A METALLOGRAPHIC AND X-RAY STUDY OF THE UO$sub 2$-ZrO$sub 2$ SYSTEM

The UO/sub 2/- ZrO/sub 2/ phase system was studied in the high temperature range of 1200 to 2350 deg C by x-ray and metallographic techniques. The results indicate a continuous face-centered-cubic solid solution from pure UO/ sub 2/ to pure ZrO/sub 2/ in the temperature range of 2300 to 2550 deg C, thus confirming the existence of a third (cubic) polymorphic form of pure ZrO/sub 2/ in addition to the tetragonal and monoclinic phases stable at successively lower temperatures. A hightemperature two-phase region, previously unreported, was found to exist above 1660 deg C in the ZrO/sub 2/-rich portion of the phase diagram. The confusion concerning single-phase compositions quenched from high temperature and resulting in cubic or tetragonal structures without an intervening two-phase region was clarified by the discovery of a diffusionless transformation which occurs on cooling compositions of 55 mole% UO/sub 2/ or less, and its approximate location was established. A phase diagram of the UO/ sub 2/- ZrO/sub 2/ system was constructed. (auth)