The ternary system uranium-boron-carbon

Phase equilibria in the ternary system U-B-C have been established by means of X-ray, metallographic and melting point analyses in the temperature range from 10000C to melting. Three ternary compounds were found to exist: besides the well known monoboroncarbide UBC two new uranium boroncarbides, UB2C and ''U5B2C7''. Ternary phase equilibria are characterized by the incompatibility of uranium metal with boroncarbide B4C and by the incompatibility of elemental boron and uranium carbides; an isothermal section of the system U-B-C at 16000C is presented. At high temperatures the crystal structure of UB2C was found to be isotypic with the homologous compound ThB2C; at temperatures below (1675±25)0C h-UB2C transforms into a low temperature modification with a new (unknown) structure type. The crystal structure of ''U5B2C7'' is closely related to the structure type of Ho5B2C6-7 as a derivative of La5B2C6. Employing the Pirani-technique, congruent melting was revealed for UBC and UB2C at (2144±25)0C and (2282±30)0C respectively. Using the clear-cross principle in studying possible phase reactions, the thermodynamic stabilities of UBC, UB2C and U5B2C7 were estimated. (orig.)