Mechanical properties and thermomechanical processing of metallic glasses -- the role of elemental distributions and size-dependent properties of shear transformation zones

某些成分的熔融金属合金可以被冷冻以产生金属玻璃。与结晶的常规金属相比，金属玻璃中的原子排列表现出明显的无序。金属玻璃的高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，这可能导致灾难性的断裂。在晶体材料中，热处理和变形处理通常用于性能改进，并且相关机制已经被理解多年。对金属玻璃中原子尺度重排的平行理解是一个科学挑战，因为基线排列未知。然而，这样的理解对于改善其性能至关重要。最近开发的一种表征小原子团簇的方法，称为剪切转变区（STZ），在金属玻璃中的变形的结果进行形状变化，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。STZ动力学将在延展性不同的类似合金之间进行比较。原子探针断层扫描，一种方法，可以产生原子尺度的成分映射，将被用来确定潜在的异质性化学成分的作用，及其与STZ人口的相关性。预期的结果包括对宏观行为模拟的现实输入。所获得的见解将有助于解释过去的一系列实验结果，并有助于延性合金的未来发展。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。技术摘要金属玻璃具有很高的强度和刚度，使其在结构应用中具有吸引力。然而，流动局部化往往导致灾难性的破坏在剪切带。与晶体材料一样，热机械处理可以改善其性能。与晶体材料相比，金属玻璃的机械性能及其对热机械处理的响应仍然缺乏原子水平的理解。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）。由于STZ的成像机制尚不清楚，其微观特性主要是在物理模拟和数值模型中进行研究，这些模型涵盖了有限的动态时间范围。最近，人们测量了不同金属玻璃的滞弹性弛豫动力学。 这些测量揭示了STZ的原子量子化层次结构，由它们所包含的原子数来分辨。从光谱中得到的潜在STZ的尺寸密度分布的形状随合金而显著变化。所提出的工作旨在获得潜在STZ的尺寸分布如何在原子尺度上理解a）与延展性、储存焓和化学不均匀性相关; B）在常规金属玻璃和具有强烈高频（β）机械弛豫的金属玻璃之间变化，这些机械弛豫已被证明是延展性的; c）受热机械处理的影响，包括热循环、循环和弹性静态变形。使用准静态和循环实验相结合，超过八个数量级的弛豫时间将被覆盖。弛豫时间谱将从数据计算。通过差示扫描量热法测定储存焓。原子探针层析成像（APT）将被用来确定偏离随机元素分布，包括富集在合金元素的集群的大小分布。将APT结果与潜在STZ的尺寸分布相关联将阐明局部化学在剪切转变中的作用。实验输入将提供宏观行为的介观模拟。拟议的工作将加强对金属玻璃加工和变形的原子尺度理解，使社区对金属玻璃性质的理解更接近晶体金属。这些结果将有助于延性合金的未来发展，从而影响未来的技术。研究生和本科生将a）学习材料基础; B）培养计划和进行组合实验和建模的能力，并交流他们的结果。PI和/或研究生将参加密歇根州资源不足地区的高中访问。

Non-technical AbstractMolten metallic alloys of certain compositions can be frozen to produce metallic glasses. In contrast to conventional metals, which are crystalline, the atomic arrangements in metallic glasses exhibit significant disorder. The high 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, which can lead to catastrophic fracture. In crystalline materials, heat and deformation treatment are routinely used for property improvement, and the associated mechanisms have been understood for many years. A parallel understanding of the atomic-scale rearrangements in metallic glasses is a scientific challenge, since the baseline arrangements are not known. However, such an understanding is crucial to improving their properties. A recently developed method of characterizing small atomic clusters called shear transformation zones (STZs), undergo shape change 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 STZ dynamics will be compared between similar alloys that differ in their ductility. Atom-probe tomography, a method that can yield atomic-scale composition mapping, will be used to determine the role of potentially heterogeneous chemical composition, and its correlation with the STZ population. The expected results include realistic input into simulations of macroscopic behavior. The insights gained will help interpret a range of past experimental results, and assist future development of ductile alloys. The proposed work will promote the education and training of graduate and undergraduate students, who will learn materials fundamentals, develop the ability 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.Technical AbstractMetallic glasses exhibit high strength and stiffness, making them attractive for structural applications. However, flow localization often leads to catastrophic failure at a shear band. As in crystalline materials, thermo-mechanical treatment can improve their properties. In contrast to crystalline materials, an atomic level understanding of mechanical properties of metallic glasses and their response to thermo-mechanical treatment is still lacking. Plastic deformation of metallic glasses is accommodated by thermally activated shear of atomic clusters, known as shear transformation zones (STZs). Because there is no known mechanism for imaging STZs, their microscopic properties had been studied mostly in physical analogues and numerical models, which cover a limited dynamic range of time. Recently, the anelastic relaxation kinetics of different metallic glasses have been measured. These measurements reveal an atomically quantized hierarchy of STZs, resolved by the number of atoms they comprise. The shape of the size-density distributions of potential STZs, obtained from the spectra, varies significantly with the alloy. The proposed work is aimed at gaining an atomic scale understanding of how the size distribution of potential STZs a) correlates with ductility, stored enthalpy and chemical heterogeneity; b) varies between conventional metallic glasses and those with intense high-frequency (beta) mechanical relaxations, which have been shown to be ductile; c) is affected by thermomechanical treatment, including thermal cycling, cyclic and elastostatic deformation. Using a combination of quasi-static and cyclic experiments, more than eight orders of magnitude in relaxation time will be covered. Relaxation-time spectra will be computed from the data. The stored enthalpy will be determined by differential scanning calorimetry. Atom-probe tomography (APT) will be used to identify deviations from random elemental distributions, including size distributions of clusters enriched in an alloying element. Correlating the APT results with the size distribution of potential STZs will elucidate the role of local chemistry in shear transformations. Experimental input will be provided for mesoscopic simulations of macroscopic behavior. The proposed work will enhance the atomic-scale understanding of processing and deformation of metallic glasses, bringing the community's understanding of metallic glass properties closer to that for crystalline metals. The results will assist the future development of ductile alloys, thus impacting future technology. Graduate and undergraduate students will a) learn materials fundamentals; b) develop the ability to plan and conduct combined experiments and modeling, and to communicate their results. The PI and/or graduate students will participate in high-school visits in under-resourced parts of the state of Michigan.