Understanding the Interactions between Recoverable and Permanent Deformations in Shape Memory Alloys

形状记忆合金是一种具有独特性能的金属材料，包括超弹性性能。这项研究旨在指导这些合金的加工和设计，以迅速扩大其在生物医学设备、航空航天和汽车部件以及其他应用中的应用。由于镍钛（NiTi）是应用最广泛的形状记忆合金，因此将对其进行研究，并且将研究结果应用于提高其在实践中的性能将具有最广泛的影响。这项研究将使用新开发的实验方法——包括通过在扫描电子显微镜中跟踪纳米颗粒组合来测量微尺度变形的能力——来绘制和理解这些合金是如何恢复大变形的，以及什么样的变形是永久的。机器学习方法将应用于将变形与原子结构联系起来，从而提高对加工效果的理解，并进一步提高建模和设计形状记忆合金部件的能力。除了这些新信息之外，本研究中开发的技术将对实验力学基础设施做出有价值的贡献，这些基础设施可用于研究广泛的材料。计划开展几项拓展活动，包括科学日和竞赛，向从小学到大学的学生介绍实验力学和材料。本研究将通过实验表征多晶形状记忆合金中应力诱导马氏体相变和位错滑移之间的相互作用，特别是微观结构对这些相互作用性质和由此产生的超弹性行为的影响。全场、高分辨率变形映射将结合高维聚类和计算机视觉方法来分割和识别相对于微观结构的变形和滑动，揭示随机和确定性的贡献。本研究将通过实验解决多晶形状记忆合金晶粒间存在应变不相容这一预测多晶形状记忆合金行为的核心难题。现有的本构模型大多是通过与试验应力-应变曲线的对比来评价的。然而，这种比较并不理想，因为它是在两个不同的长度尺度之间进行的：宏观曲线正在与基于微观力学的模型进行比较。本研究的结果将提供对形状记忆合金行为至关重要的颗粒内相互作用的实验见解，实验验证本构模型的基础，并定量地解决正在积极辩论的假设；例如，相变和塑性可以协同发生的假设，塑性提供了一种跨晶粒的桥接机制，这些晶粒不适合相变。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Shape memory alloys are metallic materials that exhibit unique properties, including superelastic behavior. This research is targeted towards guiding the processing and design of these alloys for their rapidly expanding use in biomedical devices, aerospace and automotive components, and other applications. Nickel-Titanium (NiTi) will be examined because it is the most widely used and modeled shape memory alloy, and results from this work applied to improving its performance in practice will have the broadest impact. This research will use newly developed experimental approaches - including the ability to measure microscale deformations by tracking nanoparticle assemblies in a scanning electron microscope - to map and understand how these alloys recover large deformations, and what deformation remains permanent. Machine learning approaches will be applied to relate deformations to atomic structure, enabling an improved understanding of the effects of processing and furthering the ability to both model and design shape memory alloy components. In addition to this new information, the techniques developed in this research will be valuable contributions to the experimental mechanics infrastructure that can be used in investigations of a wide host of materials. Several outreach activities are planned, including science days and competitions to introduce students from elementary school to college to experimental mechanics and materials. This research will experimentally characterize the interactions between stress-induced martensitic phase transformation and dislocation slip in polycrystalline shape memory alloys, specifically the impact of microstructure on the nature of these interactions and the superelastic behavior that results. Full-field, high resolution deformation mapping will be combined with high-dimensional clustering and computer vision approaches to segment and identify transformation and slip with respect to microstructure, shedding light on stochastic and deterministic contributions. This research will experimentally address a core difficulty in predicting the behavior of polycrystalline shape memory alloys, which is the existence of strain incompatibilities between the grains. Current constitutive models are largely evaluated by the comparison to experimental stress-strain curves. However, this comparison is not ideal in that it is between two different length scales: macroscopic curves are being compared to micromechanics based models. The results from this research will offer experimental insights into the intragranular interactions that are critical to the behavior of shape memory alloys, experimentally validate the underpinnings of the constitutive models, and quantitatively address hypotheses that are under active debate; such as the hypothesis that transformation and plasticity can occur synergistically, with plasticity providing a bridging mechanism across grains that are poorly oriented for transformation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.