Revealing Sequences of Metallic Degradation and Rupture Across Material Complexities and Thermomechanical Conditions: A Characterization of Coated and Uncoated Multi-Phase Zirconium Alloys and Single-Phase Commercial Purity Aluminum

Burden, Diana, Mechanical and Aerospace Engineering - School of Engineering and Applied Science, University of Virginia
Burden, Diana, University of Virginia

Metallic degradation is a fundamental challenge in the lifecycle of a material or component. The performance of a component is critically intertwined with its mechanism of failure. This dissertation focuses on sequences of metallic degradation occurring through rupture, fracture, or oxidation.

The first sequence is unveiled for the onset of coating fracture and the redistribution of load in a compliant, curved substrate due to external loading. A close-form, analytical solution is provided to determine load to crack initiation based on geometry, material stiffness, and loading condition. Multiple length-scales of 3D finite element (FE) models are provided to determine crack behavior on a curved Cr-Zr-alloy material. The models are validated against experimental results. The experimental set-up employs 2D digital image correlation on small scale scanning electron microscopy (SEM) imaging. The FE model and experimental results both show spontaneous and instantaneous through-thickness coating cracks at the elastic limit leading to areas of highly localized strain in the substrate under the widening cracks. Understanding this sequence of fracture is valuable for predicting the change in strain concentration when a coating is added to a component.

Through a series of quenched thermal treatments, the material and mechanical impact of a barrier coating on a thin-tubed, multi-phase metallic component is determined. The material changes are evaluated through characterization techniques such as energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and optical analysis. The mechanical impacts are directly measured and calculated through tube splitting tests and geometric changes. It is found that during the uncoated metal high-temperature oxidation, the metal undergoes spinodal decomposition to form a ferrous oxide while the single-sided coating prevents ferrous oxide formation. The coating also influences crystallographic reorientation at lower prior to oxidation. Thin oxide cracks in on the sample surfaces are useful for visualizing and understanding the stress states the underlying metallic sample is experiencing. Regarding the thermo-mechanical impact of the coating, the coated sample experiences axial tensile stress on the internal surface and axial compression on the external surface. These stresses create a smooth stress state through the component. The uncoated sample experiences radial tension on the internal surface and biaxial tension on the external surface. These stresses create a complex stress state through the thickness of the sample and lead to warping and geometric changes.

The final sequence of metallic failure is unveiled in 3D with internal imaging via XCT. Orowan alternating slip (OAS) occurs when a unique combination of impurities and loading conditions are present. The transition from void coalescence rupture mechanisms to OAS is identified as an increase in localized flow for commercial purity aluminum undergoing Mode I tearing.

PHD (Doctor of Philosophy)
Metallic Coatings, Accident Tolerant Fuels, Orowan Alternating Slip, Optimized ZIRLO, Decomposition of Zr-alloys, Fracture of Thin Films, Coatings on Curved Surfaces
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