【作者】 李卫国；

【导师】 彭向和；

【作者基本信息】 重庆大学 ， 固体力学， 2005， 博士

【摘要】 热力载荷作用下形状记忆合金(shape memory alloy, SMA)特有的热弹性马氏体相变赋予其奇异的特性,如铁弹性(ferroelasticity, FE)、形状记忆效应(shape memory effect, SME)和伪弹性(pseudoelastisity, PE)等。这些诱人的特性使形状记忆合金作为一种重要的机敏材料近年来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。形状记忆合金在高新技术领域中的迅速增长的应用要求人们对其热力学特性尤其是对其在多轴复杂加载史下的热力学特性进行全面了解和正确的描述。人们已经认识到材料的宏观变形特性与材料在各个尺度下的变形特性有着密切的关系,并从大量事实中得到了一个共识,即材料的宏观特性取决于材料的微观结构。对形状记忆合金的实验表明,其在拉伸和扭转过程中宏观响应特性的差异与其微结构的差异紧密相关,尤其是在马氏体相变和逆相变过程中其片层微结构的形成与发展不同。迄今考虑此类相变微结构的本构描述甚为鲜见,因此有必要发展一种跨层次的分析方法以研究其复杂的响应特性及其微结构依赖性,建立计及微结构特征的材料本构描述。随着研究水平的提高和新现象的不断发现(如相变局部化)要想建立更完整,适应面更广的模型,需要新的方法和新的知识。本文在分析国内外有关形状记忆合金本构研究现状的基础上,结合近年来的关于形状记忆合金实验研究得到的新现象,对实验中的微结构发展和伴随着宏观马氏体带的形成出现的应力跌落现象进行了分析,并对形状记忆合金微圆管的拉伸实验进行了有限元分析,进一步从材料细观结构的变化及两相材料的相互约束关系出发,建立了记及片层状微结构的本构模型。论文的主要工作和结论如下:(1)对有关形状记忆合金在拉伸和扭转实验中的超弹性响应的研究成果进行了回顾,特别考察了在拉伸和扭转过程中的宏观响应特性差异的物理机制、结构演化及其伴随着宏观马氏体带的形成出现的应力跌落现象,并解释了其物理机理。阐述了建立形状记忆合金本构方程时考虑其结构的必要性。(2)对形状记忆合金在拉伸下的力学特性进行了有限元分析,有限元分析中采用了三种不同分析模型,研究了不同实验阶段马氏体相和奥氏体相力学行为由于界面约束造成的变化,得出了在实验过程中两相力学行为之间的关系随马氏体体积含量的变化规律。结果显示在相变过程中,随着材料结构的演化,两相间的约束强度发生了剧烈变化。(3)基于对NiTi形状记忆合金的实验观察及有限元分析,考虑两相间的应变不协调关系,采用应变修正法建立了记及片层状微结构的本构模型,本模型考虑更多还原

【Abstract】 Shape memory alloys (SMAs) have been receiving increasing attention in recent years, due to their particular properties under thermomechanical loading, such as ferroelasticity, shape memory effect and pseudoelastisity. These properties are related to the martensitic phase transformation and are extensively used in many fields, such as industry, aviation, national defense, instruments and medical devices, etc. The rapid increasing applications of shape memory alloys require better understanding and more accurate description of the thermomechanical behavior of SMAs, especially the behavior of SMAs subjected to multi-dimensional loading complex thermomechanical loading histories. It has been recognized that the macroscopic property of a material is strongly dependent on its microstructures. The experiments on the NiTi SMAs show significant difference in the pseudoelastic behavior as well as the microstructures of the SMA under pure torsion and under pure tension, especially a sharp descent after the onset of martensitic phase transformation and the corresponding distinct martensitic band during a tensile process. However, such significant behavior has not been successfully described.In this dissertation, the differences in the responses of a NiTi SMA microtube subjected to pure tension and pure torsion are systematically investigated. A finite element analysis is performed for the significant stress drop phenomenon, taking into account the phase-transformation microstructures and their evolution observed during tension. It is found that the physical and mechanical mechanism of the distinct stress drop can be attributed to the variation of the microstructure, i.e., the initiation and growth of the martensite band that strongly reduce the constraint between phases. A constitutive model is proposed based on that a shape memory alloy is the mixture of martensite and austenite with laminated microstructure, taking the thickness of martensitic band as an important parameter. The corresponding incremental form of constitutive model is developed, and the constitutive 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 physical mechanism of the difference in the macroscopic behavior during tension and torsion, as well as the evolvement of microstructure, is更多还原