STUDY OF THE INTERFACE BEHAVIOR IN DIRECTIONALLY SOLIDIFIED LaB6-ZrB2 EUTECTICS

Open Access
- Author:
- Deng, Hongqi
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 11, 2006
- Committee Members:
- Elizabeth C Dickey, Committee Chair/Co-Chair
John Richard Hellmann Jr., Committee Member
Christopher L Muhlstein, Committee Member
Judith Todd Copley, Committee Member - Keywords:
- directionally solidified eutectic
interface - Abstract:
- Directionally solidified oxide eutectics (DSEs) are significant candidates for ultra-high-temperature structural applications as they have high melting temperature, high strength-to-weight ratios, high hardness and good physical stability up to the melting temperatures. However their low fracture toughnesses have limited their widespread use as structural materials. Directionally solidified LaB6-ZrB2 eutectics are attractive new ceramic candidates of ultra-high temperature materials because of their supreme high melting temperature (2442¡ãC), high micro-hardness (22.6~29.5 GPa), and high bend strength (up to 1320 MPa). Especially, LaB6-ZrB2 DSEs have a higher fracture toughness (>11.0 MPa* m1/2) than most oxide DSEs (<8.0 MPa* m1/2). In this thesis, LaB6-ZrB2 DSEs were investigated to understand the high fracture toughness from the aspect of the heterophase interface. Interfaces in LaB6-ZrB2 DSEs were characterized by transmission electron microscopy (TEM) and modeled by geometric models such as near coincidence site lattice (NCSL), displacement shift complete (DSC) lattice, and secondary original lattice (O2-lattice) models. The nominal crystallographic orientation relationship between the constituent phases and the interface facet corresponded to high translational symmetry. Interfacial misfit dislocations were observed on the major facet and indicated those interfaces were fully relaxed. These analyses indicated that interfaces were relaxed to relatively low-energy configurations. Thermal residual stresses resulted from the good bonded interface and the thermal expansion mismatch between LaB6 and ZrB2, were measured and modeled by X-ray diffraction techniques (XRD) and finite element method (FEM). The experimental and modeling results show considerable tensile stresses in the ZrB2 fibers and compressive stresses in the LaB6 matrix. In fibers, the radial stress is approximately 1.5 times the axial component. No evidence was found that the sample growth rate has large effect on residual stresses within the range 1.5mm/min to 6.0mm/min. lastly, qualitative assessments of the interface structure and thermal residual stress effects on the interfacial debonding were made at the end of this work.