Fundamental Structural Processes of Relaxation and Shear Transformations in Metallic Glasses

金属玻璃松弛和剪切转变的基本结构过程

• 批准号: 0904188
• 负责人: En (Evan) Ma
• 金额: $ 49.95万
• 依托单位: Johns Hopkins University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-07-15 至 2013-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0904188&HistoricalAwards=false
• 关键词: Fundamental; Structural; Processes; Relaxation; Shear

This Award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).TECHNICAL SUMMARY:The metals we are familiar with are all crystalline, and their plasticity is well known to be carried by structural defects called dislocations. In contrast, the corresponding plastic flow mechanism in amorphous metals (i.e. ?bulk metallic glasses?, or BMGs) remains largely unresolved. This project is designed to uncover the fundamental structural processes responsible for the basic relaxation events in BMGs, including the thermal relaxation events and the initial inelastic relaxation events under stresses. For example, under imposed stresses, there must be structural origins responsible for the localized basic flow events, the so called ?shear transformations? (STs). There should be preferential ?flow defects? generated (the proposed ?shear transformation zones?, STZs), which play the role of dislocations in mediating the stress-driven atomic shuffling that carries the plastic strain. The structural origin of thermal relaxations and STs will be uncovered at the atomic level using molecular dynamics simulations. The locations of fertile sites for (cooperative) STs under stresses will be identified, based on specific local structural and dynamical properties, including the degree of local order, atomic-site stresses and free volume content. The atomistic ST mechanisms (the triggering events and cooperative atomic shear/shuffling), the STZ size (number of atoms involved) and the origin of this length scale, the fertility or propensity of local atoms for STs, the evolution of the short-to-medium range order during the ST and in the flow state, and the coalescing behavior of STZs in localization leading to later shear banding, will all be investigated. The kinetic pathway of the structural processes, in particular the associated transition barrier and its dependence on local structure, will be determined in terms of the potential energy landscape. The intellectual merit of this research lies in the resolution of a key structure ? (deformation) property relationship issue for amorphous metals.NON-TECHNICAL SUMMARY:Compared with conventional metals and alloys which are all crystalline, non-crystalline (amorphous) bulk metallic glasses (BMGs) show higher strength and yet can still sustain plastic flow (permanent strains and shape changes). The internal structures for amorphous (glassy) alloys are now on the atomic scale, without the ?dislocation? defects that carry the plastic deformation in the long-range regular lattice in crystalline metals. This project is designed to uncover how such glassy structures in BMGs evolve under stresses to control the yielding and ductility of the material. For broader impact, an educational effort will be made by compiling a simulation movie to enrich and advance the teaching of several materials science courses in which the concept of dislocation is used. The movie will demonstrate how a ?dislocation-less? flow process would be like, and contrast it with a ?dislocation motion in crystals? movie to help the students broaden their view about deformation processes in general. These movies will be produced by undergraduate students (assisted by faculty/graduate students) recruited into the laboratory to complete their Senior Design course, making use of their computer skills. In general, an understanding of the structure-deformation relationship has broad implications for the intensive work on metallic glasses currently ongoing around the world, especially for identifying what kind of glass structure/compositions would have a good combination of strength and ductility. Our research trains graduate students at the cutting edge of metals research. It builds on the knowledge acquired during the PI's previous projects and should hence be an efficient use of Federal funds. The results will be disseminated at conferences and in top journals.

该奖项是根据2009年的《美国复苏与再投资法》（公法111-5）资助的。技术摘要：我们熟悉的金属都是结晶的，它们的可塑性众所周知，它们是由称为脱位的结构缺陷所携带的。相比之下，无定形金属（即？散装金属玻璃？或BMG）中的相应塑料流量机理仍未得到解决。该项目旨在揭示负责BMG中基本放松事件的基本结构过程，包括热放松事件和压力下的初始无弹性松弛事件。例如，在施加的压力下，必须有构成局部基本流动事件的结构起源，即所谓的剪切转换？ （STS）。应该有优先的？流缺陷？产生的（提出的？剪切转化区？，stzs），哪个在介导应携带塑性应变的压力驱动的原子洗牌中起着脱位的作用。使用分子动力学模拟，将在原子水平上发现热弛豫和ST的结构起源。将根据特定的局部结构和动力学特性（包括局部秩序，原子位点应力和自由体积含量的程度）确定（合作）STS的肥沃部位（合作）STS的位置。 The atomistic ST mechanisms (the triggering events and cooperative atomic shear/shuffling), the STZ size (number of atoms involved) and the origin of this length scale, the fertility or propensity of local atoms for STs, the evolution of the short-to-medium range order during the ST and in the flow state, and the coalescing behavior of STZs in localization leading to later shear banding, will all be investigated.结构过程的动力学途径，特别是相关的过渡屏障及其对局部结构的依赖，将根据势能格局确定。这项研究的智力优点在于解决关键结构？ （变形）无定形金属的性质关系问题。非技术摘要：与传统的金属和合金相比，这些金属和合金都是晶体，非晶体（无定形）散装金属眼镜（BMG），但仍显示出更高的强度，但仍然可以维持塑料流动（永久性的固定量和形状变化）。无定形（玻璃）合金的内部结构现在处于原子尺度上，而没有脱位？在晶体金属中长期常规晶格中携带塑性变形的缺陷。该项目旨在发现BMG中的这种玻璃结构如何在应力下进化以控制材料的屈服和延展性。为了获得更广泛的影响，将通过编译模拟电影来丰富和推进使用错位概念的几种材料科学课程的教学来做出教育。这部电影将演示如何无脱位？流动过程会像，并将其与晶体中的脱位运动对比？电影以帮助学生扩大对变形过程的看法。这些电影将由本科生（由教职员工/研究生的协助）制作到实验室中，以完成他们的高级设计课程，利用他们的计算机技能。一般而言，对结构信息关系的理解对目前全球正在进行的金属眼镜的密集工作具有广泛的影响，尤其是确定哪种玻璃结构/成分将具有良好的强度和延展性结合。我们的研究培训金属研究最前沿的研究生。它基于PI以前项目中获得的知识，因此应该有效利用联邦资金。结果将在会议和顶级期刊上传播。