Damage evolution of a SiC/Ti-15-3 metal matrix composite with different heat treatments

Abstract

The purpose of this thesis was to study the effects of heat treatments on the deformation mechanisms of a SiCi MMC. The damage evolution for a four ply unidirectional SiC sigma fiber/Ti 15-3 MMC was studied for a uniaxial tensile loading. Both 90 and 0 degree fiber orientations were tested, with emphasis on the 90 degree specimen. Prior to testing, 24 hour heat treatments at 450'C and 700'C were performed in an inert environment on a portion of the specimens, with the remaining specimens left in the as-fabricated condition. The specimens were subjected to isothermal non-proportional tension tests with increasing stress amplitudes at room temperature and 427'C. The elastic modulus for each unloading cycle was measured and used to identify the evolution of inelastic deformation mechanisms. Damage evolution was observed through degradation of elastic modulus as the stress level increased. Microstructural evaluations were performed after testing to determine the damage state and the microstructure. A micromechanical model using the MoriTanaka method was utilized to study the effect of damage on the stiffness of the composite. A reduction of the composite stiffness was developed as the crack density increased. This model allows for a prediction of the developed crack density at a known load level. In addition to mechanical damage, the effects of environmental damage are studied. A series of experiments were performed to determine the oxide thickness after exposure to different temperatures and times. Energy dispersion spectroscopy was then used to study the chemical concentrations in the oxide layer.