Page 136 - Fiber Fracture

FRACTURE CHARACTERISTICS OF SINGLE CRYSTAL AND EUTECTIC FIBERS 121

s 0.5
.-
C
E 0.4 0 as@ 1100°C
tj @’ 780MPa
Failure
0.3

0.2

0.1
0
0 20 40 60 80
Time, h
Fig. 8. Creep curves of A1203/Y3A15012 eutectic fibers at constant stress and temperature (vacuum
atm).

eutectic fiber has comparable creep resistance to (0001) Al2O3, is better than off-axis
A1203 but is not as good as single-crystal Y3A15012. Similar conclusions have been
suggested for bulk eutectic (Parthasarathy and Mah, 1993) of similar composition. More
creep tests on fibers at temperatures of 1400 to 1700°C have been completed by Matson
and Hecht (1999). The results showed significant variations in the stress exponent as
a function of test temperature. From TEM analysis Matson concluded that the A1203
phase was deforming by a dislocation mechanism, while the Y3Al5OI2 phase deformed
by a diffusional mechanism. This would suggest that producing eutectic fiber with very
small lamella size is desirable to increase the creep resistance. The decrease in A1203
phase thickness would lead to an increase in the backstress due to dislocation pile-up
and thereby increase creep resistance. Similar findings have been observed in direction-
ally solidified A1203/ZrO:!(Y203) eutectic (Sayir and Farmer, 2000). Any attempt to
reduce the lamella size to improve creep resistance requires increasing the pull-rate of
the fibers. This may have a limited amount of success due to the facet forming tendency
of the phases which are themselves a source of stress concentration.

CONCLUSIONS

To meet the specific requirements for tensile testing of single-crystal fibers, a new
technique based on original work by Yamaguchi’s speckle shift method was utilized to
measure the micro-strain. The Young modulus of (0001) A1203, (1 11) Y3Al5OI2 and
(1 11) Y2O3 fibers were 453, 290, and 164 GPa, respectively, and agreed well with
the literature. Single crystals of (1 11) Y203 were the weakest fibers and their strength
did not exceed 700 MPa. The fracture characteristics of single-crystal (0001) A1203,
(1 11) Y3A15012, and (1 11) Y2O3 fibers were anisotropic. All Y2O3 fibers fractured by
octahedral cleavage, and cleavage was often perpendicular to the fiber axis indicating
that this axis coincides with the (1 11) crystallographic direction as determined from