位错及界面对几种金属及合金拉伸变形影响的原子模拟

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	位错及界面对几种金属及合金拉伸变形影响的原子模拟
	谷晓玉
学位类型	硕士
导师	徐东生
	2010
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	位错界面分子动力学断裂拉伸变形
摘要	"金属的形变与断裂行为是材料科学的重要研究内容之一。借助计算机进行材料计算模拟有助于对金属变形机理更加深入的理解，尤其是可以再现某些实验无法或难以观察的过程，且高效便捷。位错和界面是金属中存在的两类主要缺陷，对其力学性能有很大的影响。本文采用分子动力学模拟研究位错在垂直于滑移面方向的拉伸变形中对晶格的弱化作用以及钛铝中不同界面的拉伸断裂机制两个基本问题。多年来，对位错的关注主要集中在位错滑移及位错同其它缺陷的相互作用等方面。近期有研究表明即使在无法滑移的情况下，刃位错对疲劳断裂也有重要影响，然而此观点缺乏定量的实验和模拟支持。本文采用原子模拟研究不同位错密度及不同温度下，位错对金属单晶拉伸断裂强度的影响。研究发现，垂直滑移面拉伸时，含位错晶体的屈服应力比理想晶体低，屈服时裂纹在位错芯处萌生，表明位错芯对晶格的弱化作用。位错密度越高屈服强度越低，对于bcc-Fe单晶，位错密度为1.86×1016 m-2，温度为300 K时屈服强度较理想晶体下降至37％。因此推断循环变形中位错的累积会极大地降低晶格强度。温度对拉伸变形起重要作用，屈服应力随温度升高而降低，且温度升高到某一临界值时，晶格会发生脆性到韧性转变。位错的存在对这种转变有促进作用，可降低转变温度。位错偶高度对上述弱化作用可能有重要影响。分子动力学模拟研究表明，位错偶高度为2d时，强度降低幅度最大，4d以上位错偶屈服强度差别不大。对比研究表明，间隙型位错偶的拉伸屈服强度低于空位型位错偶。钛铝基合金为重要的高温结构材料，但限制其应用的室温脆性问题仍然存在，目前关于钛铝基合金的一个重要方向是研究其界面对形变断裂行为的影响。对钛铝合金中含有不同界面的体系在垂直于界面方向的拉伸模拟表明，不同界面的断裂机制不同。孪晶界、伪孪晶界及120°等γ/γ同相界面，裂纹易形成于界面处，原因是屈服后位错在界面处累积引起的应力集中。而γ/a2两相界面，裂纹易在a2内部靠近晶界的地方形核并扩展。此外，γ/γ孪晶界开裂时的裂纹比较平直，而γ/a2双相界面开裂时裂纹弯曲，断口附近塑性变形严重。"
其他摘要	"The deformation and fracture of metals is one of the most important areas of investigations in material science. Material computation contributes to penetration insight into the deformation mechanisms, especially for the processes that incapable or hardly reachable experimentally; meanwhile it exhibits a combination of high efficiency and convenience. Dislocations and interfaces are two kinds of major defects in metals which have great impact on the mechanical properties. Employing molecular dynamics, the present thesis discusses two important issues, the weakening effect of dislocations under tension perpendicular to the slip plane and the tensile fracture mechanisms of different interfaces in TiAl. Over several decades, the role of dislocations is understood almost exclusively on the dislocations motion and their interaction with other defects. Recent observations show that edge dislocations influence the process of fatigue fracture, and this may even be effective without the presence of their glide. But this argument lacks the support of quantitative estimations and direct experimental verifications. So in the present thesis, atomic simulations are carried out to investigate the influence of dislocations on the tensile fracture strength of metal single crystal under tension perpendicular to the slip plane, with different densities of dislocations and at different temperatures. It is found that cracks nucleate at dislocation cores in system containing dislocations at a lower critical stress compared with perfect lattice, indicating a weakening caused by dislocations. Higher dislocation density results in lower fracture strength. For iron single crystal, the strength drops to 37% of perfect crystal at the density of 1.86×1016 m-2. So it is anticipated that the dislocations accumulation during cyclic deformation may cause weakening of lattice increasingly. The thermal effect plays an important role in tensile deformation, in that the yield strength decreases monotonically as temperature increases. There exists a critical temperature above which the lattice transforms from brittle to ductile, where dislocation nucleation and glide is favored compared to crack nucleation at low temperature, and the critical temperature for the dislocated system is lower than the perfect lattice. The height of dislocation dipole may have great influence on the above mentioned weakening of lattice. Molecular dynamics simulations show that the yield strength is the lowest when the height is 2d, and above 4d the yield strength has little difference. Comparative investigation shows the yield strength of interstitial type dipoles is lower than that of the vacancy type dipoles. TiAl-based alloys are becoming important materials for jet engine applications, but the problem of low ductility at room temperature is not yet solved. Currently, the influence of interfaces on the deformation and fracture behavior of TiAl is an important investigation. Simulations of tensile deformation perpendicular to the interface in systems with different interfaces show that the fracture mechanisms are different. Cracks nucleates from interfaces during tensile deformations of systems with twin boundary, pseudo-twin boundary and 120°rotational boundary because of the interaction of dislocations and interfaces after yielding. However cracks initialize and grow from within a2 grain during tension of system with γ/a2 interface. Moreover, the fracture surface of twin boundary is smoother than γ/a2 interface, and there is more plastic deformation around the latter."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64207
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	谷晓玉. 位错及界面对几种金属及合金拉伸变形影响的原子模拟[D]. 北京. 中国科学院金属研究所,2010.