The effect of quench rate on the microstructure, mechanical properties, and corrosion behavior of U-6 wt pct Nb
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INTRODUCTION
U R A N I U M is used in a variety of applications because of its high density (19.1 • 103 kg/m 3) and its unique nuclear properties. Poor corrosion resistance and unfavorable combinations of strength and ductility, however, are difficulties which must be overcome in order to exploit this material in engineering applications. It is known that solid solution additions of elements such as niobium, titanium, and zirconium markedly improve uranium's resistance to corrosion and substantially alter its mechanical properties.I-5 Unfortunately, these elements are extensively soluble only in the high temperature cubic 3,-phase and almost completely insoluble in the low temperature orthorhombic a-phase. This difficulty can be avoided by heat treating uranium alloys in the "y-phase field (to form uranium alloy solid solutions) followed by rapid quenching to room temperature. The high cooling rate forces the 7-phase solid solution to transform martensitically to variants of the low temperature a-phase in which the alloying elements are retained in supersaturated solid solution. These metastable a-phase solid solutions exhibit far better corrosion resistance than unalloyed uranium. These solid solutions are highly supersaturated and are amenable to subsequent age hardening, permitting a wide range of mechanical properties through the selection of aging temperature and time. Application of these principles to the U-6 pct Nb alloy* can result in corrosion rates which *All compositions are given as weight percentages.
are orders of magnitude lower than that of unalloyed uranium, 6 combined either with yield strengths as high as 1300 MPa 7 (six times that of unalloyed uranium), or with tensile elongations as high as 30 pct 7 (equivalent to that of high quality fine grained wrought unalloyed uranium).
The equilibrium microstructure of U-6 pct Nb and the properties associated with material in this condition can be understood by referring to the uranium-niobium phase diagram shown in Figure 1.5 At the solution treatment temperature of 1073 K all of the niobium is dissolved in the y]-phase. When cooled very slowly such that an approach to equilibrium is maintained, the y~-phase undergoes diffusional decomposition at 920 K to a two-phase structure consisting of essentially unalloyed a-uranium and niobium rich 7-phase. Material in this condition does not have good corrosion resistance 7 because the a-phase contains virtually no niobium in solid solution and because the two-phase structure gives rise to microscopic anodic and cathodic regions. Second-phase strengthening results in yield strengths of approximately 900 MPa, but tensile elongation is only about 12 pct. 9 The microstructure and properties associated with quenched U-6 pct Nb are markedly different and far more
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K. H. ECKELMEYER, Supervisor, Electron Optics and X-Ray Analysis Division 1822; A. D. ROMIG, Jr., Member o
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