Nb-Base FS-85 Alloy as a Candidate Structural Material for Space Reactor Applications: Effects of Thermal Aging
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INTRODUCTION
REFRACTORY metal alloys have traditionally been candidate materials for high-temperature applications, offering the potential for large improvements in performance or thermodynamic efficiency over more conventional alloys. However, due to poor oxidation resistance, their use has been limited to specific applications. One application for which there is great interest in refractory metal alloys is compact, space-based fission reactors. In this application, high-temperature stability and strength, corrosion resistance to liquid metal coolants, radiation resistance, and fabricability are key issues in materials selection. The interest in Nb-base alloys for these space applications originated in the early programs of the 1960s[1] and has continued on to the more recent Jupiter Icy Moons Orbiter (JIMO) and Prometheus programs of the U.S. National Aeronautics and Space Administration (NASA).[2] Despite a long history, the maturity of the database on Nb-base refractory metal alloys is limited. This is due in part to program cancellations and the lack of current commercial development. As reported by Wojcik,[3] niobium alloy additions to steel KEITH J. LEONARD, JEREMY T. BUSBY, and DAVID T. HOELZER, Research Staff Members, and STEVEN J. ZINKLE, Division Director, are with the Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN 378316138. Contact e-mail: [email protected] Manuscript submitted March 7, 2008. Article published online February 3, 2009 838—VOLUME 40A, APRIL 2009
and nickel-base superalloys account for approximately 98 pct of total niobium use, with the remainder used in Nb-base alloys or in pure metal form. From the start of the space reactor programs, a large number of refractory metal alloys have been developed to varying degrees. Of these refractory alloys, Nb-base alloys offer excellent formability with a reduced weight penalty, due to a lower density as compared to other refractory metals. With a premium placed on ductility and fabrication, selections of relatively weak but formable candidates were initially evaluated. Of these Nb-base alloys, Nb-1Zr was the most promising and, as a result, offers the largest database of mechanical properties, in particular, creep properties.[4] However, the high-temperature strength of Nb-1Zr is among the lowest of all the refractory alloys, thus furthering interest in the development of stronger and more creep-resistant alloys. A comparison of the creep performance of Nb-base alloys is provided in Figure 1.[4,5] Some of the stronger and more creep-resistant alloys developed by the late 1960s include C-103 (Nb-10Hf-1Ti), PWC11 (Nb-1Zr-0.1C), D-43 (Nb-10W-1Zr-0.1C), Cb-752 (Nb-10W-2.5Zr), Cb-753 (Nb-5V-1.25Zr), and FS-85 (Nb-28Ta-10W-1Zr) alloys. A comparison of the densitycompensated ultimate tensile strength (UTS) of select space reactor candidate alloys, including two Ta-base alloys, is shown in Figure 2[1,6–11] Although similar properties are observed between the Nb-1Zr and T-111 alloys up to 1100 K, the Ta-base alloys show bet
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