Fundamental Structural Processes of Relaxation and Shear Transformations in Metallic Glasses

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。技术概述：我们所熟悉的金属都是结晶的，众所周知，它们的塑性是由称为位错的结构缺陷引起的。相反，非晶态金属(即？大块金属玻璃？（或bmg）在很大程度上仍未解决。本项目旨在揭示bmg中基本松弛事件的基本结构过程，包括热松弛事件和应力作用下的初始非弹性松弛事件。例如，在施加的应力下，必须有结构起源负责局部基本流动事件，即所谓的？剪切转换?(STs)。应该有优惠吗？流的缺陷吗?生成的（建议的？）剪切转变区？STZs)，它们在介导应力驱动的携带塑性应变的原子洗刷中起位错的作用。利用分子动力学模拟，在原子水平上揭示热弛豫和STs的结构起源。将根据特定的局部结构和动力学特性，包括局部有序程度、原子位点应力和自由体积含量，确定（合作）STs在应力下的肥沃位点位置。原子性ST机制（触发事件和协同原子剪切/洗选）、STZ大小（涉及的原子数量）和这个长度尺度的起源、STZ局部原子的繁殖或倾向、STZ在ST期间和流动状态中短范围秩序的演变，以及STZ在局部化中导致后期剪切带的聚结行为，都将被研究。结构过程的动力学路径，特别是相关的过渡势垒及其对局部结构的依赖，将根据势能景观来确定。本研究的智力价值在于关键结构的解析。非晶金属的（变形）性质关系问题。非技术概述：与传统的全结晶金属和合金相比，非结晶（无定形）大块金属玻璃（bmg）具有更高的强度，但仍然可以维持塑性流动（永久应变和形状变化）。非晶态（玻璃态）合金的内部结构已达到原子尺度，没有位错。在结晶金属的长程规则晶格中携带塑性变形的缺陷。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。为了产生更广泛的影响，将作出教育努力，编写一部模拟电影，以丰富和促进使用错位概念的几门材料科学课程的教学。这部电影将展示如何脱臼？流过程就像，把它和？晶体中的位错运动？电影帮助学生拓宽他们对变形过程的一般看法。这些电影将由被招募到实验室完成高级设计课程的本科生（由教师/研究生协助）制作，利用他们的计算机技能。总的来说，对结构-变形关系的理解对于目前世界上正在进行的金属玻璃的密集工作具有广泛的意义，特别是对于确定哪种玻璃结构/成分将具有良好的强度和延性组合。我们的研究培养了金属研究前沿的研究生。它建立在PI以前项目中获得的知识的基础上，因此应该有效地利用联邦资金。研究结果将在会议和顶级期刊上发表。

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.