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年美国复苏和再投资法案（公法111-5）资助的。技术概要：我们所熟悉的金属都是晶体，它们的塑性众所周知是由称为位错的结构缺陷携带的。相反，相应的塑性流动机制在非晶态金属（即？大块金属玻璃？（BMG）仍然基本上没有解决。该项目旨在揭示BMG中基本弛豫事件的基本结构过程，包括热弛豫事件和应力下的初始非弹性弛豫事件。例如，在外加应力下，必须有结构起源负责的局部基本流事件，所谓的？剪切变换（ST）。应该有优惠吗？流动缺陷？建议（The Proposed？剪切转变区，STZ），其在介导承载塑性应变的应力驱动的原子重排中起位错的作用。热弛豫和ST的结构起源将被发现在原子水平上使用分子动力学模拟。肥沃的网站（合作）ST应力下的位置将被确定，根据特定的局部结构和动力学性质，包括当地秩序的程度，原子网站的应力和自由体积含量。原子ST机制（触发事件和合作原子剪切/洗牌），STZ的大小（涉及的原子数）和这个长度尺度的起源，生育率或ST的本地原子的倾向，ST期间的短到中等范围的顺序和流动状态的演变，以及STZ在本地化导致后来的剪切带的聚结行为，都将被调查。结构过程的动力学途径，特别是相关的过渡障碍及其对局部结构的依赖性，将根据势能景观来确定。这项研究的智力价值在于解决了一个关键结构？非技术概述：与全部为结晶的常规金属和合金相比，非结晶（非晶）块状金属玻璃（BMG）显示出更高的强度，并且仍然可以维持塑性流动（永久应变和形状变化）。非晶（玻璃态）合金的内部结构现在是在原子尺度上，没有？错位？在晶体金属的长程规则晶格中进行塑性变形的缺陷。该项目旨在揭示BMG中的这种玻璃结构如何在应力下演变以控制材料的屈服和延展性。为了更广泛的影响，将通过编辑模拟电影来丰富和推进使用位错概念的几门材料科学课程的教学，从而进行教育努力。这部电影将展示如何一个？无错位？流程会是什么样子，并将其与一个？晶体中的位错运动影片，以帮助学生扩大他们的看法有关变形过程一般。这些电影将由本科生（由教师/研究生协助）招募到实验室完成他们的高级设计课程，利用他们的计算机技能。一般来说，对结构-变形关系的理解对于目前世界各地正在进行的金属玻璃的深入研究具有广泛的意义，特别是对于确定什么样的玻璃结构/组合物将具有良好的强度和延展性的组合。我们的研究培养研究生在金属研究的前沿。它建立在PI以前的项目中获得的知识的基础上，因此应该是联邦资金的有效利用。研究结果将在会议和顶级期刊上传播。