Properties of Atomic-Scale Flow Defects in Metallic Glasses

该项目旨在探索在玻璃化转变温度以下的金属玻璃中容纳流动的缺陷的基本特性，这是充分解释和预测宏观变形行为的必要步骤。金属玻璃的塑性变形由原子团簇的热激活剪切来调节，称为剪切转变区（STZ），其也负责滞弹性。研究结果将包括非Arbian行为的细节，指导未来的原子模拟，并为实际应用的介观模拟提供现实的输入。结构弛豫状态的影响，即，加工，对STZ属性将被确定。为了涵盖广泛的时间常数，准静态和动态滞弹性松弛测量将使用弯曲应力松弛，纳米压头悬臂梁弯曲和拉伸测量在动态力学分析仪进行。将在PI开发的标准线性固体模型框架内对数据进行数值分析。通过探索潜在STZ的尺寸密度分布作为加工历史的函数，将测试自由体积或不同结构变量的潜在作用的预测。温度相关的测量将提供直接的信息，大小分辨激活障碍。进一步的探索将解决STZ的集体行为和次屈服，局部，不可逆的原子位移对STZ性能的影响。除了新的科学知识，这项工作将为合金设计提供机会，并将加强研究生和本科生教育。这将有助于激励高中生追求科学事业。非技术性总结：熔融金属的冷冻可以产生金属玻璃。金属玻璃的强度和弹性使其在医疗和其他机械设备以及体育用品中的应用具有吸引力。然而，它们表现出有限的延展性和韧性。了解这些材料中与永久变形相关的原子尺度重排是一项科学挑战，但它对于开发具有改进延展性的金属玻璃至关重要。与晶体材料不同，晶体材料中每个原子的位置都是已知的，而玻璃中的原子位置却远没有那么清楚。因此，原子重排的性质是一个积极研究的课题。最近开发的方法表征剪切转变区（STZs），小原子团簇，经历永久剪切变形的结果，在金属玻璃，在这项研究中使用。依赖于时间的形状恢复的分析揭示了STZ的不同签名，由它们所包含的原子数来分辨。将进行关键实验来解释以前的实验观察。预期的结果包括现实输入到模拟的宏观行为，有利于未来的发展延性合金。将寻求增加对缓慢、可逆变形的作用的理解，以利于制造和实验的解释。拟议的工作将促进研究生和本科生的教育和培训，他们将学习材料基础知识，培养计划和进行实验和建模的能力，并学会交流他们的结果。PI和研究生将参加密歇根州资源不足地区的高中访问。

TECHNICAL SUMMARY:This project seeks to explore fundamental properties of the defects that accommodate flow in metallic glasses below the glass transition temperature, a necessary step toward fully explaining and predicting macroscopic deformation behaviors. Plastic deformation of metallic glasses is accommodated by thermally activated shear of atomic clusters, known as shear transformation zones (STZs), which are also responsible for anelasticity. Research results will include details of non-Arrhenian behaviors, guide future atomistic simulations and provide realistic input into mesoscopic simulations for practical applications. The effect of structural relaxation state, i.e., of processing, on STZ properties will be determined. To cover a wide range of time constants, quasi-static and dynamic anelastic relaxation measurements will be conducted using bend stress relaxation, nanoindenter cantilever bending and tensile measurements in a dynamic mechanical analyzer. Data will be analyzed numerically within the framework of a standard linear solid model developed by the PI. By exploring the size-density distribution of potential STZs as a function of processing history, proposed predictions for the potential role of the free volume, or a different structural variable, will be tested. Temperature-dependent measurements will provide direct information on size-resolved activation barriers. Further explorations will address the collective behavior of STZs and the effect of sub-yield, local, irreversible atomic displacements on STZ properties. In addition to new scientific knowledge, the work will provide opportunities for alloy design and will enhance graduate and undergraduate education. It will help inspire high-school students to pursue a career in science.NON-TECHNICAL SUMMARY:The freezing of a molten metal can produce a metallic glass. The strength and elasticity of metallic glasses make them attractive for applications in medical and other mechanical devices, as well as sporting goods. However, they exhibit limited ductility and toughness. Understanding the atomic-scale rearrangements associated with permanent deformation in these materials is a scientific challenge, but it is crucial to developing metallic glasses with improved ductility. In contrast to crystalline materials, in which the position of every atom is known, atomic positions in glasses are far less well understood. Thus, the nature of atomic rearrangements is a subject of active research. A recently developed method of characterizing shear transformation zones (STZs), small atomic clusters that undergo permanent shear as a result of deformation in a metallic glass, is used in this research. Analysis of time-dependent shape recovery reveals distinct signatures of STZs, resolved by the number of atoms they comprise. Critical experiments will be performed to explain previous experimental observations. The expected results include realistic input into simulations of macroscopic behavior, benefitting future development of ductile alloys. Increased understanding will be sought for the role of slow, reversible, deformation to benefit both manufacturing and the interpretation of experiments. The proposed work will promote the education and training of graduate and undergraduate students, who will learn materials fundamentals, develop abilities to plan and conduct experiments and modeling and learn to communicate their results. The PI and graduate students will participate in high-school visits in under-resourced parts of the state of Michigan.