【作者】 皮文丽；

【导师】 彭向和；

【作者基本信息】 重庆大学 ， 固体力学， 2007， 博士

【摘要】 形状记忆合金是一种能够在温度和应力作用下发生相变的材料。由于具有独特的形状记忆效应、相变伪弹性、铁弹性等特殊功能,近年来日益受到人们巨大的关注,并迅速在工业、国防、仪表和医疗领域获得广泛的应用。作为一种机敏材料,形状记忆合金本构关系的研究对其应用和发展至关重要。人们已经认识到材料的宏观变形特性取决于材料的微结构特征及其变化。对NiTi形状记忆合金的试验研究表明,它在拉伸和扭转过程中宏观响应特性存在明显的差异,而这种差异与其相变微结构的差异紧密相关。为了合理地描述此类材料在复杂加载路径下的响应特性及微结构依赖性,有必要发展一种基于材料微结构的跨层次的分析方法。本文在分析国内外有关形状记忆合金本构行为研究现状的基础上,结合近年来有关NiTi形状记忆合金试验研究得到的新现象,设计了特殊的夹具,利用扫描电镜和体视显微镜对不同加载条件下试件相变时的表面形态进行了观察和分析。根据观察到的相变微结构对NiTi形状记忆合金的拉伸和扭转特性进行了有限元模拟,进而建立了基于相变微结构及其变化的跨层次的本构模型,对NiTi形状记忆合金的伪弹性特性进行了描述,并和实验结果作了比较。论文的主要工作和结论如下:(1)对NiTi形状记忆合金在拉伸和扭转试验中超弹性响应的研究成果进行了回顾,特别分析了在拉伸和扭转过程中的宏观响应特性差异及其微结构的演化,并针对NiTi形状记忆合金微管的扭转试验结果,初步猜测微管试件在扭转载荷作用下的相变微结构可能为细小的马氏体和奥氏体片层相间的片层状结构,NiTi形状记忆合金的拉伸和扭转特性的巨大差异与其相变微结构的巨大差异密切相关。(2)针对不同应力状态下材料相变时不同的微结构特征及其变化,设计并加工了专用的拉扭加载装置,对试件施加拉扭载荷,将其“冻结”在特定的应力状态下,进而采用扫描电镜,体视显微镜等对试件的表面形态进行观察和分析。观察发现材料在拉伸和扭转载荷作用下发生相变时试件表面微结构都是由奥氏体片和马氏体片交替叠合而成的片层状结构。在扭转载荷作用下发生相变时,其表面形成的片层结构分布较均匀,片层间距小。相反在拉伸载荷作用时,马氏体相的形核和发展都表现出高度的局部化,试件表面出现宏观马氏体带。试验观察验证了关于扭转载荷下试件相变微结构的猜测。(3)基于试验观察到的相变微结构,借助ABAQUS软件对NiTi形状记忆合金微管在拉伸和扭转载荷下的响应特性进行数值模拟,结果很好地重现了NiTi形状记忆合金微管的宏观响应特性,从而确认了相变微结构对材料宏观响应特性的影响。分析表明在应力诱发的相变过程中,随着两相材料体积分数的动态变化,相间约束将发生剧烈变化,当软相的层厚较小时,约束较强。(4)基于对NiTi形状记忆合金微薄壁管的实验观察及有限元分析,假设NiTi形状记忆合金材料由不同取向的代表性单元构成,每个代表性单元又为马氏体片和奥氏体片交替叠合的片层状结构,建立了跨微-细-宏观三个尺度的本构模型体系。假设奥氏体相为弹塑性而马氏体相为线弹性,采用简单机械模型推导了马氏体相的本构模型。进而考虑到基于以面内应变协调、面外应力协调假设为基础建立的细片层状胞元模型应用于片层间距较大场合的局限性,引入参数κ消除细片层状胞元模型应用于片层间距较大场合时存在的虚假体积应变,将细片层状胞元模型拓展到有限层厚的场合;在此基础上利用Hill自洽方法和各向异性Eshelby张量得到NiTi的弹塑性本构响应。(5)利用所建立的跨尺度的分析方法,发展了相应的数值算法和程序,对NiTi形状记忆合金微管在拉、扭及复杂载荷下的响应特性进行了分析,与试验结果的对比表明,建立的本构模型较好地描述了拉伸试验过程中的应力跌落现象和NiTi形状记忆合金材料在拉伸和扭转过程中材料伪弹性行为的巨大差异。更多还原

【Abstract】 Shape memory alloy (SMA) is a kind of material, in which phase transformation will take place at certain temperature and stress. Due to their particular properties, such as shape memory effect, pseudoelastisity and ferroelasticity, shape memory alloys have been receiving increasing attention in recent years, and are extensively used in many fields, such as industry, aviation, national defense, instruments and medical devices, etc.As a smart material, the study of constitutive relation is critical for its application and development. It has been recognized that thermomechanical properties of a material are determined by its microstructure. It was found experimentally that the pseudoelastic behavior of NiTi shape memory alloys under different loading paths is distinctly different, which can be attributed to the difference of the microstructures during transformation. It is necessary to develop a multi-scale analysis method to study response characteristic and microstructure dependence, and establish a constitutive relation, in which the microstructure characteristic is considered.In this dissertation, the differences of the responses of a NiTi SMA microtube subjected to tension and torsion are systematically investigated. Through special designed device, SEM experiment is carried out to study surface morphology under certain stress path. Finite element analysis is performed to describe mechanical behavior of tensile and torsion of NiTi SMA. The pseudoelastic behavior of a NiTi microtube is analyzed and compared with experimental results. The main work and conclusions in this dissertation are as follows:(1) The progress on the research of the pseudoelastic behavior of shape memory alloys is reviewed. The difference of the macroscopic behavior during tension and torsion and the evolvement of microstructure are analyzed. It is supposed that alternative parallel austenite and martensite fine strips uniformly distributed at the surface of the NiTi SMA microtubes subjected to torsion phase. The research close centers on this suspicion to launch.(2) In view of different phase-transformation microstructure under different loading paths, a special device is designed and processed to load sample. The surface morphologies of the deformed thin-walled NiTi SMA microtube specimens are observed with scanning electron microscope(SEM) and optical microscope, and pseudoelastic experiments are performed by same device. SEM observation shows alternative austenite and martensite striae at the surface of the NiTi SMA microtubes subjected to pure torsion. But for the microtubes subjected to pure tension, macroscopic martensite bands can be observed, which can be attributed to the localization and growth of martensite phase. The test confirmed our suspicion.(3) The mechanical response of a NiTi shape memory alloy under tension and torsion is analyzed with the commercial FE code ABAQUS/standard. The change of the mechanical response, which is the combination of the individual responses of different phases and the interphase constraint, is investigated at different stage. The result shows that interaction between phases will change remarkably with the change of volume fraction, the smaller of volume fraction of soft phase means the larger constraining force between interphase. The difference of the macroscopic behavior during tension and torsion are analyzed with different microstructure characteristic.(4) Based on test and finite element analysis, assumpe that a NiTi SMA is an aggregate of numerous cells, and each cell is composed of many alternative fine lamellas of austenite and martensite. First, constitutive equations of austenite and martensite are derived through simple mechanical model. The effect of the localization and the growth of the martensite phase are taken into account by introducing an additional local volumetric strain, which relaxes the over-constraint induced by the identical in-plane strain assumption that is accurate as the interlamellar spacing is infinitesimal. Then, the equation of representative volume element is derived with simplification supposition. Using Hill self-consistent scheme and Eshelby tensor, The elastic-plastics equation of NiTi SMA is established. The multi-scale constitutive system is get.(5) Take use of multi-scale constitutive description,the corresponding numerical algorithm is developed. The pseudoelastic behavior of a NiTi SMA subjected to pure tension and pure torsion is analyzed. It shows that the developed constitutive model can well describe the typical“stress drop”of the materials subjected tensile deformation, and the difference between the pseudoelastic responses corresponding respectively to tension and torsion.更多还原