The Structural Basis for Fracture Toughness and Elasticity of Metallic Glasses

技术：大块金属玻璃结合了金属的一些突出的机械性能，包括高强度和高硬度，以及玻璃的加工灵活性。然而，它们确实有很大的局限性，因为塑性变形在剪切带中的局部化导致在小塑性应变下的断裂，特别是在拉伸时。然而，一些金属玻璃相当坚韧(断裂能类似于高强度晶体合金)，而另一些则非常脆。这种差异的起源还没有被很好地理解。对相关基本物理机制的了解将对开发具有更好韧性和损伤容限的新合金做出重要贡献。PI试图通过一项结合实验和计算技术的研究计划来解决这一挑战，该计划旨在探索金属玻璃塑性变形和断裂之间竞争的基本方面。最近观察到的弹性性能和断裂能之间的相关性对于理解结构和断裂之间的关系是一个潜在的有用的联系。PI将详细研究脆性断裂和塑性流动之间的竞争。这项工作的中心将是利用加载过程中原位执行的高能x射线散射来详细描述裂纹尖端周围的塑性区的结构特征。这项技术可以测量金属玻璃中的弹性应变(从而测量应力)，在晶相和非晶相中都具有优异的(~10微米)空间分辨率。这将使PI能够探索在裂纹尖端附近启动剪切带的条件。作为这项工作的一部分，PI将探索三个已知的变量对脆性和韧性之间转变的影响：温度、结构松弛和合金成分。我们的目标是开发一种详细的、现实的机制模型，通过这种机制，裂尖周围会出现分布的塑性变形区域。该计划的第二个主要方面将是探索滞弹性原子重排如何影响弹性性质，特别是剪切模数。PI试图将观察到的结构在载荷下的变化与在温度、松弛和成分影响下测得的弹性常数的变化相关联。PI还将通过详细的非原位表征，包括共振X射线散射和波动电子显微镜，将这种行为与结构联系起来。通过了解原子重排(可能与塑性变形有关)影响剪切模数的方式，Pi可以开始在基本水平上理解弹性性质和断裂之间的联系。非技术性：该研究计划将通过将研究生、本科生和当地高中生整合到一个研究团队中来推进他们的教育和培训，该团队包括PI和他的研究小组的其他成员。基础科学研究成果将通过期刊出版物、会议和研讨会报告(由本科生和研究生以及首席研究员)和网络传播。高中生，包括一些来自代表不足的群体的学生，将通过巴尔的摩理工学院高中与匠心计划的合作项目参与。

TECHNICAL: Bulk metallic glasses combine some of the outstanding mechanical properties of metals, including high strength and stiffness, with the processing flexibility of glasses. They do have a significant limitation, however, in that localization of plastic deformation into shear bands leads to fracture at small plastic strains, particularly in tension. Nevertheless, some metallic glasses are quite tough (with fracture energies similar to those of high strength crystalline alloys) while others are truly brittle. The origins of this difference are not well understood. An understanding of the fundamental physical mechanisms responsible would be an important contribution to the development of new alloys with improved toughness and damage tolerance. PI seeks to address this challenge through a research program that combines experimental and computational techniques to explore fundamental aspects of the competition between plastic deformation and fracture of metallic glasses. The recently observed correlation between elastic properties and fracture energy is a potentially useful link for understanding the relationship between structure and fracture. PI will examine the competition between brittle fracture and plastic flow in detail. A centerpiece of this effort will be detailed structural characterization of the plastic zone around a crack tip using high energy x-ray scattering performed in situ during loading. This technique allows measurement of elastic strain (and thus stresses) in the metallic glass with excellent (~10 um) spatial resolution in both the crystalline and amorphous phases. This will allow PI to explore the conditions under which shear bands are initiated near crack tips. As part of this effort, PI will explore the effects of three variables known to mediate the transition between brittle and tough behavior: temperature, structural relaxation, and alloy composition. The goal is to develop a detailed, realistic model of the mechanism by which a region of distributed plastic deformation develops around a crack tip. The second major aspect of this program will be to explore how the elastic properties and the shear modulus in particular, are influenced by anelastic atomic rearrangements. PI seeks to correlate observed changes in structure under load with measured changes in the elastic constants, under the influence of temperature, relaxation, and composition. PI will also connect this behavior to structure through detailed ex situ characterization, including resonant x-ray scattering and fluctuation electron microscopy. By understanding the ways in which atomic rearrangements (which may be connected to plastic deformation) influence the shear modulus PI can begin to understand the connection between elastic properties and fracture at a fundamental level. NON-TECHNICAL: The research program will advance the education and training of graduate students, undergraduate students, and local high school students by integrating them into a research team that includes the PI and the other members of his research group. The results of the basic scientific research will be disseminated via journal publications, conference and seminar presentations (by the undergraduate and graduate students as well as the principal investigator), and the web. High school students, including some from under-represented groups, will participate through a cooperative program with Project Ingenuity at Baltimore Polytechnic High School.