CAREER: Development of a Structurally Based Plastic Flow Model to Enhance the Utilization of Bulk Metallic Glasses

块体金属玻璃（BMG）代表了一种革命性的新型工程材料，其潜在应用范围从汽车和航空航天结构到生物医学设备和体育用品。这些全金属系统表现出非凡的拉伸强度，大的弹性变形，断裂韧性值比传统玻璃高一个数量级。由于其独特的、无序的原子结构，BMG在熔化之前在高温下显著软化。这种在低应力下的“均匀流动”允许使用廉价的聚合物模塑和成型技术，这在以前对于高强度材料是闻所未闻的。这种廉价的制造技术使BMG比传统合金具有额外的竞争优势。BMG构件的结构可靠性，特别是在安全关键应用中，需要在室温下具有广义塑性流动的能力。虽然广义的，均匀的流动很容易发生在高温下，在室温下流动变得高度局部化的剪切带。单个剪切带的快速扩展可导致灾难性破坏。因此，在多个剪切带上分配流动的机制是令人感兴趣的。这需要更好地理解玻璃结构，特别是“流动缺陷”的局部结构与在宽范围的温度和应力状态下的流动行为之间的关系。由于部件尺寸的限制，金属玻璃的机械性能历来难以通过实验表征。形成早期金属玻璃带所需的高冷却速率将实验工作限制在至少一个维度上很小的试样上。具有毫米或厘米量级特征尺寸的块体成形金属玻璃的出现，使得首次能够表征在服务中预期的更复杂负载条件下的均匀和局部流动行为。金属玻璃中的流动通常被理解为扩散过程，其涉及“自由体积”、在其它致密堆积结构中的原子尺度开放空间。自由体积分布的变化导致流动缺陷的形成。PI和其他人的先前结果表明，流动过程中观察到的软化与自由体积的增加有关，与模型预测一致。然而，玻璃结构的细节，包括流动缺陷，以及在流动和剪切带形成过程中的重排还没有很好地理解。这项工作的最终目标是提供一个模型的流动行为在BMG的流动缺陷结构的现实理解的基础上，类似于可能在晶体材料的描述。拟议的计划侧重于三个领域的技术优点：（一）表征的均匀和局部的流动行为作为温度，应变率和应力状态的函数;（二）通过新的实验技术的玻璃和流动缺陷的原子结构的识别;（三）通过计算机模拟流动过程中的原子结构演变的描述。这种实验技术与计算机模拟的结合将为预测和验证这类独特材料中流动的结构-性能关系提供必要的反馈框架。该计划将产生广泛的影响：（i）作为加速开发和优化新型BMG合金、结构、和加工技术，以及（ii）通过培养新一代的科学家和工程师。拟议的工作将通过澄清BMG的局部原子结构以及识别流动过程中发生的结构变化来深入了解玻璃形成能力。这些知识将反过来加速制造工艺的发展，包括连接技术，这对于这些独特材料在结构应用中的广泛采用至关重要。在教育价值方面，BMG以其新颖的行为和体育应用激发了学生的兴趣。教育活动将强调这种先进材料的设计如何影响社会和日常生活。这些努力对于非工程专业的学生来说尤其必要，因为材料科学的学科对公众的曝光率仍然很低。在实验室，拟议的招聘工作将侧重于鼓励年轻妇女，特别是那些从女子学院，考虑在工程研究生工作。直接参与这项研究的学生将接触到尖端科学，同时获得工业和学术界就业所需的材料科学基础的坚实基础。

Bulk metallic glasses (BMGs) represent a revolutionary new class of engineering materials with potential applications ranging from automotive and aerospace structures to biomedical devices and sporting goods. These fully metallic systems exhibit extraordinary tensile strengths, large elastic deflections, and fracture toughness values an order of magnitude higher than traditional glasses. Due to their unique, disordered atomic structure, BMGs soften considerably at elevated temperatures prior to melting. This "homogeneous flow" at low stresses permits the use of inexpensive polymer molding and forming techniques, previously unheard of for high strength materials. Such inexpensive manufacturing techniques give BMGs an additional competitive advantage over traditional alloys. Structural reliability of BMG components, particularly in safety critical applications, requires the capability for generalized plastic flow at room temperature. While generalized, homogeneous flow occurs easily at high temperature, at room temperature flow becomes highly localized in shear bands. Rapid propagation of a single shear band can cause catastrophic failure. Mechanisms for distributing flow over multiple shear bands are therefore of interest. This requires a better understanding of the relationship between the glass structure, particularly the local structure of "flow defects", and the flow behavior over a wide range of temperatures and stress states. The mechanical behavior of metallic glasses has been historically difficult to characterize experimentally due to component size limitations. The high cooling rates required to form the early metallic glass ribbons constrained experimental work to specimens that were small in at least one dimension. The advent of bulk forming metallic glasses, with characteristic dimensions on the order of millimeters or centimeters, makes it possible for the first time to characterize both homogeneous and localized flow behavior under the more complex loading conditions expected in service. Flow in metallic glasses is typically understood as a diffusional process involving the "free volume", atomic scale open spaces in the otherwise densely packed structure. Variations in the free volume distribution result in the formation of flow defects. Prior results by the PI and others indicate that the observed softening during flow is associated with an increase in free volume, consistent with model predictions. However, the details of the glass structure, including flow defects, and the rearrangements during flow and shear band formation are not well understood. The ultimate goal of this work is to provide a model for the flow behavior in BMGs based on a realistic understanding of the flow defect structure, similar to the descriptions possible in crystalline materials. The proposed program focuses on three areas of technical merit: (i) characterization of the homogeneous and localized flow behavior as a function of temperature, strain rate, and stress state; (ii) identification of the glass and flow defect atomic structure through novel experimental techniques; (iii) description of the atomic structure evolution during flow through computer simulations. This combination of experimental techniques with computer simulations will provide the feedback framework necessary to predict and validate the structure - property relationship for flow in this unique class of materials.This program will have broad impact (i) as the foundation for the accelerated development and optimization of new BMG alloys, structures, and processing techniques and (ii) through the preparation of a new generation of scientists and engineers. The proposed work will provide insight into glass forming ability by clarifying the local atomic structure of BMGs as well as identify structural changes that occur during flow. This knowledge will in turn accelerate the development of manufacturing processes, including joining techniques, critical for the widespread adoption of these unique materials in structural applications. In terms of educational value, BMGs excite student interest with their novel behavior and sports applications. Educational activities will emphasize how the design of such advanced materials impacts society and everyday life. These efforts are especially necessary for non-engineering students because the discipline of materials science remains underexposed to the general public. In the laboratory, proposed recruiting efforts will focus on encouraging young women, particularly those from women's colleges, to consider graduate work in engineering. Students directly involved in this research will be exposed to cutting-edge science while obtaining the solid foundation in the fundamentals of materials science required for employment in industry and academia.