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MAS-osumilite is a metastable double-ring silicate which is attractive for use in glass-ceramics because of its low thermal expansion coefficient. The phase can be crystallized from glass powders of MAS-osumilite composition containing small amounts of BaO. The nucleation and crystallization process has been investigated by means of TEM and SEM. Crystallization starts with the precipitation of (3-quartz, of composition MgAl,O,*nSiO,. BaO concentrates in a residual glass phase between the @quartz, crystals. In a second stage small amounts of Ba-osumilite (BaMg,Al,-Si,O,) nucleate and crystallize from this glass phase. In a third stage BaO-free MAS-osumilite crystals are formed by epitactic growth on the Ba-osumilite surfaces and grow into adjacent P-quartz,, grains. 'hvo factors are essential for the suppression of undesired further phases to develop a monophase (besides some Ba-osumilite) MAS-osumilite glassceramic: a residual glass phase with a composition close to Ba-osumilite, and P-quartz,, crystals having a composition close to that of MAS-osumilite.

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Flat and curved, basal and nonbasal planar defects are frequently observed in sintered AIN ceramics. To prove the existence of inversion domains, TEM techniques sensitive to crystal polarity were used. Dark-field imaging in certain multiple-beam situations with g = (0002) (where g is operating reflection) yielded strong domain contrast giving clear evidence of an inverted crystal structure within the domains. This was confirmed by the dynamic contrast in the disks of convergent-beam electron diffraction. AH planar defects observed were found to be associated with inversion domains. The formation of inversion twins may be nucleated by accidental growth or by nucleation and growth within the bulk of a preexisting AIN grain. Both mechanisms are supposed to be associated with the accommodation of impurities. [Key words: aluminum nitride, domains, defects, electron diffraction, transmission electron microscopy.]

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