密排六方金属纳米线超塑性和伪弹性的分子动力学研究

发布时间：2018-03-19 21:47

  本文选题：超塑性　切入点：伪弹性　出处：《上海交通大学》2015年博士论文　论文类型：学位论文

【摘要】：塑性是材料的重要的力学性能,决定了材料可实现的变形量。提高材料的塑性一直是结构材料研究的重点。伪弹性则是形状记忆材料中的重要性能,高的可回复应变可以使之吸收更多机械功,故提高可回复应变一直是形状记忆材料的研究重点。与粗晶材料相比,纳米线具有超高的塑性。已报道通过应力诱发相变或孪生方式变形,纳米线的延伸率可以达到50%左右,即所谓的超塑性。此外,纳米线由于表面效应,变形后可以回复到初始状态,实现伪弹性。因此纳米线成为获得超塑性和超高可回复应变的合适体系。目前纳米线的力学性能研究主要侧重于面心立方和体心立方结构体系,密排六方体系的研究结果则较少而且存在一些争议。由于滑移系少,一般认为密排六方金属塑性较差,通常粗晶态的密排六方金属延伸率不到10%。但是在纳米线材料中,密排六方金属的力学性能仍需要进一步探索。本工作根据变形机制的不同,选取两大类密排六方金属的纳米线作为研究对象——应力诱发相变类和应力诱发孪生类。前者选取的代表为纯钴和钴-铁合金纳米线,后者选取的代表为纯镁纳米线。通过分子动力学模拟纳米线的单轴拉伸和卸载过程,从而预测纳米线超塑性和伪弹性的可行性,主要的发现为:(一)在钴纳米线中,通过HCP→FCC→HCP的两步结构相变,可以达到80%左右的延伸率,并且变形后的纳米线可以回复至初始构型,可恢复应变约为71%。形变过程中的相变可以通过静态方法计算能垒,从而进一步解释相变的合理性。该能垒计算的方法进过进一步改进,将纳米线能量表示为应变的函数,计算钴-铁合金纳米线中各种构型之间相互转换的能量关系,并且用于判断应力诱发相变的顺序以及相变类型。(二)在镁纳米线中,通过应力诱发二次孪晶,使纳米线的延伸率可以达到60%左右,且该形变也能通过去孪晶化回复,即在镁纳米线中发现超塑性和伪弹性。二次孪晶中的孪生模式为{11(?)1},为首次在镁中发现,该孪晶的产生是由于初次孪晶界附近的应力集中而造成的。此外,镁纳米线中的{11(?)1}孪晶发现属于非对称结构。由此结果出发进一步拓展,研究了1(?)00对称倾斜晶界的结构能量,发现了两类新的稳定结构:(1){11(?)3}孪晶确定为一种稳定结构且其结构也是非对称的;(2){11(?)6}孪晶中发现晶粒的再取向现象。1(?)00对称倾斜晶界中新结构的发现对于理解晶界结构及其演化过程提供新的例子。总之,本文通过分子动力学模拟提出在HCP金属中达到较高塑性和可回复应变的可能途径,对材料变形机制的研究以及高塑性材料的开发提供新的思路。
[Abstract]:Plasticity is an important mechanical property of materials, which determines the amount of deformation that can be realized. Improving the plasticity of materials has always been the focus of the study of structural materials, and pseudoelasticity is an important property in shape memory materials. High recoverable strain can absorb more mechanical work, so increasing recoverable strain has always been the focus of shape memory materials. Nanowires have super plasticity. It has been reported that by stress-induced phase transformation or twinning deformation, the elongation of nanowires can reach about 50%, which is called superplasticity. In addition, nanowires are due to surface effects. After deformation, nanowires can be restored to the initial state to achieve pseudo-elasticity. Therefore, nanowires are suitable systems for obtaining superplasticity and ultra-high recoverable strain. At present, the mechanical properties of nanowires are mainly focused on face-centered cubic and body-centered cubic structures. Due to the lack of slip systems, the ductility of dense hexagonal metals is generally considered to be poor, and the elongation of dense hexagonal metals in coarse crystalline states is usually less than 10. However, in nanowires, the ductility of dense hexagonal hexagonal metals is less than 10. The mechanical properties of dense hexagonal metals still need to be further explored. Two kinds of dense hexagonal metal nanowires were selected as the object of study-stress-induced phase transition and stress-induced twinning. The former was represented by pure cobalt and cobalt-ferroalloy nanowires. The latter is represented by pure magnesium nanowires. The feasibility of superplasticity and pseudoelasticity of nanowires is predicted by simulating uniaxial stretching and unloading process of nanowires by molecular dynamics. The main findings are: (1) in cobalt nanowires, through HCP. 鈫扚CC. 鈫扵he two-step structural phase transition of HCP can reach an elongation of about 80%, and the deformed nanowires can be restored to the initial configuration, and the strain can be restored to about 71.The phase transition during deformation can be calculated by static method. The energy of nanowires is expressed as a function of strain, and the energy relations between different configurations in cobalt-ferroalloy nanowires are calculated. And it is used to judge the sequence of stress-induced phase transition and the transformation type. (2) in magnesium nanowires, the elongation of nanowires can reach about 60% by stress-induced secondary twinning, and the deformation can be recovered by de-twin crystallization. In other words, superplasticity and pseudoelasticity were found in magnesium nanowires. It is found for the first time in magnesium that the twin is caused by the stress concentration near the primary twin boundary. The twins are found to be asymmetric structures. Two kinds of new stable structures have been found in the structure energy of symmetrical inclined grain boundary. The twin is determined to be a stable structure and its structure is also asymmetric. It is found that the reorientation phenomenon of the grains in the twin crystal is. 1? The discovery of new structures in symmetric inclined grain boundaries provides a new example for understanding grain boundary structures and their evolution. In summary, a possible way to achieve high plasticity and recoverable strain in HCP metals is proposed by molecular dynamics simulation. The research on deformation mechanism of materials and the development of high-plastic materials provide new ideas.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O341;TB383.1
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本文编号：1636144