【作者】 李静；

【导师】 康国政； 于超；

【作者基本信息】 西南交通大学 ， 力学， 2020， 硕士

【摘要】 随着纳米科技的不断发展以及材料器件等逐步趋向小型化,纳米晶体NiTi合金在电子、机械、航空、能源、交通、医疗等诸多领域得到了广泛应用。NiTi形状记忆合金独特的热-力学特性由其马氏体相变和重定向机制决定,因此,在原子尺度上揭示相变、重定向微观机制一方面可为高性能NiTi形状记忆合金的设计与制备提供理论指导,另一方面也能为基于微观机理本构模型的建立提供信息,具有重要的科学和工程意义。虽然目前在NiTi合金应力/温度诱发马氏体相变的分子动力学模拟方面取得了较大的进展,但已有的工作很少涉及单程形状记忆效应中特有的一个过程,即应力诱发马氏体重定向,且没有探究单程形状记忆效应对加载条件的依赖性。本文采用基于第二近邻修正型嵌入原子势的分子动力学方法对纳米单晶NiTi合金单程形状记忆效应进行模拟,揭示其在温度诱发马氏体相变和马氏体状态下应力诱发马氏体重定向过程中的晶体结构特征及微结构演化,探究原子尺度下NiTi合金马氏体相变和马氏体重定向在不同峰值应力和不同加卸载速率下的变化规律,此外进一步分析边界条件和尺寸因素所产生的影响。本文主要开展了以下研究工作:(1)建立纳米单晶NiTi形状记忆合金分子动力学模型,对周期性边界条件下的块状单晶NiTi合金的形状记忆效应进行模拟,此外分析了应力峰值和应力加卸载率因素对NiTi合金形状记忆效应的影响。(2)将模型的边界改为非周期性边界,以y方向为周期性边界,x、z方向为非周期性边界,对NiTi合金单晶纳米柱的形状记忆效应进行模拟,讨论自由表面所造成的影响。(3)改变原模型的长宽高比例,建立三种不同尺寸的NiTi单晶纳米柱模型,讨论在非周期性边界条件下尺寸因素对单晶纳米柱温度诱发相变及形状记忆效应的影响。本文的研究工作可深入揭示NiTi形状记忆合金原子结构微观演变机理,阐明相变过程中所涉及的原子迁移过程(如马氏体成核和生长、晶体孪生行为以及不可恢复应变的发生),这对NiTi形状记忆合金产品的性能优化、寿命评估和安全设计具有重要实践指导意义。更多还原

【Abstract】 With the continuous development of nanotechnology and the progressive miniaturization of materials and devices,nanocrystalline NiTi alloys have been widely used in many fields such as electronics,machinery,aviation,energy,transportation,and medical treatment.The unique thermo-mechanical properties of NiTi shape memory alloy are determined by its martensite transformation and reorientation mechanism.Therefore,revealing the phase transition and reorientation microscopic mechanism on the atomic scale can not only provide theoretical guidance for the design and preparation of high-performance NiTi shape memory alloy,but also provide information for the establishment of constitutive model based on microscopic mechanism,which has important scientific and engineering significance.Although much progress has been made in molecular dynamics simulation of NiTi alloy stress/temperature-induced martensite transformation,the existing work rarely involves a process unique to the one-way shape memory effect,namely stress-induced martensite reorientation,and the dependence of the one-way shape memory effect on loading conditions is also not explored.In this paper,the one-way shape memory effect of nanocrystalline NiTi alloy was simulated by the molecular dynamics method based on the second nearest neighbor modified embedded atomic potential,and the crystal structure characteristics and microstructure evolution in the process of temperature-induced martensitic transformation and stress-induced martensite reorientation were revealed.The change of NiTi alloy martensitic transformation and martensite reorientation at atomic scale caused by different peak stresses and different loading and unloading rates was investigated.In addition,the influence of boundary conditions and size factors was also analyzed.The main work of this thesis is summarized as follows:(1)A molecular dynamics model of nano-single-crystal NiTi shape memory alloy was established,the shape memory effect of bulk single-crystal NiTi alloy under periodic boundary conditions was simulated,and the influence of peak stress and stress rate on the shape memory effect of NiTi alloy was analyzed.(2)Adopting a non-periodic boundary,the shape memory effect of the NiTi single crystal nano-pillar was simulated.The influence caused by free surface was further discussed.(3)Three NiTi single crystal nano-pillar models with different sizes were constructed by changing the length,width and height ratio of the original model.The effect of size on the temperature-induced phase transition and shape memory effect of single crystal nano-pillars under non-periodic boundary conditions were discussed.The research work in this paper is of great significance for further revealing the microscopic evolution mechanism of NiTi shape memory alloy.This study can clarify the atomic migration processes involved in the phase transition process(such as martensite nucleation and growth,crystal twinning behavior and the occurrence of unrecoverable strain),all of which have important practical guiding significance for the performance optimization,safety design and life evaluation of NiTi shape memory alloy products.更多还原