Mechanical properties of advanced SiC fiber composites irradiated at very high temperatures

Six different composite materials of various near-stoichiometric silicon carbide (SiC) fiber reinforcements and pyrolytic carbon or SiC/pyrolytic carbon multilayer interphases were neutron-irradiated to ∼6 × 1025 n/m2 (E > 0.1 MeV) at nominal temperatures of 800 °C and 1300 °C, and tested for tensile properties at room temperature. Only insignificant or very minor modifications to the tensile strength were realized. However, statistical analysis on relatively large specimen populations revealed minor but statistically significant strength degradation for some composites. From 50 to 150 nm appeared to be within the optimum PyC interphase thickness range for the SiC fibers used in terms of tensile properties. The misfit stresses present in the unirradiated samples were significantly reduced after irradiation. The change in misfit stress may be attributed to the irradiation-induced modification of coefficient of thermal expansion or potential differential swelling between the fibers and the matrix. True matrix cracking stress estimated from the proportional limit stress and misfit stress did not appear to degrade by neutron irradiation.