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 carbide former such as Ti in an aluminum matrix at a relatively high temperature (1200-1400-degrees-C). The highly exothermic process is moderated by means of a carrier gas and leads to a fine distribution of carbides of size 0.1-3 mum. A nucleation and growth study was carried out to understand the decomposition of the methane, distribution and subsequent reaction with an aluminum-titanium alloy to form the titanium carbide. It was observed that formation of TiC occurs in stages. Following the CH4 decomposition, the solid carbon particles are distributed and trapped in the alloy. The reaction to form TiC is probably limited by diffusion of titanium to carbon and thereafter the carbide. After inoculating carbon in the alloy, the reaction can be completed in solid 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 proceeds, the first phase to form is aluminum carbide or an aluminum-titanium carbide of the form. Given sufficient time for completion, the reaction proceeds to form the most stable carbide, ie. TiC.