Date

2018

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Vinci, Richard P.

Abstract

Relative surface energies of low-index planes and the effect of Europium segregants on grain boundary structure and fracture strength of magnesium aluminate spinel (MgAl2O4) bicrystals were evaluated by micro scale fracture tests and high-resolution electron microscopy. Single crystal specimens with {111}, {110}, and {100} boundary planes were bonded together using hot pressing to make {111}/{100} and {100}/{110} interfaces. Certain of the resulting specimens were doped with Eu. Micro cantilever deflection tests were employed to measure fracture toughness within each single crystal and at both bicrystal boundaries. Correlating surface energy with fracture energy measurements, the surface energies of {111}, {110}, and {100} planes were found to have a decreasing trend. High-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) was utilized to characterize grain boundary structure and chemistry. Differences in Eu segregation behavior on the two grain boundaries resulted in differences in grain boundary structure and differences in corresponding interfacial fracture strength. Eu segregated more uniformly to the {111}/{100} interface where it bonded strongly to the {111} plane but not to the {100} plane. The doped {100}/{110} boundary was characterized by a lack of uniform segregation. Corresponding fractography work and an in-situ foil fracture test was carried out in addition at the interface, aiding the structure and fracture behavior analysis. Here, we demonstrate how micro-cantilever fracture toughness measurements on single crystal and individual grain boundaries can indicate surface energy trends. And by combining micro fracture tests and HAADF-STEM analysis, a method to investigate the correlation between the grain boundary structure and fracture strength was established to interpret how rare earth segregation behavior affects intrinsic toughening mechanisms of magnesium aluminate spinel.In recognition of the shortcomings of the microcantilever bend fracture test, a new micro-scale fracture test that uses a bowtie-shaped micro-beam specimen with a chevron notch was designed and employed in transparent ceramic toughness testing. This clamped-clamped specimen can produce stable crack growth in brittle materials. Cyclic loading causes progressive crack extension, thereby producing multiple fracture toughness results in one experiment. The symmetric geometry eliminates the mixed mode fracture that exists in single-ended cantilevers. A 3D finite element analysis (FEA) model built in ANSYS Mechanical APDL and Altair Hypermesh was used to relate the crack length to the beam compliance. Full analysis of the bowtie chevron specimen geometry sensitivity has been carried out with FEA. A detailed crack stability analysis was conducted combining different nano-mechanical testing system, loading conditions, FEA analysis and TEM experimental methods. MATLAB programming was utilized to process large experimental data and to apply a polynomial fit in establishing a compliance and crack extension length relationship. The fracture energy could then be evaluated using an energy approach (‘Work of Fracture’) by combining FEA and experimental data. The results of tests using fused quartz and a glass-ceramic material match very well with published fracture toughness values. This validates the new micro scale testing method that possesses a combination of advantages not available in any other testing methods.

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