Page 117 - Fiber Fracture

102 M.-H. Berger

Fig. 17. Microstructure of Nextel 720 composed of a continuum of mosaic mullite grains enclosing
a-alumina round or elongated grains.

larger a-alumina particles which become intergranular with a bimodal distribution in
size (Hay et al., 1999; DelCglise et al., 2001). Some of the elongated a-alumina grains
show exaggerated growth. This evolution of the microstructure is accelerated by the
combination of temperature and load. When tested in creep, these fibres show much
reduced creep rates when compared to pure a-alumina fibres and creep rates of the order
of lop6 s-' have been measured at 1400°C. This remarkable behaviour is attributed to
the high creep resistance of mullite and to the presence of the elongated and oriented
a-alumina grains (Fig. 18). The fibres are however very sensitive to alkaline-containing
environments. Mullite decomposes in the presence of a low concentration of alkalines
to form alumino-silicate phases of melting points lower than 1200°C. Under load fast
growth of large alumina grains occurs by liquid transportation, which is detrimental for
the fibre strength and tensile strength is seen to vary with the loading rate suggesting
a slow crack growth process (DelCglise et al., 2002). The failure surface shown in
Fig. 19 presents an intergranular zone that fans out symmetrically from platelet grains
and an intragranular zone corresponding to a fast propagation preceding failure. This
observation could seriously limit the use in real environments at high temperature of
fibres based on the A1203-Si02 system.

OTHER OXIDE FIBRES

Polycrystalline-alumina-based fibres can at present not compete with silicon-carbide-
based fibres when low creep rates are required. Fibres with higher resistance to creep by
dislocation motion could be provided by oxides with high melting point and complex
crystal structure, a tendency to order over long distances and the maintenance of this
order to high fractions of the melting temperatures (Kelly, 1996). Experimental devel-
opment of monocrystalline fibres by Czochralski-derived techniques from chrysoberyl