EFFECT OF COMPOSITION ON PHASE EVOLUTION AND MICROSTRUCTURE OF REACTION BONDED MULLITE (RBM)

Compositions ofa120j+si, si02+ai and ai+si systems were prepared to study the effectofreaction bonding process on the mulliteformation. the composition ofeach system was adoptedaccording to mullite stoichiometery and sintered in 700-1600°c range. results showed that theformation of reaction bonded mullite starting from ai20j+si mixtures, proceeded in two partiallyoverlapping steps, the oxidation ofsi to si02• and the reaction of sio: and al203 to form mullite.in this system, up to 1400°c, conversion ofsi to si02 was taken place and cristobalite formed, butmullite formation was not observed. mullite phase started toform at 1450°c.results indicated that complete reaction was not occurred up to 1600°c and 2 hours soakingtime. xrd patterns ofsamples in al+si02 system showed that the reaction through sequences:(a)reduction of si02 by ai, (b) formation of a-a1203 and sial-derived si oxidation, and (c) mulliteformation. x-ray diffraction patterns ofheat-treated al+sisystem showed that reaction between aland oxygen at 900°c was occurred with the reaction product being a-ai20j. oxidation of si andformation ofmullite were not detected in this system.sem micrographs showed that both ai203+si and si02+al systems have similar microstructures.which consisted of a-ai203, mullite and free si. the microstructures of the samples in al+sisystem consisted ofa-ai20 j. free al and si with intermetallic al-si compound.