Fabrication of Refractory Intermetallic Cr₂Ta by Reducing Metal Oxides with Calcium Hydride

High-temperature Cr-Cr₂Ta composites are promising candidates for replacing nickel-based precipitation-hardening superalloys for producing turbine blades. Segregation at the solidification of such alloys leads to a dendritic structure, which has a negative impact on the mechanical properties. In order to avoid such issues, powder metallurgy is considered a suitable method. In this work, the theoretical and practical aspects of producing Cr₂Ta by powder metallurgy using metallothermic (calcium-hydride) synthesis of a refractory intermetallic compound from oxide raw materials have been studied. Using X-ray and electron microscopic analysis, it has been established that the formation of the Cr₂Ta phase occurs due to solid-state diffusion of chromium particles in Ta. The apparent activation energy of this process is 291 ± 69 kJ/mol. It has been shown that the diffusion reaction actively occurs at a temperature of only 0.56 T_m of Cr₂Ta. Thermodynamic calculations and thermal analysis have established that such a low temperature of formation of Cr₂Ta is due to heat releasing during the exothermic reduction reaction of Cr₂O₃ oxide by calcium. The optimal synthesis parameters have been determined, and their influence on the phase composition of the powder has been studied. The developed technology makes it possible to obtain Cr₂Ta intermetallic powder containing 95 ± 5 vol pct of the target phase, with a sponge particle morphology and a size of < 20 μm. Typical impurity content has been determined as follows: < 0.28 wt pct for oxygen, < 0.07 wt pct for nitrogen, and < 0.02 wt pct for hydrogen. Thus, the resulting powder can be used for the production of Cr-Cr₂Ta composites.