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

技术摘要形状记忆合金 (SMA) 是一种具有源自马氏体转变的卓越性能的材料。 马氏体转变的晶体学方面已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一些基本方面尚不清楚。 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通过诱导额外的变换变体来实现。 文献中有大量关于可塑性的证据，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。通过结合微观机械测试、原位和死后扫描透射电子显微镜 (STEM) 以及多尺度计算建模工作，当前的研究旨在开发出作为循环函数的微观结构演变的更详细图片。 通过观察与小体积材料（微柱）中个体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。 通过表征这些孤立的转变事件期间产生的缺陷，可以阐明可塑性和转变之间耦合的本质。 微机械测试的结果还与通过原位 STEM 实验在纯热循环和组合热机械循环中观察到的子结构发展进行了比较。 这项实验工作得到了各种长度尺度的微观结构敏感建模的补充，包括对特定马氏体模式产生的局部应力的埃谢尔比型、变量水平预测。 在之前发表的工作中，该模型的输出与实验观察到的主动滑移系统表现出显着的一致性。 各种新颖的实验和计算技术的结合使人们能够前所未有地深入了解镍钛基 SMA 中驱动功能疲劳的基本机制，并最终为未来的应用带来更耐疲劳的合金设计。 非技术摘要形状记忆合金 (SMA)，例如 NiTi，是一种独特的材料，能够在循环加热或加载后保留其原始形状的“记忆”。 这使得这些材料对于医疗行业（例如支架和手术器械）、汽车行业中的固态执行器以及技术领域中的微机电系统（MEMS）应用极具吸引力。 不幸的是，这些卓越的性能随着重复循环而迅速退化，使得它们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镍钛基形状记忆合金的功能疲劳机制。通过采用微机械测试、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微观结构如何随着机械和热循环而演变。 具体来说，这包括观察和分析材料的行为以及在不同长度尺度下的纯机械循环、纯热循环和组合热机械条件下所产生的缺陷积累。 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抗疲劳合金的开发，这种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.