GOALI: Understanding Light-weight Transparent Ceramic Mechanical Response: From Single Grain Boundary to Bulk Material

透明多晶轻质陶瓷具有独特的可加工性、光学性能和机械性能，适用于透明装甲、航空光学和潜艇高压玻璃等应用。不幸的是，这些材料在强度上往往比其他玻璃表现出更大的变化，这使得它们不太适合这种高可靠性的应用。这个GOALI学术联络机会(GOALI)项目是基于这样一个假设，即材料晶体之间的化学成分的微小变化对整体机械行为有很大的影响。了解在这些边界上哪些化学成分是有利的，可以改进材料的工程设计，并为高强度光学元件的应用创造新的市场。实现这一目标将促进实验力学的进步，促进国家的健康、繁荣和福祉，并确保国防安全。为了建立这一假设，必须对大小从人类头发宽度的1/100(以测量单晶边界)到整个组件大小的样品进行机械测试和模拟。直到最近，新开发的测试方法才使这种实验变得可行，从而创造了理解这个问题的新机会。作为该项目的一部分，工业合作伙伴将提供学生实习机会，研究组将招募未被充分代表的本科生，并将通过女孩学习材料营活动来激励未来的女性学生。大尺寸镁铝尖晶石的机械强度表现出相当大的变异性，假设是由于晶界化学计量和化学的局部变化造成的。然而，颗粒的几何形状、晶界的各向异性以及晶界化学的变化使得像平均晶界成分和平均强度这样的简单度量变得具有挑战性。为了从机理上了解尖晶石的力学行为，这项工作在实验上建立了相关的长度尺度：从测量单一边界的断裂性能和晶界化学到预测大样本的断裂强度及其性能分布。该项目开发了数字粒子跟踪方法，用于绘制在透射电子显微镜中原位进行的单晶界断裂实验期间的演变应变。从相关的有限元分析得到的粘结带模型可以与测量的晶界化学成分和取向错误相关联。在介观尺度(几十个颗粒)可以观察到作为各向异性函数的晶界和块体性质分布。这使我们能够定义用于宏观样品的连续分析的代表性体积元素。在这个尺度上，实验上合理的成分梯度、残余应力梯度和晶界特性分布将被用来预测宏观强度的分布。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Transparent polycrystalline light-weight ceramics provide a unique combination of processibility, optical properties, and mechanical properties making them suitable for applications as transparent armor, optics for aviation, and high-pressure glass for submarines. Unfortunately, these materials often exhibit greater variations in strength than other glasses, making them less suitable for such high-reliability applications. This Grant Opportunities for Academic Liaison with Industry (GOALI) project is based on the hypothesis that minor changes in chemistry at the boundaries between material crystals have a large effect on the overall mechanical behavior. Understanding what chemistry is favorable at these boundaries could allow for improved engineering of the material and create new markets for applications as a high strength optical element. Achieving this goal will progress experimental mechanics; advance the national health, prosperity, and welfare; and secure the national defense. To establish this hypothesis, mechanical testing and simulation must be performed on samples ranging in size from 1/100th of the width of a human hair (to measure single crystal boundaries) to the size of the entire component. Newly developed testing methods have only recently made such experiments feasible, thus creating new opportunities to understand this problem. As part of the project, student internships will be provided at the industrial partner, underrepresented undergraduate students will be recruited in the research group, and prospective female students will be motivated through Girls Learning about Materials Camp activity.The mechanical strength of large-format MgAl2O4 spinel exhibits considerable variability that is hypothesized to result from local variations in grain boundary stoichiometry and chemistry. However, the granular geometry, grain boundary anisotropy, and variations in grain boundary chemistry make relating simple metrics like average boundary composition and average strength challenging. To develop a mechanism-informed understanding of the mechanical behavior of spinel, this work bridges experimentally relevant length scales: from measuring fracture properties and grain boundary chemistry on single boundaries to predicting the fracture strength of large specimens and their property distributions. The project develops digital particle-tracking methods for mapping evolving strain during single grain boundary fracture experiments performed in situ in a transmission electron microscope. Cohesive zone models developed from associated finite element analyses can be correlated with measured grain boundary chemistries and misorientations. Grain boundary and bulk property distributions as a function of anisotropy can be observed at the mesoscale (dozens of grains). This allows us to define representative volume elements for use in a continuum analysis of the macroscopic samples. At this scale, experimentally reasonable compositional gradients, residual stress gradients, and grain boundary property distributions will be used to predict the distribution of macroscopic strength.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.