Abstract
Grain boundaries in hot-pressed SiC with a sintering additive, Al2O3, were investigated using both high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) with a view to explaining the correlation between high-temperature strength at 1773 K and the grain boundary segregation of both Al and O atoms.High-resolution transmission electron micrographs showed no thin secondary phase at the grain boundary interface. EDS line scans using a 3 nm probe showed the segregation of both Al and O atoms to grain boundaries. This grain boundary segregation lowers grain boundary strength and causes a transition from transgranular cleavage to intergranular fracture. We concluded that the high-temperature strength was degraded by the grain boundary weakening caused by the grain boundary segregation.Strengthening has been tried by controlling grain boundary composition according to equilibrium segregation theory. The high-temperature strength is improved from 200 MPa to 700 MPa, which is equivalent to room-temperature strength, by decreasing the grain boundary concentration of both Al and O.
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