Ceramic-matrix composites strengthened by suitable fiber additions are being developed for high-temperature use, particularly for aerospace applications. New oxide-based fibers, such as mullite, are particularly desirable because of their resistance to high-temperature oxidative environments. Mullite is a candidate material in both fiber and matrix form. The primary objective of this work was to determine the growth characteristics of single-crystal mullite fibers produced by the laser-heated floating zone method.
Directionally solidified fibers with nominal mullite compositions
of 3Al2O3·2SiO2 were grown by the laser-heated floating
zone method at the NASA Lewis Research Center. SEM analysis revealed
that the single-crystal fibers grown in this study were strongly
faceted and that the facets act as critical flaws, limiting fiber
strength. The average fiber tensile strength is 1.15 GPa at room
temperature. The mullite fibers exhibit superior strength retention
(80 percent of their room temperature tensile strength at 1450
°C). Examined by transmission electron microscopy, these
mullite single crystals are free of dislocations, low-angle boundaries,
and voids. In addition, they show a high degree of oxygen vacancy
ordering.
High-resolution digital images from an optical microscope furnish
evidence of the formation of a liquid-liquid miscibility gap during
crystal growth. These images represent the first experimental
evidence of liquid immiscibility for these compositions and temperatures.
Continuing investigation with controlled seeding of mullite single
crystals is planned.
Strongly faceted single-crystal growth from planar interface
(note loss of axial symmetry).
Sayir, A.; and Farmer, S.C.: Directionally Solidified Mullite Fibers. Ceramic Matrix Composites: Advanced High-Temperature Structural Materials, Mat. Res. Soc. Symp. Proc., R.A. Lowden, ed., Materials Research Society, Pittsburgh, PA, vol. 365, 1995, pp. 11-21.
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