Abnormal grain growth in ultrafine grained metals under high cycle loading

大多数技术上重要的金属都是多晶的，是由许多称为晶粒的小簇构成的。这些颗粒的大小是一个关键参数，因为颗粒大小对材料的强度有很大的影响。同样的金属，如果它的颗粒小，强度就会大得多。因此，晶粒尺寸小于1微米的超细金属因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势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裂纹。因此，在超细晶结构金属的整个寿命期内保持小晶粒尺寸是至关重要的；否则，它的机械性能会灾难性地下降。在这个项目中，pi正在发展对晶粒生长机制的新认识，从而可以适当地控制晶粒尺寸。pi特别关注异常的晶粒生长，其中一小部分晶粒比其他晶粒长得又大又快，从而消耗其他晶粒。虽然这种现象在高温和高应变下得到了很好的理解，但在很少探索的高循环加载范围（在室温下施加大量低应变循环）中，对异常晶粒生长知之甚少。该项目的推广活动包括暑期材料科学与工程丰富计划，目标是来自STEM领域代表性不足群体的高中生，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：本提案的总体目标是对室温下高周载荷下超细晶金属异常晶粒生长的根源有一个基本的了解。本研究的中心假设是，在室温高周加载下，弹性各向异性效应主导了晶粒生长的驱动力，导致晶粒生长行为异常。pi通过在配备电子背散射衍射的扫描电子显微镜内高通量表征循环载荷诱导的超细颗粒金属晶粒生长来验证这一假设。六种具有不同弹性各向异性、面心立方或体心立方结构的不同金属薄膜的制备和测试是本研究的重点。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，在不同的应变振幅（高达1%）。pi使用微观力学和相场建模来确定应变能密度和晶界迁移率方面的热力学驱动力，以导致异常晶粒生长。采用综合实验和模型，确定了室温下面心立方和体心立方金属中控制异常晶粒生长的主要因素和加载范围，包括其动力学。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.