Solid solution directionally solidified eutectics: Model systems for structure-property relationships in interfacial fracture

The next generation of high temperature materials for application in aerospace and power generation systems will be required to withstand temperatures well in excess of 1200°C, often in oxidizing atmospheres. Oxide-oxide directionally solidified eutectics (DSE's) have shown promise as high temperature ceramic materials, only to be limited by their lack of fracture toughness at room temperature. In the case of DSE oxide materials, the interfacial fracture behavior has been blamed for the poor performance in the past and is the subject of interest in this work.; In this thesis, the solid solution, directionally solidified quaternary eutectic (SS-DSE), Co_1−xNi_xO/ZrO₂(CaO), is developed as a model system for the study of interfacial fracture in oxide-oxide DSE's. A variety of structural and mechanical characterization techniques are applied to investigate structure-property relationships for interfacial fracture behavior.; The optical floating zone technique was employed for growing both the eutectic crystals and their single crystal counterparts, Co_1−x Ni_xO. Co_1−xNi_xO/ZrO₂(CaO) was shown to possess the necessary structural elements to serve as a model system for interfacial fracture. Lamellar microstructures were observed for all compositions. The crystallographic relationships between phases evolved as a model solid solution. Interdiffusion of chemical species was minimal, allowing the layers to treated independently.; The core of this thesis is dedicated to studying the nature of interfacial fracture behavior in oxide eutectics. This study is motivated by the novel observation of extensive interfacial delamination for the system CoO/ZrO ₂(CaO). A transition from interfacial delamination to interfacial penetration is observed for compositions of Co_1−xNi_xO/ZrO ₂(CaO) with x > 0.2. The residual stress state in these materials was investigated using X-ray and neutron diffraction-based techniques. The role of plasticity in interfacial fracture was explored using a combination of indentation experiments, including spatially resolved nanoindentation testing. Constitutive laws for describing the elastoplastic behavior of the Co _1−xNi_xO phase were developed as a result of these measurements. These empirical laws are combined with dislocation theory to propose a dislocation-based mechanism for crack nucleation, which correctly describes the salient features of the interfacial fracture problem in this system.