Enhancement of Uranium Oxidation Resistance by Molybdenum Implantation
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ENHANCEMENT OF URANIUM OXIDATION RESISTANCE BY MOLYBDENUM IMPLANTATION
E. N. KAUFMANN,* R. G. MUSKET,* C.A. COLMENARES,* AND B. R. APPLETON** *Lawrence Livermore National Laboratory, Livermore, CA 94550; **Oak Ridge National Laboratory, Oak Ridge, TN 37830
ABSTRACT Initial studies of the oxidation resistance imparted to uranium metal through the implantation of Mo ions have shown that a significantly increased resistance can be 1 6 2 achieved for doses of 5 x 10 /cm . No enhancement of 16 oxidation resistance was found for doses below 1 x 10 /cm We report results of weight-gain measurements, ion-beam analysis, and x-ray diffraction, as well as observations of the way in which the protective implanted layer fails at long exposure times.
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INTRODUCTION Pure a-phase uranium is known to be highly susceptible to lowtemperature oxidation, particularly in humid atmospheres [1]. Alloys of uranium with additives such as Ti, Nb, or Mo display much-improved oxidation resistance. Bulk alloys, however, also differ from the pure a-phase in crystal structure, density, and mechanical properties, and may for these reasons be less suitable in specific applications. A surface-protection method that prevents oxidation while maintaining bulk properties could prove useful. Of the many surface-modification techniques available, ion implantation is one that does not result in dimensional changes in a finished part, that suffers from no filmadherence problems (since the modified layer is integral with the surface), and that can overcome diffusion barriers and metallurgical incompatibilities. A preliminary indication that 350-keV Mo implanted in a-uranium to 16 2 a fluence of 3 x 10 /cm provides oxidation resistance was reported by Anthony [2]. This observation is consistent with the behavior of the known alloy U-10wt%Mo, which can be retained metastably in the cubic y-phase and which is quite corrosion-resistant. Anthony's test 0 conditions [80 C in air with 266-torr H2 0 vapor (75% relative humidity)] were considerably less severe than those routinely applied at Lawrence Livermore National Laboratory in evaluating coatings [800C in N2 with 100-torr H20 vapor (27% relative humidity)], primarily because the presence of gaseous 02 (in air) at greater than 50 ppm inhibits the corrosive action of H2 0 vapor [3]. Oxygen content of the N2 gas used in our experiments was less than 5 ppm. Our intent was to determine if Mo-implanted uranium showed enhanced resistance under the more severe conditions and to reveal the responsible underlying mechanisms.
EXPERIMENTAL PROCEDURES AND RESULTS Pure depleted-uranium metal disks of 0.565-in. diam and 0.050-in. thickness, with a small hole near the periphery, were machined, mechanically lapped, and finally electropolished for use in this study. The disks were initially implanted, using a rastered beam, on one face 2 1 6 /cm with 180-keV 96Mo+ ions to a maximum fluence of 5 x 10 (projected range %300 A). After implantation, the unimplanted face, the edge, and a portion of the implanted face were coate
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