Page 124 - Fiber Fracture

FRACTURE CHARACTERISTICS OF SINGLE CRYSTAL AND EUTECTIC FIBERS 109

INTRODUCTION

The concept of using single-crystal fibers as an active component or load bearing
constituent has potential in a variety of applications. In many cases properly processed
single-crystal fibers provide high crystalline perfection and sometimes near theoretical
strength. The main objective of this paper is to examine the fracture characteristics
of single-crystal fibers from a variety of crystal systems and a related directionally
and
solidified eutectic system. Aluminum oxide (cL-A~~O~), its single-crystal form as
sapphire (trigonal), yttrium sesquioxide Y2O3 (bcc-yttria), and yttrium aluminum garnet
(Y3A15012; cubic - YAG) will be discussed. The fracture characteristic of single-crystal
Y203 and Y3A15012 will be contrasted with single-crystal Al2O3.
The shortcomings of single-crystal fibers A1203 and Y3A15012, specifically low
toughness and slow crack growth, can be overcome by growth of directionally solidified
eutectic fibers. In this approach, the eutectic architecture of a continuous reinforcing
phase within a higher volume phase or matrix, can be considered as a naturally
occurring in-situ composite. This work reports the results of experiments aimed at
identifying the sources of high levels of strength retention and creep resistance in a
two-phase A1203/Y3A15012 eutectic system. Examination of the fracture characteristics
of the individual end members of the A1203-Y3A15012 region of the A1203-Y203
phase diagram (Viechnicki and Schmid, 1969) provides needed insight for discussion of
fracture characteristics of the directionally solidified eutectic fibers.

EXPERIMENTAL

All fibers tested in this study were produced using the laser-heated float zone
technique (LHFZ) which has been described in Sayir and Matson (1991) and Farmer et
al. (1993). Fibers were grown to -24 cm in length. For the dynamic studies at different
strain rates, cold grips were used with a total fiber gauge length of 23 cm. The fibers
were tested in air by placing them in a MoSi2 furnace (CM Inc., Bloomfield, NJ) with a
hot zone of 2.5 cm. All fibers fractured within this 2.5 cm length. The strain rates were
calculated from the total gauge length and cross-head speed of the test frame (Model
4502, Instron Corp., Canton, MA). This value can be considered a relative strain rate
if test conditions remain the same, i.e. temperature and grip method (Sayir, 1993). For
static load (stress rupture) studies, the fibers were dead weight loaded in high vacuum
atm) at 1400°C in a tantalum element furnace with a hot zone of 2 cm.
To accurately measure the micro-strain of the small-diameter single-crystal fibers, a
new technique was utilized. This technique was based on original work by Yamaguchi
and modified to measure strain on small areas (Barranger, 1990; Lant and Barranger,
1990 Sayir et al., 1994). Yamaguchi’s speckle-shift technique requires no specimen
surface preparation and allows micro-strain measurements of an extremely small gauge
length (on the order of 0.1 mm). It provides a strain resolution of approximately 15
micro-strain. Details about the application of this technique on small-diameter fibers
can be taken from previous work (Sayir et al., 1994). This technique requires the
reflection of coherent laser light from an optically rough surface that results in a speckle