CAREER: Shear Banding in Bulk Metallic Glasses

技术摘要：金属玻璃表现出超过最强高温合金的巨大屈服强度，但其作为结构材料的使用受到其低延展性的限制，这是剪切条带的直接结果。这一提议的长期研究目标是表征大块金属玻璃中的剪切带传播，以便这类独特材料的非凡强度能够被用于结构应用。为了达到这一目标，我们的研究目标是检验如下假设：1)大块金属玻璃中的剪切带同时传播，而不是在样品中逐渐传播；2)锯齿状流动中剪切带速度通常为每秒毫米数量级；3)剪切带速度是施加应变率的函数。最后的破坏事件将被描述，剪切带灾难性传播的条件将被确定。研究方法要求在几个数量级的应变率下进行宏观和微观测试，同时进行时间和空间分辨率的应变测量。将采用的表征技术包括单轴压缩测试期间的高速视频成像和从应变片上获取的数据、使用纳米压痕的微压缩测试、扫描电子显微镜以及剪切带事件的声发射测量。非-技术摘要：金属玻璃？-这个名称意味着一些非常了不起的事情。金属玻璃和传统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没有长程有序。其结果是，金属玻璃的力学行为与其晶体玻璃的力学行为有根本的不同。最值得注意的是，金属玻璃非常坚固，但不幸的是，它们在宏观上表现出脆性行为。由于大块金属玻璃是一类相对较新的材料，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塑性变形的微观机制方面。该方案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。圣克拉拉大学(SCU)为工程专业的第一代本科生举办了一个以实验室为基础的关于金属玻璃的研讨会，将在这些学生的学术生涯早期增强他们的文化资本，以最大限度地增加他们在大学获得丰厚和成功的机会。这个工作坊将用于SCU为社区中未得到充分服务的高中生所做的其他外展工作。从对第一代大学生和代表性不足群体成员的外联评估中获得的结果和见解将有助于制定最佳做法，促进这些学生参与科学、技术、工程和数学(STEM)领域。在地方一级，这些努力将增加STEM专业的第一代和代表性不足群体的学生数量。

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.