:（6） Based on the principle of thermal processing map, high temperature compressed deformation datas were used to obtain the processing maps of Ag-6.92SnO₂-3.69Sb₂O₃ and Ag-9.12SnO₂-1.46Sb₂O₃ materials under the true strain of 0.03, 0.2, 0.4, 0.6 and 0.9. The adiabatic shear bands, flow localization, wedge crack, particle breakage and voids formed at oxide particle were observed on some damage samples. Ag-6.92SnO₂-3.69Sb₂O₃ materials processing temperature range is 720℃-840℃and strain rate range is 0.01s^-1-0.1s^-1. Ag-9.12SnO₂-1.46Sb₂O₃ materials processing temperature range is 790℃-845℃and strain rate range is 0.01 s^-1-0.18s^-1.（7） Ag-6.32SnO₂-3.69Sb₂O₃ and Ag-9.12SnO₂-1.46Sb₂O₃ materials are prepared by internal oxidation+powder hot extrusion technology. Their relative density is 99.55% and 99.89%, hardness is 93HB and 85HB, electrical conductivity is 71IACS % and 69IACS % . At the same time, Ag-6.32SnO₂-3.69Sb₂O₃ and Ag-9.12SnO₂-1.46Sb₂O₃ materials are prepared by internal oxidation+hot forging technology. Their relative density is 98.89% and 99.35%, hardness is 84.3HB and 74.8HB, electrical conductivity is 61IACS% and 66IACS%. Because the hot forging technology retains the intact shape of powders, but the powder hot extrusion technology can not only destroy the intact shape but also get streamline structure. So the microstructure and properties of Ag-SnO₂-Sb₂O₃ materials fabricated by internal oxidation+ powder hot extrusion technology is better than that by internal oxidation+ hot forging technology.更多还原