晶体相场方法模拟空位结构及其演化过程

发布时间：2018-04-03 11:14

本文选题：晶体相场方法　切入点：应变　出处：《广西大学》2015年硕士论文

【摘要】：纳米金属材料微观结构决定了材料的性能,掌握纳米金属材料的晶体结构和缺陷(空位、位错)、晶界以及晶粒大小对探究纳米金属材料的塑性变形的机制是具有重要的指导意义。对纳米金属材料,位错可作为其塑性变形机制的主导作用,但当晶粒尺寸很小只有几纳米时,空位也将是其塑性变形机制中不可或缺的一种变形机制。由于纳米金属材料晶粒尺寸非常小,在实验室实验中时时观察材料塑性变形的过程是具有一定难度,但计算机模拟实验可以弥补实验室实验中的不足。晶体相场模型作为一种全新的计算机模拟实验的模型,凭借其自身的优越性已被广泛的应用到研究晶粒结构领域中,利用该模型将空位和位错同时作为研究纳米材料的塑性变形机制是十分有意义的。因此,本文应用空位晶体相场方程,首次研究了空位晶体相场方程在不同的晶体相中所表述的二维周期性形貌图,并从温度和压应力两个外界因素分别讨论了对单晶和多晶空位浓度的影响,本文所做的创新工作和取得主要成果如下：1、通过选取位于不同相的平均原子密度数值,呈现的是不同形貌的二维周期性晶体结构的形貌图,主要有六角“疙瘩”相、条状相和六角“凹坑”相。当体系的平均原子密度数值足够小时,体系中将有空位的出现；当平均原子密度数值为某些数值时,制备的为几种相共存的形貌图。这些不同相形貌图可以用于解释实验室实验中制备纳米薄膜形貌图。2、无论是单晶体系还是多晶体系,温度对晶体中的空位浓度影响很大,并且体系呈现的形貌图也是有差别的。当所处环境的温度越高时,体系达到稳定状态时空位饱和浓度也就越高,但空位浓度的数量级仍保持在10-3～10-4数量级。当体系所处的环境温度越高时,体系有空位出现所需的时间也越短,即体系中空位的形成速率大。说明空位浓度受温度的影响较大。3、在相同的生长环境下,单晶中空位饱和浓度比多晶中空位饱和浓度要小,由于含有晶界的多晶更有利于空位的形成,并且空位会优先选择在晶界位错处生长。在单晶中,晶粒达到稳定状态后只有空位的存在,而多晶体系,当晶体达到稳定状态会有空位和空位团的出现。4、无论是单晶还是多晶,晶体在压应力作用后晶体继续产生大量空位,并且空位的数量远大于受温度影响产生的空位数量。多晶体系在压应力的作用下产生新空位,所需的应力值要小于单晶体系。以上研究思路与结论,以期有助于利用晶体相场模型揭示空位在纳米金属材料的塑性变形过程中演变行为,进而探讨其作用规律并对该模型今后应用到具体材料中具有重要的指导意义。
[Abstract]:The microstructure of nanometallic materials determines the properties of materials, and the crystal structure and defects of nanometallic materials are mastered.Dislocation, grain boundary and grain size are of great significance in exploring the mechanism of plastic deformation of nanometallic materials.For nanometallic materials, dislocation can play a leading role in the plastic deformation mechanism, but when the grain size is very small and only a few nanometers, the vacancy will also be an indispensable deformation mechanism in the plastic deformation mechanism.Because the grain size of nanometallic materials is very small, it is difficult to observe the process of plastic deformation in laboratory experiments, but computer simulation experiments can make up for the deficiency in laboratory experiments.As a new computer simulation model, the crystal phase field model has been widely used in the research of grain structure by virtue of its own superiority.It is very meaningful to study the plastic deformation mechanism of nanomaterials by using the model to study both vacancies and dislocations.Therefore, in this paper, the two-dimensional periodic morphologies of the vacancy crystal phase field equations in different crystal phases are studied for the first time by using the vacancy crystal phase field equation.The effects of temperature and compressive stress on the concentration of single crystal and polycrystalline vacancy are discussed respectively. The main results of this paper are as follows: 1. The average atomic density in different phases is selected.There are hexagonal "pimples", stripe and hexagonal "pit" phases.When the average atomic density of the system is small enough, there will be vacancies in the system, and when the average atomic density is some values, the morphologies of several coexisting phases will be prepared.These morphologies can be used to explain the morphologies of nanocrystalline films prepared in laboratory experiments. Whether in single crystal system or polycrystalline system, the temperature has a great influence on the vacancy concentration in the crystal, and the morphologies of the system are also different.The higher the temperature of the system is, the higher the vacancy saturation concentration is when the system reaches the stable state, but the order of vacancy concentration is still in the order of 10 ~ (-3) ~ (-3) ~ (-4) ~ 10 ~ (-4).The higher the ambient temperature of the system is, the shorter it takes for the vacancies to appear in the system, that is, the higher the rate of the formation of vacancies in the system.Under the same growth environment, the vacancy saturation concentration in single crystal is smaller than that in polycrystalline, because the polycrystal with grain boundary is more favorable for the formation of vacancy.And the vacancy will be preferred to grow at the grain boundary dislocation.In a single crystal, there is only a vacancy when the crystal reaches a stable state, while in a polycrystalline system, when the crystal reaches a stable state, there will be vacancies and vacancy groups.A large number of vacancies continue to be produced after compressive stress, and the number of vacancies is much larger than that caused by temperature.A new vacancy is produced in polycrystalline system under compressive stress, and the required stress value is smaller than that of single crystal system.The above research ideas and conclusions are helpful to reveal the evolution of vacancies in the plastic deformation of nanometallic materials by using the crystal phase field model.Furthermore, it is of great significance for the application of the model to the concrete materials in the future.
【学位授予单位】：广西大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TB383.1

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