纳晶双峰材料的断裂韧性研究

发布时间：2018-02-11 23:12

本文关键词： 纳晶双峰材料断裂韧性位错粘聚力模型晶粒尺寸　出处：《华北电力大学》2015年硕士论文　论文类型：学位论文

【摘要】：纳晶材料与相应的传统粗晶材料相比具有较高的屈服强度和较好的耐磨性能,但其断裂韧性很低,限制了纳晶材料的应用。为了克服纳晶材料低断裂韧性的缺陷,人们提出了许多解决纳晶材料低断裂韧性的方法,其中最普遍有效的一种就是制备双峰结构,在这样的结构中纳晶晶粒提供高强度,而粗晶晶粒增加材料的断裂韧性,使纳晶材料在强度损失较小的情况下,韧性相对大幅提高,我们把这种双峰结构的纳晶材料称为纳晶双峰材料。为了解释纳晶双峰材料比相应纳晶材料具有更高断裂韧性的机理,本文在充分理解纳晶双峰材料变形机制的基础上建立了位错粘聚力模型,其目的是研究纳晶双峰材料的断裂韧性K BB IC BB;模型只考虑了一种特殊的情况,即裂纹尖端位于纳晶晶粒之间,与粗晶晶粒相交,粘聚力区长度被假设为纳晶晶粒尺寸d。裂纹的钝化和扩展由粘聚力和位错联合控制。位错从裂纹尖端发射到粗晶对面的晶界处并堆积,位错的发射过程对裂纹产生屏蔽效应,从而增加材料的断裂韧性。本文的研究不仅具有重要的科学价值,而且也为设计既具有高强度又具有高断裂韧性的纳晶材料提供了理论基础,推动了机械理论的发展和纳晶材料的工程应用。主要研究内容和结论总结如下:(1)在纳晶双峰材料的位错粘聚力模型中,从裂纹尖端发射的位错最大数量N随着粗晶晶粒尺寸D增大而增多,但最大发射位错数N的值相对很小且变化速率逐渐减小。N值相对很小且变化速率逐渐减小是因为随着粗晶晶粒尺寸D的增大,位错从裂纹尖端发射距离也会增大,施加在位错上的驱动力会越来越小,而且位错到达粗晶晶界后被阻止,使得后来发射的位错的滑动变得越来越困难。(2)在模型中,本文以铜为例分析了纳晶双峰材料裂纹尖端的断裂韧性与纳晶晶粒尺寸d及其与粗晶晶粒尺寸D的关系。分析结果显示:当纳晶晶粒尺寸d固定不变时,纳晶双峰材料的断裂韧性会随着粗晶晶粒尺寸D的增大而增大,而且这种变化是很明显的;当粗晶晶粒尺寸D固定不变时,纳晶双峰材料的断裂韧性也随着纳晶晶粒尺寸d的增大而增大,但增大的不明显。根据断裂韧性和两种晶粒尺寸之间的关系可知:纳晶双峰材料比相应纳晶材料具有更高的断裂韧性。
[Abstract]:The nanocrystalline material has higher yield strength and better wear resistance than the traditional coarse crystalline material, but its fracture toughness is very low, which limits the application of nanocrystalline material, in order to overcome the defect of low fracture toughness of nanocrystalline material, Many methods have been proposed to solve the low fracture toughness of nanocrystalline materials. One of the most effective methods is the preparation of bimodal structure in which nanocrystalline particles provide high strength while coarse grains increase the fracture toughness of the materials. When the strength loss of nanocrystalline materials is relatively small, the toughness of nanocrystalline materials is greatly improved. This kind of nanocrystalline material with bimodal structure is called nanocrystalline bimodal material. In order to explain the mechanism that nanocrystalline bimodal material has higher fracture toughness than the corresponding nanocrystalline material, On the basis of fully understanding the deformation mechanism of nanocrystalline bimodal materials, a dislocation cohesion model is established, the purpose of which is to study the fracture toughness of nanocrystalline bimodal materials K BB IC BBB.The model only considers one special case. That is, the crack tip is located between nanocrystalline grains and intersects with coarse crystals. The length of cohesive zone is assumed to be nanocrystalline size d. The crack passivation and propagation are controlled by cohesive force and dislocation. The dislocation emits from the crack tip to the grain boundary opposite to the coarse grain and accumulates, and the emission process of the dislocation produces shielding effect on the crack. This study not only has important scientific value, but also provides a theoretical basis for the design of nanocrystalline materials with both high strength and high fracture toughness. It promotes the development of mechanical theory and the engineering application of nanocrystalline materials. The main research contents and conclusions are summarized as follows: 1) in the dislocation cohesion model of nanocrystalline bimodal materials, The maximum number of dislocations emitted from the crack tip N increases with the increase of coarse grain size D, However, the maximum number of emission dislocations N is relatively small, the rate of change is gradually decreasing, and the rate of change is decreasing because with the increase of coarse grain size D, the distance of dislocation emission from the crack tip also increases. The driving force applied on the dislocation becomes smaller and smaller, and the dislocation is stopped when it reaches the coarse grain boundary, making the sliding of the later emitted dislocation more and more difficult in the model. In this paper, the relationship between fracture toughness at crack tip of nanocrystalline bimodal material and the size d of nanocrystalline grain and the size of coarse grain D is analyzed. The results show that when the size of nanocrystalline grain d is fixed, The fracture toughness of nanocrystalline bimodal material increases with the increase of coarse grain size D, and this change is obvious. The fracture toughness of nanocrystalline bimodal material also increases with the increase of nanocrystalline size d. According to the relationship between fracture toughness and two grain sizes, the nanocrystalline bimodal material has higher fracture toughness than the corresponding nanocrystalline material.
【学位授予单位】：华北电力大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TB34

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