Evidence for High-Temperature Forms of Zirconium and Tantalum Monocarbides

Evidence has been found which suggests the existence of high-temperature crystalline modifications of both zirconium carbide and tantalum carbide. The high-temperature inversion in zirconium carbide, indicated by a sharp decrease in spectral emisivity, in electrical resistivity, and in density, was reversible. No permanent changes in microhardness or in X-ray diffraction patterns were found after cooling to room temperature. Emissivity measurements showed a hysteresis effect of approximately 400° C for zirconium carbide. A similar, although less sharply defined, effect was noted for tantalum carbide. No such effect was found in solid solutions of these two monocarbides.