XRD PEAK BROADENING EFFECTS IN U-Mo α″ PHASE E. Dabush, J. Sariel, I. Dahan and G. Kimmel Nuclear Research Center, Negev, P.O. Box 9001, Beer-Sheva, 84190 Israel ABSTRACT The U-Mo system has been extensively studied due to its use in the past as a nuclear fuel. This system has several metastable phases, depending on the Mo content and the cooling rate after solution treatment at high temperature. One of these phases α″ is characterized by its high strength and ductility. X-ray diffraction of mechanically polished specimens of U-Mo exhibits broadened lines associated with the surface grinding. Broadened diffraction lines are unfavorable for structural analysis. Yet, the broadening effect indicates a valuable information about the microstructure. It was found that the broadening effect is sensitive to the cooling rate and that the broadening as a function of the cooling rate has an opposite trend for mechanically and electrolytically polished. The presentation of this phenomenon together with its implications is given in this work. INTRODUCTION In equilibrium, above 650°C, U-Mo system has a cubic structure (cI2) denoted as γ phase, which is a solid solution in the range of 0-40% at. Mo. Below 650°C there is a separation into two phases: U-α which dissolve less than 0.1%at. Mo and an ordered MoU 2 - γ s intermediate phase. However, it is easy to obtain single phases at room temperature in a wide solubility range by rapid cooling from the high temperature solid solution to the room temperature. These phases are metastable with various crystal structures depending on the Mo content and cooling rate. In this work we are concentrating on the monoclinic α″ which has a deformed U-α structure. The α″ phase is obtained by rapid cooling from temperature of 800-900°C in the composition rang of 6-12 at. % Mo. Due to the small penetration of X-rays in uranium base alloys it is necessary to conduct electrolytic polishing in order to expose the bulk structure. In order to inspect the effectiveness of the electrolytic polishing procedure we decided to examine X-ray diffractograms from specimens underwent mechanical polish (MP) with those from specimens underwent electrolytic polish (EP). The structure of α″ phase is closely related to U-α orthorhombic. The four atoms in the unit cell as has been given by Stewart and Williams [1], can be generated in an artificial space group C2 1 /m [2] from the asymmetric unit which include one atom. The natural space group should be P2 1 /m with two atoms in the unit cell. Since some programs cannot handle unusual settings, a conversion between both settings for U 9%at. Mo is shown in Table 1. Copyright (c)JCPDS-International Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume 45. 146