基于等径角挤扭的SiC-Al粉末微—纳结构和力—热性能

发布时间：2018-01-11 22:00

本文关键词：基于等径角挤扭的SiC-Al粉末微—纳结构和力—热性能　出处：《合肥工业大学》2015年博士论文　论文类型：学位论文

【摘要】：SiCp/Al复合材料不仅密度远低于传统钢结构用材,而且其刚度、强度和尺寸稳定性等力学性能均符合结构件服役需要；同时具有良好的耐磨性能、耐蚀性能以及热学、电学性能,因此在某些环境下还可以用作功能材料。该材料传统制备工艺往往存在需要二次加工或工艺成本高等诸多不足或限制。为此,本课题提出一种制备该材料的新工艺：将SiC(经过预处理)和Al的混合粉末装进黄铜包套进行预压,然后将包套放入自行设计的等径角挤扭(ECAP-T)模具垂直通道中,对其在一定温度下进行挤压,从而使混合粉末在变形过程中实现固结。该工艺可以提供较大静水压力,从而改善SiC颗粒分布、细化Al晶粒,同时有效闭合孔隙、减少裂纹源。此外,此工艺可以使材料从松散粉末直接固结成致密块体,无需二次加工,提高了制备效率；并且变形温度低于传统方法所需温度,从而节省了能耗。在众多大塑性变形(SPD)工艺中,ECAP-T作为一种在等径角挤压(ECAP)和挤扭(TE)基础上改进的新工艺,不仅能保证足够的变形量,而且还能改善材料的变形均匀程度。现阶段已经有相关研究探讨了SiC和Al的混合粉末通过ECAP-T变形致密的可行性,初步观察了材料变形后组织,测试了材料的部分力学性能,但缺乏对变形后复合材料的SiC与Al之间的界面、基体材料的晶体结构、材料的力学与热学综合性能等相关研究。本课题先通过有限元数值模拟方法得到了带有包套的粉坯在ECAP-T变形过程中的流线网络图和速度场分布云图,研究了粉坯在不同位置处的变形程度和粉坯在不同变形阶段的流动特点。在此基础上,对含有不同质量分数(10 wt%,20 wt%和40wt%) SiC的混合粉末开展了在不同温度(150℃、250℃和350℃)下的多道次ECAP-T变形实验,对变形后材料进行相对密度测试和金相组织观察。结果表明：增加变形道次以及降低SiC含量能够使材料整体相对密度提高,内部SiC颗粒团簇数量下降,SiC颗粒分布更为均匀；变形温度(250℃以上)对试样相对密度和SiC颗粒分布的影响很小,但如果变形温度过低(150℃),材料会有明显的孔隙和SiC团聚现象。借助X射线光电子能谱仪(XPS),透射电子显微镜(TEM)和扫描电子显微镜(SEM)等多种手段针对在250℃下1道次ECAP-T变形后所制得的复合材料界面进行观察表征。结果表明：基体Al和增强体SiC之间存在元素互扩散现象,促使SiC颗粒表面非晶态SiO2层与Al基体之间发生了保护性反应,生成了Al2O3,避免了有害相Al4C3的产生。随着变形道次的增加,元素互扩散程度不断提高,界面反应程度也不断加大,界面结合更加牢固。对250℃下ECAP-T变形固结后的含10wt.%SiC复合材料的X射线衍射峰形进行研究,发现了Al基体内晶体结构与变形程度之间的联系：随着变形道次的增加,Al基体晶粒内的亚结构平均尺寸不断减小,显微应变和位错密度不断提高。与ECAP-T变形固结后的纯Al材料相比,变形后的复合材料Al基体亚结构随变形道次的增加平均尺寸的降幅和显微应变的增幅均小于纯Al；且复合材料Al基体内位错密度小于纯Al内位错密度。采用单因素变量法对不同实验条件下所制得的变形试样进行力学和热学性能测试。结果表明,增加ECAP-T变形道次和增加SiC含量能够提高试样的强度、刚度和硬度,降低试样的热导率,使试样更容易获得较低的热膨胀系数CTE；而改变ECAP-T变形温度对试样的力学和热学性能影响并不明显。但是要注意避免变形温度过低(150℃)的情况,试样会因为大孔隙的存在,使得综合性能被严重削弱。本文从多个角度(材料致密程度、SiC颗粒分布、SiC/Al界面以及Al基体晶体结构)进行分析,阐明了三种实验参数(变形道次、变形温度和SiC含量)影响试样力学与热学性能的机制。
[Abstract]:SiCp/Al composite not only density is far lower than the traditional steel structure material, and its stiffness, strength and dimensional stability and mechanical properties are in accordance with the structure of service needs; at the same time has good wear resistance, corrosion resistance and thermal properties, therefore, also can be used as functional materials in certain environments. The traditional preparation process often there need to be two times the processing process or the high cost of many deficiencies or limitations. Therefore, this paper puts forward a new process for preparing the material: SiC (pretreated) mixed powders and Al into brass clad pre pressing, then pack into the designed channel angular extrusion and twist (ECAP-T) die vertical channel, the extrusion at a certain temperature, so that the implementation of mixed powder consolidation in the deformation process. This process can provide high hydrostatic pressure, so as to improve the distribution of SiC particles, fine Al grain, and effectively closed pores, reduce crack source. In addition, this process can make the material into a dense block from the loose powder consolidation, without the two processing, improve the preparation efficiency; and the deformation temperature is lower than that of the traditional method of the required temperature, thereby saving energy. The large deformation of many (in the process of ECAP-T SPD), as one of the ECAP (ECAP) and twist extrusion (TE) process based on improved, can not only ensure enough deformation, but also can improve the deformation uniformity. At present there have been discussed the feasibility research of mixed powder of SiC and Al by ECAP-T deformation density, a preliminary observation of the microstructure of materials after deformation, some mechanical properties of material were tested, but the lack of between SiC and Al composites after deformation interface, crystal structure of matrix material, the mechanical and thermal properties and other related materials Research on this topic. First through the finite element numerical simulation method has been covered with powder billet in ECAP-T deformation process streamline network diagram and velocity field distribution nephogram, flow characteristics of powders in different positions of the deformation degree and powders in different deformation stages. On this basis, with different the mass fraction (10 wt%, 20 wt% and 40wt%) mixed powders of SiC have been carried out at different temperatures (150 C, 250 C and 350 C) under the multi ECAP-T deformation experiment, relative density test and microstructure observation of the deformed materials. The results show that the increase of deformation and decrease the content of SiC can make the whole material relative density increased, the internal SiC particle clusters decreases and the distribution of SiC particles is more uniform; the deformation temperature (250 DEG C) is very small on the relative density of the sample and the SiC particle distribution, but if the deformation temperature is too low (150 C), The material has obvious pores and aggregation of SiC. By using X ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and other means for the interface of the composite material at 250 DEG C for 1 times after ECAP-T deformation produced were observed. The results showed that the matrix characterization Al and enhanced interdiffusion phenomena exist between SiC, the surface of SiC particles occurred in non protective reaction between amorphous SiO2 layer and the Al substrate, the formation of Al2O3, to avoid the harmful phase Al4C3. With deformation times increasing, interdiffusion degree increasing, interfacial reaction degree is also increasing, interface with more firmly. To study the temperature below 250 DEG ECAP-T after the consolidation deformation of composite material with 10wt.%SiC X ray diffraction peak, found between the Al matrix in the crystal structure and deformation degree of contact with the increase of deformation The average size of the sub structure, Al matrix grains decreases, the micro strain and dislocation density increase. Compared with the ECAP-T after the consolidation deformation of pure Al material, Al composite substrate sub structure after deformation with the deformation times increasing decline and the average size of the micro strain increase was less than that of the pure Al and the composite; the matrix material Al dislocation density less than that of the pure Al. The dislocation density in the deformed specimens by single factor variable method under different experimental conditions are obtained in the thermal and mechanical properties test. The results show that the increase of ECAP-T deformation times and increasing the content of SiC can improve the tensile strength, stiffness and hardness, reduce the thermal conductivity the samples are easier to obtain, low coefficient of thermal expansion of CTE and ECAP-T; change the effect of deformation temperature the mechanical and thermal properties of the specimen is not obvious. But we should pay attention to avoid the deformation of low temperature (150 DEG C). Because of the large pore condition, the sample will exist, the comprehensive performance is severely weakened. This paper (material density degree, the distribution of SiC particles, SiC/Al interface and Al matrix crystal structure) analysis of the three kinds of experimental parameters (deformation, deformation temperature and SiC content) the influence mechanism of mechanical and thermal performance.

【学位授予单位】：合肥工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB333

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