CAREER: Shear Banding in Bulk Metallic Glasses

技术概述：金属玻璃显示出巨大的屈服强度，超过了最强的高温合金，但其作为结构材料的使用受到其低延展性的限制，这是剪切带的直接后果。该提案的长期研究目标是表征大块金属玻璃中的剪切带传播特性，以便这种独特材料的非凡强度可以用于结构应用。为了实现这一目标，研究目标是验证以下假设：1)大块金属玻璃中的剪切带是同时传播的，而不是在样品中逐渐传播的；2)剪切带速度在锯齿状流动中通常在毫米/秒的数量级上；3)剪切带速度是施加应变速率的函数。最终的破坏事件将被表征，并确定剪切带灾难性传播的条件。研究方法需要宏观和微观尺度的应变速率测试跨越几个数量级，与时间和空间分辨应变测量相一致。采用的表征技术包括高速视频成像和单轴压缩测试期间应变片的数据采集，使用纳米压痕的微压缩测试，扫描电子显微镜和剪切带事件的声发射测量。非技术总结：？金属玻璃吗?——这个名字暗示了一些很不寻常的东西。金属玻璃和传统的金属合金通常是几种元素的组合，如铝、铜、镍和钛。但是，传统金属中的原子排列成有序的晶体阵列，而金属玻璃中的原子排列却没有长程顺序。结果是，金属玻璃的力学行为与晶体玻璃的力学行为根本不同。最值得注意的是，金属玻璃非常坚固，但不幸的是，它们表现出宏观脆性。由于大块金属玻璃是一种相对较新的材料，目前对其力学行为的理解远不如晶体金属复杂，特别是在原子水平和微观塑性变形机制方面。本课题的研究目标是通过力学测试、材料表征和建模相结合的方法来研究金属玻璃的微观变形机制，从而充分发挥金属玻璃作为结构材料的潜力。为圣克拉拉大学（SCU）第一代工程专业本科生举办的金属玻璃实验室研讨会将在这些学生学术生涯的早期阶段增强他们的文化资本，最大限度地提高他们在大学里获得丰富和成功的机会。该研讨会将用于其他SCU的推广工作，以帮助社区中服务不足的高中生。从对第一代大学生和代表性不足群体成员的拓展评估中获得的结果和见解将有助于制定促进这些学生参与科学、技术、工程和数学（STEM）领域的最佳实践。在地方一级，这些努力将增加来自第一代和代表性不足的STEM专业群体的学生数量。

TECHNICAL SUMMARY: Metallic glasses exhibit enormous yield strengths that exceed those of the strongest superalloys, but their use as structural materials is limited by their low ductility, the direct consequence of shear banding. The long-term research goal of this proposal is to characterize shear band propagation in bulk metallic glasses so that the extraordinary strengths of this unique class of materials can be exploited for use in structural applications. In pursuit of this goal, the research objectives are to test the hypotheses that 1) shear bands in bulk metallic glasses propagate simultaneously rather than progressively across a sample, 2) shear band velocities are typically on the order of millimeters per second during serrated flow, and 3) shear band velocity is a function of the imposed strain rate. The final failure event will be characterized, and the conditions under which a shear band propagates catastrophically will be determined. The research approach requires macro and microscale testing at strain rates spanning several orders of magnitude in concert with temporally and spatially resolved measurements of strain. Characterization techniques to be employed include high-speed video imaging and data acquisition from strain gages during uniaxial compression testing, microcompression testing using nanoindentation, scanning electron microscopy, and acoustic emission measurements of shear banding events.NON-TECHNICAL SUMMARY: ?Metallic glass? - the name suggests something quite remarkable. Metallic glasses and conventional metal alloys are typically combinations of several elements, such as aluminum, copper, nickel, and titanium. But whereas the atoms in conventional metals are arranged in orderly, crystalline arrays, the atomic arrangements in metallic glasses have no long-range order. The result is that the mechanical behavior of metallic glasses is fundamentally different from that of their crystalline counterparts. Most notably, metallic glasses are extraordinarily strong, but unfortunately, they exhibit macroscopically brittle behavior. Since bulk metallic glasses comprise a relatively new class of materials, the current understanding of their mechanical behavior is far less sophisticated than that of crystalline metals, particularly with respect to the atomic level and microscopic mechanisms of plastic deformation. The research goal of this proposal is to investigate the microscopic deformation mechanisms in metallic glasses through a combination of mechanical testing, materials characterization, and modeling so that the potential of metallic glasses as structural materials can be fully realized. A laboratory-based workshop on metallic glasses for first generation undergraduates majoring in engineering at Santa Clara University (SCU) will enhance the cultural capital of these students at an early point in their academic careers to maximize their opportunities for enrichment and success in college. This workshop will be adapted for use in other SCU outreach efforts for underserved high school students in the community. Results and insights gained from assessments of outreach to first generation college students and members of underrepresented groups will contribute to the development of best practices for promoting the participation of these students in Science, Technology, Engineering, and Mathematics (STEM) fields. At a local level, these efforts will increase the number of students from first generation and underrepresented groups majoring in STEM fields.