A new class of hypereutectic carbide composites.

Extremely high thermal shock resistance in the Group IV-A carbides (TiC, ZrC, HfC) has been achieved by the fabrication of carbide-graph ite composites using vacuum arc melting and casting techniques. High purity materials were employed to ensure a low interstitial content. Gravity and skull casting have been employed to produce test shapes. The eutectic composition between the monocarbide and terminal graphite phase has been denned in each system. When carbon content exceeds the eutectic composition, the excess precipitates from the liquid as platelets within the eutectic matrix. Mechanical properties have been determined at room temperature on Ti-C and Zr-C materials. A sharp demarcation has been noted at the eutectic point where, in the Ti-C system, compressive strengths of 240,000 psi for hypoeutectic material decrease to 30,000 psi for hypereutectic material. Similarly, flexure strength drops from 30,000 to 10,000 psi. Thermal shock resistance was evaluated in both 200-kw and 5-Mw air arc tunnels at heating rates of 700,1800, and 2400 Btu/ft2/sec. No evidence of failure was noted in hypereutectic materials within any of the three systems. In comparative testing with tungsten and graphite, HfC-C showed marked superiority by exhibiting no measurable recession whatsoever when exposed to a heat flux of 700 Btu/ft2/sec.