264
Some Phase Equilibria in the Uranium-Nitrogen System’ R. Benz and M. G . Bowman
Contributionfrom the Los Alamos Scienti3c Laboratory, University of California, Los Alamos, New Mexico. Received August 23, 1965 Abstract: Portions of the U-N phase diagram were determined by X-ray, metallographic, chemical, a n d thermal analyses of phases equilibrated with Nz (up to 5 atm.) at temperatures u p to 2850”. The U-UN portion of the diagram is a eutectic with the eutectic composition near p u r e U. T h e UN phase region has a U-rich boundary with an N:U ratio equal to or less than 0.96 i 0.02 at 1500” a n d 0.92 i 0.02 at 1800”. The N-rich boundary has a n N:U ratio of 1.04 i 0.02 at 1600”. T h e mononitride melts congruently with the N:U ratio 0.96 =t0.03 at 2835 i 30’. The body-centered-cubic hexagonal transition of the sesquinitride was found to occur at 1120”, but the transition temperature may b e dependent on composition. The composition of the hexagonal phase is UN1.47 o.02 near the decomposition pressure at 1315 ”.
and co-workers2 first characterized in the U-N system as NaC1-type facecentered-cubic (f.c.c.) UN, Mnz03-typebody-centeredcubic (b.c.c.) UzN3, and CaF2-type f.c.c. UN2. They found the U-UN and UN-U2N3 regions to be two phase over an unspecified temperature range. From variation of the UN, lattice parameter with composition, they concluded U2N3 and UN, form a continuous series of solid solutions in agreement with previously determined N2-pressure isotherms for the composition range UNl.66-UN~.65,Later, the hexagonal form of U2N3, isostructural with Th2N3,4 was identified.5je Recently, Nz-pressure dependence of the UN “apparent melting p ~ i n t ’ ’ and ~ , ~ the temperature dependence of nitrogen solubility in liquidg were reported. In what follows, information on these phases is extended and summarized in a phase diagram.
Ru:fkes
Experimental Section Materials. Ca-reduced U metal was 99.9 & 0.01 pure. The particular metal used in determination of U melting point and solid-solid transition temperatures contained impurities, in parts per million:
