Degree of short-to-medium-range order in metallic glasses

大多数使用中的金属和合金都是晶体，其内部结构以长程有序为特征。在过去的世纪里，人们对它们的结构进行了深入的研究，为其性质提供了具体的基础。这样的结构-性质范式在材料科学领域已经很好地建立起来。相反，通过快速冷却其母液获得的金属玻璃（MG）具有类似液体的非晶结构，内部没有可辨别的微观结构。虽然缺乏长程周期性，但MG确实具有结构，在最近邻原子协调和位置相关的水平上，在几个纳米的长度尺度上。这种结构性组织很难描述，而且人们对它的了解仍然很少。因此，对镁合金中原子结构和性质之间联系的理解远远落后于对传统金属的理解。本计画旨在利用电脑模拟之方法，解决磁导之中短程序问题。详细的结构信息有望揭示这些玻璃的宏观行为，从而在设计其应用时做出更好的判断。在教育方面，本研究对丰富材料科学重点学科，特别是材料结构核心学科具有重要意义。许多计算机分子动力学模拟适合本科生参加高级设计/研究经验课程，提供有趣的案例研究，与以前在课堂上学到的熟悉的晶体对称性进行比较。技术摘要本项目的目标是1）量化金属玻璃（MG）中可实现的短程有序（SRO）的程度和范围，2）可靠地识别不同类型MG中的中程有序（MRO）的特征模式。这种基本的理解，在无定形固体中的排序是一个基本的材料科学问题，但迄今为止一直难以实现，因为MG配置构建到目前为止，无论是通过拟合散射数据或液体淬火在分子动力学（MD）模拟，本质上是远远低于实验玻璃有序。该项目旨在开发有效的MD模拟路线，能够产生具有势能，原子密度，弹性常数的MG结构，因此与真实世界的MG相似。一个重点是算法开发，以加速MD在更深的能源盆地的搜索。该计划是使用1）延长老化，2）循环应力辅助老化，以及3）新的删除-添加协议，建立在从模拟晶体崩溃或气相沉积获得的无定形结构的基础上。初步结果表明，这些方法（及其组合）可以访问低能量的玻璃态沿着的方式走向最终的结晶。所捕获的配置提供了前所未有的机会，以揭示SRO/MRO在化妆和饱和度的水平，类似于那些在实验室玻璃，允许连接与原子机制的基础上的剪切转换，松弛和其他玻璃性能。 结果预计将是一个重大的一步，朝着建立具体的结构-性能关系的MG。

Nontechnical abstractMost metals and alloys in use are crystals, with an internal structure characterized by long-range order. Their structures have been well studied over the past century, providing concrete basis of their properties. Such a structure-property paradigm is well established in the field of materials science. In contrast, metallic glasses (MGs) obtained by fast cooling of their parent liquids have a liquid-like amorphous structure with no discernable microstructure inside. Though lacking long-range periodicity, MGs do have structure, at the level of nearest-neighbor atomic coordination and positional correlations on the length scale of a couple of nanometers. This structural organization is difficult to characterize and remains poorly understood. As a result, the understanding of the links between the atomic structure and properties in MGs lags far behind that achieved for conventional metals. This project aims to resolve the short-to-medium range order in MGs, using computer simulation approaches. The detailed structural information is expected to shed light on the macroscopic behavior of these glasses, enabling better judgement in designing their applications. In terms of education, this research has significance in enriching key materials science subjects, in particular the core subject of Structure of Materials. Many of the computer molecular dynamics simulations are amenable to the participation of undergraduate students in Senior Design/Research Experience courses, providing interesting case studies to compare with the familiar crystal symmetry learnt previously in the classroom. Technical abstractThe goal of this project is to 1) quantify the degree and extent of short-range order (SRO) realizable in metallic glasses (MGs), and 2) reliably identify the characteristic patterns of medium-range order (MRO) in different types of MGs. This basic understanding of the ordering in amorphous solids is a fundamental materials science issue, but has been difficult to achieve so far because the MG configurations constructed to date, either by fitting to scattering data or by liquid quenching in molecular dynamics (MD) simulations, are inherently far less ordered than experimental glasses. The project aims to develop efficient MD simulation routes that are capable of producing MG structures having potential energy, atomic density, elastic constants, and therefore degree of order all similar to real-world MGs. A focus is algorithm development to accelerate MD in the search of deeper energy basins. The plan is to use 1) prolonged aging, 2) cyclic-stress-assisted aging, and 3) a new deletion-addition protocol, building upon amorphous structures obtained from simulated collapse of crystals or vapor deposition. Preliminary results suggest that these approaches (and their combination) can access low-energy glassy states along the way towards eventual crystallization. The configurations captured provide an unprecedented opportunity to uncover the SRO/MRO at a level of make-up and saturation that resembles those in laboratory glasses, to allow connections with atomistic mechanisms underlying shear transformations, relaxation and other glass properties. The results are expected to be a major step-forward towards establishing concrete structure-property relations in MGs.