GRAVITY-INDUCED MICROSTRUCTURAL NONUNIFORMITIES DURING COMBUSTION SYNTHESIS OF INTERMETALLIC-CERAMIC COMPOSITE-MATERIALS

Combustion synthesis of intermetallic-ceramic composite materials usua lly involves high combustion temperature as well as high. temperature gradient. Gravity may play an important role in such systems when the Liquid phase is present, and the intermetallic and ceramic phases have different densities. The combustion characteristics and microstructur e may both be affected by gravity. Combustion synthesis of(1 - x)Ni3Al + xTiB(2) composites, with the weight fraction x varying from 0.05 to 0.8, was carried out under normal gravity conditions. Both the combus tion temperature and wave propagation velocity increased, and the prop agation mode changed from unstable (x less than or equal to 0.2) to st able (x greater than or equal to 0.4), as the TiB2 content increased. The combustion temperatures were higher than the melting point of Ni3A l for samples with x greater than or equal to 0.4, resulting in a comp osite material consisting of ceramic TiB2 particles dispersed in a Ni3 Al matrix. Owing to buoyancy of TiB2 particles in the denser molten Ni 3Al phase, gravity was found to affect the microstructure of the compo site, yielding a nonuniform distribution of phases. The phase separati on distance calculated by using Stokes' law compared well with. measur ements.