MICROSTRUCTURE AND PROPERTIES OF AL2O3-AL(SI) AND AL2O3-AL(SI)-SI COMPOSITES FORMED BY IN-SITU REACTION OF AL WITH ALUMINOSILICATE CERAMICS

Al2O3-Al(Si) and Al2O3-Al(Si)-Si composites have been formed by in sit u reaction of molten Al with aluminosilicate ceramics. This reactive m etal penetration (RMP) process is driven by a strongly negative Gibbs energy for reaction, In the Al/mullite system, Al reduces mullite to p roduce alpha-Al2O3 and elemental Si, With excess Al (i.e., x > 0), a c omposite of alpha-Al2O3, Al(Si) alloy, and Si can be formed. Ceramic-m etal composites containing up to 30 vol pct Al(Si) were prepared by re acting molten Al with dense, aluminosilicate ceramic preforms or by re actively hot pressing Al and mullite powder mixtures. Both reactive me tal-forming techniques produce ceramic composite bodies consisting of a fine-grained alumina skeleton with an interpenetrating Al(Si) metal phase. The rigid alumina ceramic skeletal structure dominates composit e physical properties such as the Young's modulus, hardness, and the c oefficient of thermal expansion, while the interpenetrating ductile Al (Si) metal phase contributes to composite fracture toughness. Microstr uctural analysis of composite fracture surfaces shows evidence of duct ile metal failure of Al(Si) ligaments, Al2O3-Al(Si) and Al2O3-Al(Si)-S i composites produced by in situ reaction of aluminum with mullite hav e improved mechanical properties and increased stiffness relative to d ense mullite, and composite fracture toughness increases with increasi ng Al(Si) content.