Page 135 - Fiber Fracture

120 A. Sayir and S.C. Farmer

Fast Fmture Strength of
AI,O,NAC at I1(M°C
k
E
a
%
L
3;
a2
...I
111
UY
9
b
3
.CI
a
I
loJ IO-^ io-' loo ioi io2 lo3 io4
Time to Failure, h

Fig. 7. Time to failure of A1203/Y3A15012 eutectic fibers as a function of applied stress at 1100°C (vacuum
atm). Circles represent the time to failure at fracture and squares represent experiments typically
interrupted at 100 h without fracture.

The directionally solidified A1203/Y3A15012 eutectic has a very small lamella size
of 0.1 to 0.4 km and hence has a large area of interphase boundaries. The nature of the
interface boundaries is very different than grain boundaries of polycrystalline materials.
An important characteristic of the A1203/Y3A15012 eutectic interphase boundary from
the fracture point of view is its coherent and strong nature (Matson and Hecht, 1999).
The toughness values determined using fracture mechanics analysis (Broek, 1986)
assuming entrapped voids as the initial flaw were about 4.6 MPa m-1/2. The degree of
microstructural anisotropy, although a beneficial condition in promoting improvements
in high-temperature mechanical properties, was insufficient to achieve higher toughness
and the fine eutectic lamellae have little effect in diverting the path of the fracture crack.
The moderate fracture toughness of A1203/Y3A15012 (4.6 MPa m-1/2) is greater than
that of either constituent alone (KI, (A1203) = 2.4 MPa m-'l2) and KI, (Y3A15012)
= 1.7 MPa m-1/2). Yang et al. (1996) estimated that KI, = 2 MPa m-'l2 using an
indentation technique and in regions of exaggerated coarsening. The different estimates
of KI, for A1203/Y3A15012 are probably due to different estimates of the flaw size. The
stress concentration at and around the exaggerated coarsened regions can be greater than
the geometric size of the region (Matson and Hecht, 1999).
The strong interface bonding of Al203/Y3A15012 eutectic is highly beneficial
to increased creep resistance by load transfer from one phase to another. For
A1203/Y3A15012 eutectic fibers, typical time-dependent creep strain data measured
in vacuum are shown in Fig. 8. The creep rate constantly decreases with time, giving the
appearance of a primary or transient creep stage up to 0.4% or more strain. Fiber fracture
occurs during this stage at 1100°C. A comparison of creep rate with available literature
values for single-crystal (0001) A1203 and Y3A15012 indicates that the A1203/Y3A15012