Nonequilibrium synthesis of nbai 3 and Nb-Ai-V
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INTRODUCTION
NIOBIUM aluminum alloys are presently under investigation for high-temperature structural applications which require a high strength-to-weight ratio as well as superior oxidation resistance. Unfortunately, the ordered NbxAly alloys are particularly brittle at room temperature and, for most compositions, have extremely poor oxidation resistance at high temperature. Recent efforts have been made to improve the limited ductility of these materials via alloying and rapid solidification processing. I~1 One of these processing methods is the technique of laser cladding, where the inherent cooling rates of up to 106 ~ provide the opportunity to produce nonequilibrium materials which possess an increased range of solid solubilities. Macroalloying can also be useful in cases where there is a beneficial effect on ductility while the alloy retains good resistance to oxidation. Of the various niobium-based alloys studied by Svedberg, t2] the niobium aluminide, NbAI3, had the slowest oxidation rate. This composition was shown to have the minimum aluminum content necessary to allow a continuous external layer of alumina to form that will protect the underlying metal. Svedberg t2] observed that arc-melted NbAI3 isothermally oxidized in air at 1200 ~ forms an oxide comprised of an outer layer of both alumina and NbA104 over an inner layer of only alumina adjacent to the alloy/oxide interface. Layered oxide structures have also been observed to form in this temperature range in other studies, t3,4,51 Perkins e t a l . , t3] however, found that under similar conditions to that of R.T. HAASCH, Research Associate, C.M. LOXTON, Head of Surface Analysis Facility, Materials Research Laboratory, S.K. TEWARI, Graduate Student, and J. MAZUMDER, Professor, are with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. S. SIRCAR, Principal Scientist, is with the Lockheed Engineering and Sciences Company, NASA-White Sands Test Facility, Johnson Space Center, Las Cruces, NM 88004. Manuscript submitted February 28, 1991. METALLURGICAL TRANSACTIONS A
Svedberg, a NbA13 alloy formed by the pack aluminization of Nb had oxidized with the development of alternating layers of NbA104 and A1203, with an A1203 layer at the oxide/alloy interface and at the outer surface. Preliminary studies for oxidation at 1200 ~ of laser-clad Nb and NbAI alloys have shown 16] that, in agreement with these studies of the stoichiometric alloys, the cladding with nominal composition of NbA% has the lowest oxidation rate. This oxidation resistance occurred despite the fact that the oxide contained significant Nb205 along with the protective A1203, although to a lesser extent than with Nb and Nb3AI. Efforts to improve the ductility of the brittle NbA13 by partial replacement of Nb with other elements of groups IVA, VA, or VIA are in progress, t~] Vanadium appears to be a suitable macroalloying choice and is found to improve the ductility, although for V additions above 5 at. pct V, the dual-ph
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