Fabrication and evaluation of Nb/Nb 5 Si 3 microlaminate foils
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I. INTRODUCTION
A NUMBER of metal-silicide alloys are currently being evaluated for use at high temperatures beyond those presently attainable with nickel-based superalloys.[1–4] One such alloy that holds considerable promise consists of bcc Nb and the tetragonal line compound Nb5Si3. Both phases are chemically stable to temperatures exceeding 1660 8C.[5] In this binary alloy, the Nb phase provides room-temperature toughness, and the Nb5Si3 phase provides high-temperature strength and creep resistance.[6,7] Alloys of Nb/Nb5Si3 also have densities comparable to that of existing nickel-based superalloys and can be modified to enhance their oxidation resistance.[8] Metal silicide alloys, such as Nb/Nb5Si3, can be produced using several fabrication methods, resulting in various geometries of the Nb and Nb5Si3 phases. Casting Nb and Si produces a dispersion of Nb in a silicide matrix that has a room-temperature fracture strength greater than pure Nb.[9] Extruding cast Nb/Si alloys elongates and aligns the Nb particles in the extrusion direction within the silicide matrix. This “in situ composite” geometry further improves the strength and fracture toughness of the alloy.[7,10] Heat treating these in situ composites at 1500 8C for 100 hours produces minimal coarsening, which is indicative of the alloy’s microstructural stability. A composite laminate structure of Nb/ Nb5Si3 has also been fabricated by hot pressing Nb foils and Si wafers.[11] This layered composite, with a bilayer thickness of approximately 280 mm, exhibited ductility during tensile tests at 1100 8C, and the Nb layers bridged cracks that formed in the Nb5Si3 layers. While Nb/Nb5Si3 alloys have been produced by a variety of methods, the geometry of the phases and its effect on physical properties has not been examined in detail. This A.J. GAVENS, Engineer, is with Krolls Atomic Power Lab, Schenectady, NY, 12309. D. VAN HEERDEN, Postdoctor, and T.P. WEIHS, Assistant Professor, are with the Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218. T. FOECKE, Materials Scientist, is with the Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. Manuscript submitted December 15, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
effect has been examined, though, in two closely related alloy systems. Chen et al. compared the strength and fracture toughness of MoSi2 reinforced with 20 vol pct of Nb particles, Nb fibers, or Nb layers, and found that the laminated structure had the highest strength and fracture toughness.[12] Similarly, Heathcote et al. observed a higher resistance to fatigue crack growth in microlaminates of Nb/Nb3Al as compared to in situ Nb/Nb3Al composites.[13] Thus, by fabricating a laminate structure of Nb and Nb5Si3, the potential exists for improving the physical properties of Nb/Nb5Si3 alloys. Laminate phase geometries can be fabricated by hot pressing foils or by physical vapor deposition (PVD). In general, PVD produces laminate materials with fewer impuriti
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