Effect of mechanical alloying on sinterability and phase evolution in pressure-less sintered TiB₂‒TiC ceramics

Phase relation and microstructure evolution in the pressure-less sintered TiB₂‒TiC ceramics preceded with mechanical alloying were systematically studied by a combination of SEM analysis. WC debris from milling balls promotes sintering by dissolving into the TiC phase to achieve dense microstructures at 1600 °C. Variation of W solution in TiC grains exposes two types of core‒rim structures, with no or more W in dark and white cores respectively but with common medium W in both rims. Diminishing white-cores reveal an exchange reaction between WC and TiC via mechanical alloying to form the Ti_1-zW_zC phase prior to sintering. The dark-cores inherit from the as-milled TiC power to further enable the reprecipitation of rims from a mixed liquid-phase, which facilitated also the anisotropic growth of TiB₂ grains. The dark-cores grow persistently in the second-step at 2000 °C enabled by this liquid-phase, which coarsens the TiB₂ grains too. With more alloyed phase, sintering was insufficient at 1500 °C with only the surface fluidity from the primary powders, and the second-step sintering increased the fluidity in the liquid-phase to fully densify the binary microstructure. Re-distribution of the alloyed W by two-step sintering rationalizes the evolution process of the binary microstructures and leads to better understanding of the mechanical behaviors.