多晶体材料微观结构演化的晶体相场法研究

发布时间：2019-03-26 19:42

【摘要】：真实材料的微观演化行为一直是人们所感兴趣的研究方向,特别是纳观尺度下的缺陷运动情况。受制于设备的限制,一直以来,人们只能通过实验得到的图案,结合一些数据结果,来推测材料的行为。但是当研究的材料结构或材料所属的环境很复杂时,这时候就需要采用计算机模拟实验。晶体相场(PFC,Phase-Field-Crystal)法是一种可以模拟晶体纳米尺度结构的研究方法,通过确定研究对象的密度场和最小化自由能函数,结合相应的动力学方程,即可模拟纳米尺度下晶体材料的演化过程。目前已发展成为一种成熟的模拟纳米材料的研究手段,与传统的相场方法相比,晶体相场方法能够胜任描述原子尺度结构和大的扩散时间尺度的研究。工程使用的金属材料一般是多晶体。材料加工过程中,多晶体会受到各种作用,如挤压,腐蚀,高温等,这直接影响到材料内部微结构的变化。所以多晶体的塑性变形研究一直受到人们的广泛关注。鉴于此,本文将采用PFC法研究纯物质多晶材料内部在单、双方向应力和动、静态载荷的作用下晶界演化过程,探究晶界上位错运动、位错反应与温度,应力应变速率大小、方向和应力形式的关系,研究晶界运动的演化过程,分析内部畸变能的变化情况,揭示纳米多晶材料受应力作用的微观机理。得到如下结论:1、采用PFC法模拟多晶体塑性变形,观察到晶粒旋转、晶粒吞并、大小角晶界迁移等现象。晶粒旋转主要发生在取向差较小的两晶粒之间,而晶粒吞并现象发生在大晶粒与小晶粒之间。2、温度的改变使得晶界出现不同程度的预熔化。低温下的晶粒旋转困难,位错容易滑移进入晶粒内部,补偿了晶粒取向的差异。而温度较高时,晶界迁移速度更快,更容易出现晶粒吞并现象。3、应力方向的改变使得晶粒生长方向也随之改变,晶粒生长倾向于向垂直于压力轴的方向发展,而晶界倾向于平行或者垂直于压力轴的方向发展。4、增加了应变速率后,晶界迁移与位错运动的速率变快。静态双轴应力作用下,多晶体材料在前期发生剧烈变化,出现位错的发射与分解。对比动态双轴应力作用下样品的演化,静态双轴应力在前期能形成更高的位错密度。
[Abstract]:The micro-evolution behavior of real materials has always been an interesting research direction, especially the defect motion on a nanoscale scale. Because of the limitation of the equipment, people can only infer the behavior of the material by the pattern obtained from the experiment and the result of some data. But when the material structure or the environment of the research is very complex, computer simulation experiment is needed at this time. The crystal phase field (PFC,Phase-Field-Crystal) method is a kind of research method which can simulate the nano-scale structure of crystal. By determining the density field and minimizing the free energy function of the research object, the corresponding kinetic equations are combined. The evolution process of crystal materials at nanometer scale can be simulated. At present, it has developed into a mature research method for simulating nano-materials. Compared with the traditional phase-field method, the crystal phase-field method can describe the atomic-scale structure and the large diffusion time-scale. The metal materials used in engineering are usually polycrystals. In the process of material processing, polycrystals will be subjected to various effects, such as extrusion, corrosion, high temperature and so on, which directly affect the change of microstructure inside the material. So the research on plastic deformation of polycrystals has been paid more and more attention. In view of this, the PFC method will be used to study the grain boundary evolution process of pure material polycrystalline materials under the action of single, biaxial stress and dynamic and static loads. The dislocation motion, dislocation response and temperature, stress and strain rate on grain boundary will be investigated. The relationship between direction and stress form is studied, the evolution process of grain boundary movement is studied, the variation of internal distortion energy is analyzed, and the micro-mechanism of stress action on nano-polycrystalline materials is revealed. The conclusions are as follows: 1. PFC method is used to simulate the plastic deformation of polycrystals. The phenomena of grain rotation, grain annexation, grain boundary migration and so on are observed. The grain rotation occurs mainly between the two grains with smaller orientation difference, while the phenomenon of grain annexation occurs between the large grain and the small grain. 2, the change of temperature makes the grain boundary pre-melt in varying degrees. It is difficult to rotate the grain at low temperature, and the dislocation slip easily into the grain, which compensates the difference of grain orientation. When the temperature is higher, the grain boundary migration speed is faster, and it is easier to appear the phenomenon of grain annexation. 3, the direction of grain growth changes with the change of stress direction, and the grain growth tends to develop in the direction perpendicular to the pressure axis. The grain boundary tends to develop parallel or perpendicular to the pressure axis. 4. When the strain rate is increased, the velocity of grain boundary migration and dislocation movement becomes faster. Under the action of static biaxial stress, the polycrystalline material changes dramatically in the early stage, resulting in the emission and decomposition of dislocations. Compared with the evolution of the samples under dynamic biaxial stress, the static biaxial stress can form a higher dislocation density at the early stage.
【学位授予单位】：广西大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O76

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