GOALI: Micromechanical Experiments and Modeling of Shape Memory Response in Ni-Ti Based Alloys

形状记忆合金（SMA）是具有源于马氏体转变的显著特性的材料。 马氏体相变的晶体学方面已经在许多系统中进行了计算和验证;然而，形状记忆和伪弹性行为的基本方面还不清楚。 其中最主要的是矩阵如何适应与变换相关的大应变。 理论上，调节可以通过基质可塑性或诱导额外的转化变体来实现。 随着文献中塑性的大量证据，理解缺陷产生的机制已成为减轻热机械循环应用中功能疲劳的关键组成部分。通过结合微机械测试，原位和事后扫描透射电子显微镜（STEM）和多尺度计算建模工作，目前的研究旨在开发一个更详细的图片的微观结构的演变作为循环的功能。 通过观察与小体积材料（微柱）中单个马氏体转变模式相关的塑性变形，促进了缺陷产生和增殖的基础研究。 通过表征在这些孤立的转变事件期间产生的缺陷，塑性和转变之间的耦合的性质被照亮。 微观力学测试的结果也进行了比较，在纯热和结合热机械循环通过原位STEM实验观察到的子结构的发展。 这项实验工作正在补充微观结构敏感的建模在不同的长度尺度，包括Eshelby型，变量水平的预测特定的马氏体模式开发的局部应力。 在以前发表的工作中，该模型的输出显示出显着的协议与实验观察到的主动滑移系统。 这种结合各种新的实验和计算技术允许一个前所未有的洞察力的基本机制驱动功能疲劳NiTi为基础的SMA，并最终导致更多的耐疲劳合金设计为未来的应用。 非技术概述形状记忆合金（SMA），例如NiTi，是能够在循环加热或加载之后保持其原始形状的“记忆”的独特材料。 这使得这些材料在医疗行业中的应用非常有吸引力，例如支架和手术器械，在汽车行业中作为固态致动器，以及在技术领域用于微机电系统（MEMS）。 不幸的是，这些显著的性能随着重复循环而迅速退化，使其不适合许多潜在的应用。 目前的工作主要是通过使用各种新的实验技术研究功能疲劳的机制，在镍钛基形状记忆合金。通过采用微机械测试，原位和死后扫描透射电子显微镜（STEM）和计算建模，该研究旨在更详细地了解微观结构如何随着机械和热循环而演变。 具体来说，这包括对材料的观察和分析。的行为和由此产生的缺陷积累的纯机械循环，纯热循环，并结合热机械条件在不同的长度尺度。 此外，该计划还促进了与通用汽车研发部、德国波鸿鲁尔大学等机构的工业和国际合作。 这项工作的结果最终将有助于开发耐疲劳合金，这些合金在苛刻的应用中可以超越当前最先进的材料。

TECHNICAL SUMMARYShape memory alloys (SMAs) are materials with remarkable properties that stem from a martensitic transformation. The crystallographic aspects of the martensitic transformation have been calculated and verified in a number of systems; however, there are fundamental aspects of shape memory and pseudoelastic behavior that are not understood. Principal among these is how the matrix accommodates the large strain associated with the transformation. Theoretically, accommodation may be achieved either by matrix plasticity or by inducing additional transformation variants. With plentiful evidence for plasticity in the literature, understanding the mechanism of defect generation has become a critical component for mitigating functional fatigue in thermomechanical cycling applications.By combining micromechanical testing, in situ and post mortem scanning transmission electron microscopy (STEM), and multi-scale computational modeling efforts, the current study aims to develop a more detailed picture of the microstructural evolution as a function of cycling. The fundamental study of defect generation and multiplication is being facilitated by the observation of plastic deformation associated with individual martensite transformation modes in small volumes of material (micropillars). By characterizing the defects generated during these isolated transformation events, the nature of the coupling between plasticity and the transformation is being illuminated. The results from micromechanical testing are also compared to the substructure development observed in pure thermal and combined thermomechanical cycling via in situ STEM experiments. This experimental work is being supplemented by microstructure-sensitive modeling at various length scales, including an Eshelby-type, variant-level prediction of the local stresses developed by specific martensite modes. In previously published work, this model's output has shown remarkable agreement with the active slip systems observed experimentally. This combination of a variety of novel experimental and computation techniques allows for an unprecedented insight into the fundamental mechanisms driving functional fatigue in NiTi-based SMAs and will eventually lead to more fatigue-resistant alloy design for future applications. NON-TECHNICAL SUMMARYShape memory alloys (SMAs), such as NiTi, are unique materials that are able to retain a "memory" of their original shape after cyclic heating or loading. This makes these materials extremely attractive for applications in the medical industry such as stents and surgical devices, in the automotive industry as solid-state actuators, and in the technological world for use in micro-electro-mechanical systems (MEMS). Unfortunately, these remarkable properties rapidly degrade with repeated cycling, making them unsuitable for many potential applications. The current work is focused on studying the mechanisms of functional fatigue in NiTi-based shape memory alloys through the use of a variety of novel experimental techniques. By employing micromechanical testing, in situ and post mortem scanning transmission electron microscopy (STEM), and computational modeling, the study aims to develop a more detailed understanding of how the microstructure evolves with both mechanical and thermal cycling. Specifically, this includes observation and analysis of the material?s behavior and resultant defect accumulation for pure mechanical cycling, pure thermal cycling, and combined thermomechanical conditions at different length scales. In addition, this program facilitates industrial and international collaborations with General Motors Research and Development, the Ruhr University in Bochum, Germany, and others. The results of this work will eventually aid in the development of fatigue-resistant alloys that can out-perform current state-of-the-art materials in demanding applications.