Page 132 - Fiber Fracture

FRACTURE CHARACTERISTICS OF SINGLE CRYSTAL AND EUTECTIC FIBERS 1 I7

which govern high-temperature strength remain to be elucidated. Two proposed possible
mechanisms, both thermally activated, are atomistic level crack propagation due to
lattice trapping (Hsieh and Thomson, 1973; Lawn, 1975), and/or dislocation assisted
crack shielding. The practical conclusions of this study are therefore twofold. First,
an understanding of fracture characteristics is needed to improve the high-temperature
strength retention of single-crystal A1203. It should be pointed out that this is not a
limitation of the state of the art of crystal growth, but rather a direct result of extensive
work that produced fibers with exceptionally high strength, yet were drastically weaker
at high temperatures, without any clear explanation. Second, there is a clear need to find
a new class of fibers that do not exhibit large strength degradation at high temperatures.
This impetus led to the development of directionally solidified A1203/Y3A15012 eutectic
fibers (Sayir and Matson, 1991).

Directionally Solidijied A1&/Y3A15012 Eutectic Fibers

The main driving force for the study of directionally solidified A1203/Y3A15012
eutectic fibers arose from the shortcomings of single-crystal AI203 fibers as discussed
in the previous section. Accordingly, the major objective in the eutectic work was
to examine the time-dependent failure of A1203/Y3A15012 eutectic fibers at elevated
temperatures. First, the tensile strength at 25 and 1100°C of the A1203/Y3A15012
eutectic fibers were determined. Second, the strain rate-dependent tensile strength of
single-crystal A1203 fibers and directionally solidified AI203/Y3A15012 eutectic fibers
at 1100°C were compared.
The fast-fracture tensile strengths of as-grown fibers tested at 25 and 1100°C are
shown as Weibull-probability plots in Fig. 5. A typical primary fracture origin is shown
in Fig. 6 as well as corresponding longitudinal fiber surface morphologies. The room
temperature tensile strength of A1203/Y3A15012 eutectic fibers was 2.4 GPa (f0.336
GPa), considerably lower than the tensile strength of single-crystal (OOO1) A1203 fibers.
Confocal micro-Raman spectroscopy was performed on a number of A1203/Y3A15012
eutectic fibers in an attempt to identify YA103 precipitates. Raman spectra did not reveal
any YA103 phase in the A1203/Y3AI5012 eutectic fibers. This is significant because the
strength controlling flaws are either pores and/or the facet forming tendencies of the
components as determined by SEM characterization, Fig. 6.
The tensile strength of A1203/Y3A15012 eutectic fibers was 1.3 GPa at 1100°C
and was superior to single-crystal (0001) A1203. To understand why A1203/Y3A15012
eutectic fibers are stronger than (0001) A1203 at elevated temperature, it is necessary
to compare slow crack growth characteristics of the A1203/Y3A15012 eutectic with
single-crystal (0001) A1203. The time-dependent failure of A1203/Y3A150,2 eutectic
fibers was first evaluated using tensile strength results taken at different strain rates.
The strain rate-dependent tensile strengths of directionally solidified eutectic fibers are
compared with those of c-axis sapphire in Fig. 3. The tensile strength of directionally
solidified A1203/Y3A15012 eutectic fibers did not change with change of strain rate
over four orders of magnitude at 1100°C. The tensile strength of the A1203/Y3A15012
eutectic fibers was 1.39 GPa at 1 100"C, a loss of approximately 40% from its room
temperature value. At some conditions the tensile strength of (0001) A1203 showed