| Single crystal ceramic oxides have excellent oxidation resistance and creep properties, vital to aerospace and high temperature applications. Directionally-solidified eutectic ceramic fibers have high strength and improved flaw tolerance over single crystal fibers but often have lower toughness due to the high degree of interfacial bonding between the phases which prevents crack deflection. The toughness is affected by the residual stresses in these in-situ composites, arising from thermal expansion mismatch between the constituent phases. The magnitude and sign of the stresses depend upon the relative thermal expansion behavior of each material, the degree of interfacial bonding, and the temperature difference from the stress-free temperature. Since the extent of interfacial bonding and the exact stress-free temperature cannot be known a priori, the thermal residual stresses in these composites must be experimentally measured and coupled with theoretical modeling to be fully described.; In this study, residual thermal stresses in Al2O3-YAG and Al2O3-ZrO2(Y2O3) directionally solidified eutectics are measured using x-ray diffraction single crystal techniques at ambient and elevated temperatures and modeled using finite element analysis methods (FEM). Cross-sectional microstructures, in the form of two-dimensional Scanning Electron Microscopy images, are used to form a template for the phase volumes, resulting in models with improved accuracy over those using idealized geometries.; Residual stresses measured in the Al2O3-YAG eutectic are low, under 290 MPa (+/- 170 MPa), as is expected. The finite element model for this system shows volume-averaged stresses on the order of 100 MPa. Residual stresses measured in the Al2O phase of the Al2O 3-ZrO2(Y2O3) hyper-eutectic are on the order of -250 to -400 MPa (+/- 200 MPa) at room temperature, rising to 1150 to 1300 MPa (+/- 310) at 900°C. The hydrostatic stress-free temperature for the Al2O3 phase is calculated to be at 330°C (+/- 200°C). Averaged theoretical stresses for the Al2O3 phase of Al2O-ZrO 2(Y2O3) at room temperature are on the order of -175 to -530 MPa. |
