Deformation via the Transformation of Hierarchical Microstructures

形状记忆合金的微观结构是分层排列的孪晶，孪晶在孪晶中形成孪晶等等。在机械加载时，双胞胎在所有层次上集体重新排列以适应应变。孪晶界运动受局部应力场控制，而局部应力场又依赖于孪晶界在所有长度尺度上的有序重排。虽然孪晶是形状记忆合金的主要变形机制，但由于热形状记忆合金中存在较大的驱动力（超过孪晶应力），自组织和协调重排机制很少受到关注。磁性形状记忆合金（MSMA）的发现引起了人们对孪晶组织重排的极大兴趣，因为对于给定的单晶，孪晶应力变化超过两个数量级，并且磁驱动力相对较小，因此，磁力学性能对孪晶微观结构高度敏感。智力优势：分层孪晶微观结构的变形力学将进行实验和数值研究。在变形实验中，将通过透射电子显微镜、原子力显微镜和光学显微镜三种方法对孪晶的重排进行现场观察，覆盖从纳米到厘米的七个量级的长度尺度。数值模拟将为分层孪晶力学提供理论见解。通过实验研究，得到孪晶组织的缺陷含量。斜向偶极子，即具有剪切位移场的介观线缺陷，代表了孪晶在所有层次上的位移场。将开发一个偏斜动力学代码并应用于大规模的数值研究，这将产生微观结构和力学行为之间的相关性。通过为数值研究提供实验数据，并通过系统地改变实验和数值参数，将产生对具有复杂分层双微观结构的（磁性）形状记忆合金的力学（和磁力学）特性的定量基本理解。更广泛的影响：基于MSMA的执行器的优势之一是10%的巨大行程。由于循环作动过程中的故障，这种作动器的发展受到阻碍。然而，具有密集孪晶微结构的MSMA具有较长的使用寿命，可显著减少脑卒中。通过本研究获得的定量理解将允许开发具有大行程和长寿命的MSMA致动器。该项目将生产将MSMA从研究实验室引导到工业技术所需的计算工具，并通过nanoHUB.org传播。研究生和本科生将接受与纳米技术和微电子工业相关的尖端表征方法的培训。美国学生将在博伊西州立大学进行实验研究，他们将在每年夏天访问德国波洪鲁尔大学的国际合作伙伴一到三个月，学习计算方法。他们将获得国际经验，并获得实验和数字技术相关的专门知识。该课程将为学生在要求苛刻和充满挑战的全球技术市场中成功地做出贡献和竞争做好准备。学生们将参加幼儿园至六年级的学校参观活动，向孩子们介绍迷人的现代科技世界。成功的招生方法是积极招收贫困学生和代表性不足群体的学生。这些招聘方法包括从麦克奈尔项目招聘和从工程入门课程招聘。

The microstructure of shape-memory alloys consists of hierarchically arranged twins, where twins are formed within twins within twins and so on. Upon mechanical loading, the twins collectively rearrange on all hierarchical levels to accommodate strain. Twin-boundary motion is controlled by local stress fields, which in turn depend on the organized rearrangement of twins on all length scales. While twinning was recognized as the dominating deformation mechanism in shape-memory alloys, the mechanics of self-organization and coordinated rearrangement received little attention due to large driving forces (exceeding the twinning stress) in thermal shape-memory alloys. The discovery of magnetic shape-memory alloys (MSMA) has sparked significant interest regarding the organized rearrangement of twins because the twinning stress varies over two orders of magnitude for a given single crystal, and the magnetic driving forces are comparatively small, and hence, the magneto-mechanical properties are highly sensitive to the twin microstructure.Intellectual merit: The deformation mechanics of hierarchical twin microstructures will be studied experimentally and numerically. During deformation experiments, the rearrangement of twins will be observed in-situ with three methods – transmission electron microscopy, atomic force microscopy, and optical microscopy – covering seven orders of length-scale: from nanometer to centimeter. Numerical simulations will provide theoretical insight into the hierarchical twinning mechanics. The defect content of the twin microstructure will be obtained from the experimental study. Disclination dipoles, i.e. mesoscopic line defects with a shear displacement field, represent the displacement fields of twins on all hierarchical levels. A disclination dynamics code will be developed and applied to a large scale numerical study, which will yield a correlation between microstructure and mechanical behavior. By feeding the numerical study with experimental data, and by systematically varying experimental and numerical parameters, a quantitative fundamental understanding of the mechanical (and magneto-mechanical) properties of (magnetic) shape-memory alloys with complex hierarchical twin-microstructures will be generated.Broader impact: One of the advantages of MSMA based actuators is the gigantic stroke of 10%. Development of such actuators is hindered due to failure during cyclic actuation. MSMA with densely twinned microstructures achieve a long lifetime, however, with significant reduction of stroke. The quantitative understanding gained with this study will allow developing MSMA actuators with large stroke and long lifetime. This project will produce the computational tools required for leading MSMA from the research laboratory to industrial technologies and disseminate them via the nanoHUB.org. Graduate and undergraduate students will be trained in cutting edge characterization methods relevant for the nanotechnology and microelectronic industries. The US students, who will perform the experimental studies at Boise State University, will visit the international partner at Ruhr University Bochum, Germany for one to three months each summer to study the computational methods. They will gain international experience and acquire experimental and numerical, technology-relevant expertise. This program will prepare the students to successfully contribute to and compete in the demanding and challenging global technology market. Students will participate in K-6 school visits to introduce children to the fascinating world of modern technology. Successful recruiting approaches are in place for actively recruiting underprivileged students and students of underrepresented groups. These recruiting approaches include recruiting from the McNair program and recruiting from introductory courses in engineering.