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Title:Multiscale characterization of ferroelastic deformation in ceramic materials
Author(s):Smith, Charles Sheridan
Director of Research:Krogstad, Jessica A
Doctoral Committee Chair(s):Krogstad, Jessica A
Doctoral Committee Member(s):Dillon, Shen J; Lambros, John; Sottos, Nancy R
Department / Program:Materials Science and Engineering
Discipline:Materials Science and Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Ceramic materials offer a variety of useful properties that make them desirable for a wide range of engineering applications, however, ceramics are limited in their utility by low toughness. Ferroelastic deformation provides a mechanism through which ceramics are intrinsically toughened, but the effect of microstructure on the deformation behavior has yet to be fully understood. In this present examination, the behavior of ferroelastic deformation was evaluated on a range of length scales, specifically highlighting the influence of several variables on the domain nucleation behavior. Ferroelastic domain nucleation was first evaluated in micro-scale single crystals. The stress required for domain nucleation was measured while crystal orientation was tracked. Domain nucleation was observed to not follow a critical resolved shear stress criterion, suggesting that orientation alone cannot be used to predict the deformation behavior. Furthermore, multiple types of deformation were observed to act in concert with ferroelastic deformation. This suggests that domain nucleation is a complex process that may involve multiple potential mechanisms of deformation. Domain nucleation in bulk polycrystals was also examined. Statistics collected on grain sizes that more frequently contain mechanically nucleated domains show that larger grains in close proximity to finer grains more frequently deform. The deformation behavior in polycrystals was contrasted against the domain nucleation behavior in single crystal nanopillars. The nanopillars exhibited high deformation stress, while prolific domain nucleation without fracture was observed in polycrystals. These results suggest that local constraints imposed by microstructure play a key role in locally increasing shear stresses responsible for domain nucleation. To design microstructures with specific characteristics, ceramic processing routes must also be developed to control microstructural development during fabrication. To this end, spark plasma sintering (SPS) offers a promising processing route for fabricating dense nanostructured ceramics. The densification mechanisms associated with ceramic processing using SPS have also been investigated in the present work. Results collected on many samples that were processed under identical processing control conditions convey significant variability in the resulting material properties between and within individually produced samples. Furthermore, the results indicate that electric current plays an important role in densifying ionic conducting ceramics during sintering using SPS. Overall, the research presented in this dissertation shows that ferroelastic domain nucleation is a complex process involving several competing and cooperating mechanisms, and that domain nucleation is affected by different microstructural variables. Domain nucleation cannot be predicted based solely on crystal orientation, however, other microstructural variables including grain size do significantly impact the ferroelastic deformation behavior. Microstructures with large ferroelastic grains embedded in a more finely grained matrix promote ferroelastic deformation even without fracture, and the deformation is sensitive to the stress state being applied. Several processing routes presented here result in these favorable bimodal grain size distributions and may be tested more thoroughly in the future to explore the effect that such microstructures have on the intrinsic toughness.
Issue Date:2020-06-18
Rights Information:Copyright 2020 Charles Smith
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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