FORMATION OF TIC IN INSITU PROCESSED COMPOSITES VIA SOLID-GAS, SOLID-LIQUID AND LIQUID-GAS REACTION IN MOLTEN AL-TI

A novel technique to generate fine single crystal TiC platelets in an aluminum based matrix has been developed (M. J. Koczak and K. S. Kumar , US Patent 4,808,372 (1989)). The process involves decomposition of a carbonaceous gas (CH4) and reaction of nascent carbon with a strong c arbide former such as Ti in an aluminum matrix at a relatively high te mperature (1200-1400-degrees-C). The highly exothermic process is mode rated by means of a carrier gas and leads to a fine distribution of ca rbides of size 0.1-3 mum. A nucleation and growth study was carried ou t to understand the decomposition of the methane, distribution and sub sequent reaction with an aluminum-titanium alloy to form the titanium carbide. It was observed that formation of TiC occurs in stages. Follo wing the CH4 decomposition, the solid carbon particles are distributed and trapped in the alloy. The reaction to form TiC is probably limite d by diffusion of titanium to carbon and thereafter the carbide. After inoculating carbon in the alloy, the reaction can be completed in sol id or liquid state. Transmission electron microscopy studies confirmed the presence of 40-50 nm amorphous carbon particulates in the alloy. It is also postulated that, in liquid state, once the reaction proceed s, the first phase to form is aluminum carbide or an aluminum-titanium carbide of the form. Given sufficient time for completion, the reacti on proceeds to form the most stable carbide, ie. TiC.