Deformation via the Transformation of Hierarchical Microstructures

通过分级微观结构的转变产生变形

• 批准号: 172377318
• 负责人: Privatdozent Dr. Steffen Brinckmann
• 金额: --
• 依托单位: Boise State University
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2010
• 资助国家: 德国
• 起止时间: 2009-12-31 至 2014-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/172377318?language=en
• 关键词: Deformation; via; Transformation; Hierarchical; Microstructures

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.

形状记忆合金的微观结构由分层排列的孪晶组成，其中孪晶在孪晶内形成，孪晶在孪晶内形成等等。在机械载荷下，孪晶在所有分层水平上集体重新排列以适应应变。孪晶边界运动受局部应力场控制，而局部应力场又取决于孪晶在所有长度尺度上的有组织重排。虽然孪生被认为是形状记忆合金中的主要变形机制，但由于热形状记忆合金中的驱动力很大（超过孪生应力），自组织和协调重排的机制很少受到关注。磁性形状记忆合金（MSMA）的发现引起了人们对孪晶有序重排的极大兴趣，因为对于给定的单晶，孪晶应力的变化超过两个数量级，并且磁驱动力相对较小，因此，磁机械性能对孪晶微结构高度敏感。本文将从实验和数值模拟两个方面研究分级孪晶微结构的变形机理。在变形实验中，将用三种方法原位观察孪晶的重排-透射电子显微镜，原子力显微镜和光学显微镜-覆盖七个长度级别：从纳米到厘米。数值模拟将提供理论上的洞察分层孪生机制。通过实验研究，可以得到孪晶组织的缺陷含量。向错偶极子，即具有剪切位移场的介观线缺陷，代表所有层次上的孪晶的位移场。一个向错动力学代码将被开发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。通过给数值研究提供实验数据，并系统地改变实验和数值参数，将产生对具有复杂分层孪晶微结构的（磁性）形状记忆合金的机械（和磁机械）性能的定量基本理解。更广泛的影响：基于MSMA的致动器的优点之一是10%的巨大冲程。这种致动器的发展由于在循环致动期间的故障而受到阻碍。然而，具有密集孪生微结构的MSMA实现了长寿命，同时显著减少了冲程。通过这项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。该项目将产生从研究实验室到工业技术的领先MSMA所需的计算工具，并通过nanoHUB.org传播它们。研究生和本科生将接受与纳米技术和微电子行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，每年夏天将访问德国波鸿鲁尔大学的国际合作伙伴，学习计算方法。他们将获得国际经验，并获得实验和数字，技术相关的专业知识。该计划将帮助学生成功地为苛刻和具有挑战性的全球技术市场做出贡献并参与竞争。学生将参加K-6学校参观，向孩子们介绍现代技术的迷人世界。成功的招聘方法已经到位，积极招聘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计划招聘和从工程入门课程招聘。