STUDIES ON U-C CONSTITUTIONAL DIAGRAM BETWEEN UC AND UC2

The U-C system from UC1.0 to UC2.4 (4.8 to 10.8 wt % carbon), with major emphasis on the region between 1820 and 1275 °C, was studied by identifying the phases present in arc-cast and heat-treated specimens that had been cooled to room temperature. Uranium sesquicarbide was found to have a line composition very close to UC1,50 and an upper stability limit of about 1800 °C. Below 1800 °C, the uranium mono-carbide lattice accommodated a maximum of about 1.02 carbon atoms per uranium atom. The dicarbide phase existed over a much wider composition range, from about UC1.7 to UC1.9 at 1765 °C; this range narrowed as the temperature decreased. The maximum carbon content of the cubic dicarbide phase decreased with temperature from 1.89 carbon atoms per uranium atom at the melting point to a minimum of 1.85 at about the cubic-to-tetragonal transformation temperature. Combined-carbon/uranium atom ratios above 1.85 could be obtained at room temperature only by quenching specimens containing less than 0.2 wt % free carbon because the free carbon acted as precipitation centers in the metastable solutions. When cooled from the cubic to the tetragonal region, the dicarbide disproportionated into monocarbide and a tetragonal dicarbide that had an estimated composition of UC1.89 at room temperature. No thermal decomposition of the tetragonal uranium dicarbide was observed after heat-treatment at 1260 to 1600 °C. © 1969.