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

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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.