Page 137 - Fiber Fracture

122 A. Sayir and S.C. Farmer

X-ray characterization. The moderate tensile strength of single-crystal (1 11) Y3A15012
was controlled by facet forming tendency of the cubic garnet structure and in some
cases by the precipitation of cubic perovskite phase YA103. The tensile strength of
single-crystal (0001) A1203 fibers was 6.7 GPa and is among the highest tensile strength
values reported in the literature for continuous fibers. The strength controlling defects
were processing flaws related to facet forming tendency.
High-strength single-crystal (OOOI) A1203 fibers did not retain their strength at
elevated temperatures. The data suggest that single-crystal (0001) A1203 failure was
dependent on slow crack growth at elevated temperatures. A phenomenological descrip-
tion has been provided to predict the time to failure, but the microscopic mechanism
controlling the high-temperature strength of A1203 remains to be elucidated.
The room temperature tensile strength of A1203/Y3A15012 eutectic fibers was 2.39
GPa, considerably lower than the tensile strength of single-crystal (OOO1) A1203 fibers.
The high-temperature tensile strength of A1203/Y3A15012 eutectic fibers is superior to
sapphire (1.3 GPa at 1 100°C) and demonstrably less prone to slow crack propagation.
The A1203/Y3A15012 eutectic interphase boundary is of a coherent nature with strong
bonding. Fracture occurred in a brittle manner. The A1203/Y3A15012 eutectic has a high
degree of microstructural anisotropy combined with strong bonding between the phases
and possesses superior creep resistance.

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