| The high-temperature rheology of unidirectional (1D), continuous SiC-fiber/calcium aluminosilicate (CAS) glass-ceramic matrix composites was studied through experiments that optimized the contribution of sliding on the fiber-matrix interface (interphase flow in this case) to the bulk creep strain-rate ({dollar}dotvarepsilonsb{lcub}ss{rcub}{dollar}). Composite rheology was studied in compression as a function of the misorientation angle ({dollar}varphi{dollar}) of the fibers to the applied stress (-{dollar}sigmasb1{dollar}). From the data, which revealed a strong dependence of {dollar}dotvarepsilon sb{lcub}ss{rcub}{dollar} on {dollar}varphi{dollar} and temperature, a rheological model was postulated. Composite rheology can be effectively described by the superposition of three modes of deformation, the contribution of each being a function of {dollar}varphi{dollar}. For {dollar}40spcirc0.05{dollar}, however, the growth of pre-existing microcracks within the ply having the greatest misorientation to {dollar}sigmasb1{dollar} contributed to the overall composite strain. The effect of such cracking is quite dramatic for composites with {dollar}psi=20spcirc{dollar}.; The high-temperature creep of the two fine-grained ({dollar}dsim3{dollar}-{dollar}4 mu{dollar}m) CAS (anorthite) matrix materials was also studied at ambient pressure and at high confining pressure ({dollar}Psim300{dollar} MPa) to characterize mechanisms of deformation. CAS-II, in which the anorthite is tabular, showed extensive cavitation during creep at ambient pressure; CAS-III, with equiaxed anorthite showed no such porosity development. Specimens deformed at high confining pressure reveal a dramatic drop in effective viscosity. The behavior results from the disposition of secondary mullite: at high pressure, mullite residing at anorthite grain-boundaries reacts to form sillimanite and corundum. This microstructural change allows for easier deformation. |
