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

形状记忆合金（sma）是一种由马氏体相变产生的具有显著性能的材料。马氏体相变的结晶学方面已经在许多体系中得到计算和验证；然而，形状记忆和伪弹性行为的一些基本方面还没有被理解。其中最主要的是基体如何适应与相变相关的大应变。理论上，调节可以通过基体塑性或诱导额外的转变变体来实现。由于文献中有大量关于塑性的证据，了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。通过结合微观力学测试、原位和死后扫描透射电子显微镜（STEM）以及多尺度计算模型的努力，目前的研究旨在开发更详细的微观结构演变作为循环函数的图像。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。通过描述这些孤立的转变过程中产生的缺陷，阐明了塑性与转变之间耦合的本质。显微力学测试的结果也与原位STEM实验中观察到的纯热循环和综合热-机械循环的子结构发展进行了比较。这一实验工作还得到了不同长度尺度的微结构敏感模型的补充，包括eshelby型的、由特定马氏体模式产生的局部应力的变水平预测。在先前发表的工作中，该模型的输出与实验观察到的活动滑动系统显示出显著的一致性。各种新颖的实验和计算技术的结合，使人们对niti基sma中驱动功能疲劳的基本机制有了前所未有的了解，并最终将为未来的应用带来更耐疲劳的合金设计。形状记忆合金（sma），如NiTi，是一种独特的材料，能够在循环加热或加载后保持其原始形状的“记忆”。这使得这些材料在医疗行业（如支架和外科设备）、汽车行业（如固态致动器）和技术领域（用于微机电系统（MEMS））的应用中极具吸引力。不幸的是，这些卓越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。目前的工作重点是通过使用各种新颖的实验技术来研究niti基形状记忆合金的功能疲劳机制。通过采用微力学测试、原位和死后扫描透射电子显微镜（STEM）以及计算模型，该研究旨在更详细地了解机械和热循环过程中微观结构的演变。具体来说，这包括对材料的观察和分析。在不同长度尺度下，纯机械循环、纯热循环和复合热-机械条件下的S行为和由此产生的缺陷积累。此外，该项目还促进了与通用汽车研发中心、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抗疲劳合金，这种合金在苛刻的应用中可以胜过当前最先进的材料。

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.