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

形状记忆合金是具有独特性能的金属材料，包括超弹性行为。该研究旨在指导这些合金的加工和设计，以快速扩大其在生物医学设备，航空航天和汽车零部件以及其他应用中的应用。镍钛合金（NiTi）将被检查，因为它是最广泛使用和建模的形状记忆合金，并从这项工作的结果应用于提高其性能在实践中将有最广泛的影响。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粒子组装来测量微尺度变形的能力-来绘制和了解这些合金如何恢复大变形，以及什么变形保持永久性。机器学习方法将被应用于将变形与原子结构相关联，从而能够更好地理解加工的影响，并进一步提高建模和设计形状记忆合金部件的能力。除了这些新的信息，在这项研究中开发的技术将是有价值的贡献的实验力学基础设施，可用于调查的各种材料。计划开展一些推广活动，包括科学日和竞赛，向小学到大学的学生介绍实验力学和材料。 本研究将实验表征应力诱导马氏体相变和多晶形状记忆合金中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的性质和超弹性行为的影响。全场，高分辨率变形映射将与高维聚类和计算机视觉方法相结合，以分割和识别相对于微观结构的转换和滑移，揭示随机和确定性的贡献。本研究将从实验上解决多晶形状记忆合金行为预测的一个核心难点，即晶粒间存在的应变不相容性。目前的本构模型主要通过与实验应力-应变曲线的比较来评估。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：宏观曲线与基于微观力学的模型进行比较。从这项研究的结果将提供实验见解的晶内相互作用，是至关重要的形状记忆合金的行为，实验验证的本构模型的基础，并定量地解决假设，正在积极辩论;例如，假设转化和可塑性可以协同发生，该奖项反映了NSF的法定使命，并通过使用基金会的智力价值进行评估，更广泛的影响审查标准。

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.