Abnormal grain growth in ultrafine grained metals under high cycle loading

非技术总结大多数技术上重要的金属是多晶的，由许多称为晶粒的小簇组成。这些晶粒的尺寸是一个关键参数，因为晶粒尺寸对材料的强度有很大的影响。同样的金属，如果它的晶粒更小，它的强度就更大。因此，晶粒尺寸小于1微米的超细晶粒金属由于其特别高的强度而成为一类非常重要的结构材料，在航空航天和核工业等领域具有关键应用。小晶粒金属的另一个显著优点是它们往往具有更好的疲劳性能，并且在循环载荷下不太可能形成有害的裂纹。因此，在超细晶粒结构金属的整个寿命期间保持小的晶粒尺寸至关重要;否则其机械性能可能会灾难性地下降。在这个项目中，PI正在开发对晶粒生长机制的新理解，以便可以适当地控制晶粒尺寸。PI特别关注异常晶粒生长，其中一小部分晶粒与其他晶粒相比生长得非常大和快，并因此消耗其他晶粒。虽然这种现象在高温和高应变下得到了很好的理解，但对很少探索的高循环载荷范围内（在室温下以低应变施加大量循环）的异常晶粒生长知之甚少。该项目的推广活动包括材料科学和工程的夏季充实计划，针对STEM领域代表性不足的高中学生，并让高中教师，研究生和本科生参与制定课程并实施该计划。技术概述本提案的首要目标是实现对室温下高循环载荷下超细晶粒金属中异常晶粒生长的起源的基本理解。该建议的中心假设是，在室温下的高周加载制度中，弹性各向异性效应占主导地位的晶粒生长的驱动力，导致异常的晶粒生长行为。PI通过在配备电子背散射衍射的扫描电子显微镜内对超细晶粒金属中循环负载诱导的晶粒生长进行高通量表征来测试这一假设。制备和测试六种不同的金属薄膜，具有不同程度的弹性各向异性，具有面心立方或体心立方结构的工作的重点。这些实验可以表征晶粒尺寸分布和取向的演变作为应用周期的函数，对于各种应变幅度（高达1%）。PI使用微观力学和相场模型来确定的热力学驱动力的应变能密度和异常晶粒生长的晶界迁移率。综合实验和建模被用来确定控制异常晶粒生长的主要因素和加载范围，包括其动力学，在室温下的面心立方和体心立方金属。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

NON-TECHNICAL SUMMARYMost technologically important metals are polycrystalline, and are made of many small clusters called grains. The size of these grains is a key parameter, as grain size has a strong influence on a material’s strength. The same metal is much stronger if its grains are smaller. For this reason, ultrafine grained metals, with grain sizes less than one micrometer, are a very important class of structural materials due to their particularly high strength, with critical applications in the aerospace and nuclear industry, among others. Another significant advantage of small grained metals is that they tend to have better fatigue properties and are less likely to form detrimental cracks under cyclic loading. It is therefore crucial to keep small grain sizes throughout the lifetime of an ultrafine grained structural metal; otherwise its mechanical properties can degrade catastrophically. In this project, the PIs are developing new understanding of the mechanics of grain growth, so that grain size can be properly controlled. The PIs focus specifically on abnormal grain growth, where a small fraction of grains grow drastically large and fast compared to other grains and as a result consume other grains. While this phenomenon is well understood at high temperatures and high strains, little is known about abnormal grain growth in the rarely explored range of high cycle loading (applying a large number of cycles with low strain at room temperature). The outreach activities in this project include a summer enrichment program in material science and engineering, targeting high school students from underrepresented groups in the STEM fields, and involving high school teachers, graduate and undergraduate students to develop the curriculum and implement the program. TECHNICAL SUMMARYThe overarching goal of this proposal is to achieve a fundamental understanding of the origins of abnormal grain growth in ultrafine grained metals under high-cycle loading at room temperature. The central hypothesis of this proposal is that the elastic anisotropy effect dominates the driving force for grain growth in the high-cycle loading regime at room temperature, resulting in abnormal grain growth behavior. The PIs test this hypothesis through high-throughput characterization of cyclic-load-induced grain growth in ultrafine grained metals inside a scanning electron microscope equipped with electron back scattered diffraction. Fabrication and testing of six different metallic films with varying degrees of elastic anisotropy, with face-centered cubic or body-centered cubic structures are the focus of the work. These experiments can characterize the evolution of grain size distribution and orientation as a function of applied cycles, for various strain amplitudes (up to 1%). The PIs use micromechanics and phase field modeling to determine the thermodynamic driving forces in terms of strain energy densities and the grain boundary mobilities for abnormal grain growth. The integrated experiments and modeling are being used to identify the predominant factors and loading ranges controlling abnormal grain growth, including its kinetics, in face-centered cubic and body-centered cubic metals at room temperature.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.