| The crystallization behavior of mullite growing from the melt within a siliceous glassy phase has been investigated. Initial studies examined single crystal, nominally continuous mullite fibers grown in bundles by directional solidification using a modified version of the Edge-Defined Film-Fed Growth (EFG) process (GE-AE/Saphikon, Inc.). These fibers are intended to be recovered by dissolution of the glass matrix and used as reinforcements of ceramic matrix composites. Subsequent experiments employed small-scale crucible-based solidification experiments conducted at the UCSB Materials Processing Lab.; The EFG approach yielded ribbons containing bundles of aligned single-crystal mullite with relatively small diameters embedded in a silicate glass matrix. Because of interactions between the growing crystals, however, the fibers recovered are actually discontinous crystals and hence termed "fibrils." Ribbons were produced from SiO{dollar}sb2{dollar}-{dollar}rm Alsb2Osb3{dollar}-MgO melts at solidification rates ranging from 2.5 to 61 cm/h. Typically, mullite fibers grew in the (001) direction and had distinct facets on the {dollar}{lcub}{dollar}110)-type planes. The mullite fibrils exhibited a variety of cross-sectional profiles--including various forms of glass-filled hollows, internal splintering, and lateral dendritic growth--which resulted largely from morphological instability during growth, and thus depended on the alloy composition and solidification parameters. Morphological instabilities became more pronounced with increasing growth velocity. Increasing the MgO content or the {dollar}rm Alsb2Osb3{dollar}/SiO{dollar}sb2{dollar} ratio reduced these instabilities, indicating the role of alloy chemistry in modifying the transport properties in the melt, and hence the ease of solute redistribution.; The crucible experiments examined the role of melt chemistry further, paying particular attention to the effects of various modifying additions to the SiO{dollar}sb2{dollar}-{dollar}rm Alsb2Osb3{dollar} melt (MgO, BaO, Na{dollar}sb2{dollar}O, and K{dollar}sb2{dollar}O). The choice of modifying oxide had a significant effect on the fibril morphology and scale, such that modifiers with smaller cationic diameter tended to produce larger fibrils and more stable microstructures. However, high concentrations of modifier also appeared to reduce the effectiveness of these additions, suggesting that the modifiers eventually destabilize the growth interface as impurities.; Results from both experiments implied the critical role of faceting constraints on determining the final morphology. Examination of several fiber growth tips indicated that the preferred tip shape was a flat facet on the (001) plane. Inadequate solute redistribution at the tip, however, lead to the formation of reentrant hopper-shaped tips which subsequently produced hollow fibers. |
