Microstructure and mechanical behavior of spray-deposited High-Li Al-Li alloys
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I. INTRODUCTION
THE density of Al is reduced by approximately 3 pct for each wt pct addition of Li, while the Young’s modulus is increased by approximately 6 pct.[1,2] In principle, the weight savings in aircraft structural parts manufactured with Li-containing alloys could reach 15 pct.[3] One of the major problems associated with Al-Li alloys is the decreased ductility resulting from the presence of the shearable strengthening phase d8 (Al3Li). To that effect, considerable efforts have been directed toward improving the ductility and fracture resistance of Al-Li alloys through the use of rapidly solidified powder metallurgy (PM) and ingot metallurgy (IM) techniques.[4–9] Some of the approaches taken for improving toughness in Al-Li alloys involve encouraging dislocation cross-slip or bypass by modifying the lattice parameters to increase mismatch in the Al-Al3Li system, introducing secondary precipitation systems, and using a dispersionhardening system. Other approaches include grain refinement through additions of Mn, Zr, Cr and Co and minimization of tramp elements through composition control.[10,11] The major difficulties that are associated with the processing of high-Li Al-Li alloys are the strong tendency of macro-and microsegregation of Li and the limited solubility of transition elements in the Al matrix. For ingot-processed Al-Li alloys, the concentration of Zr (a major grain-refining element) is typically limited to less than 0.12 wt pct, due to the low equilibrium solid solubility of Zr in Al.[12,13,14] These segregation problems may be alleviated, to a certain extent, by rapid solidification (RS) PM processing.[15] The L. DEL CASTILLO, Graduate Student Researcher, H.M. HU, Postdoctoral Scientist, and E.J. LAVERNIA, Professor and Chair, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California at Irvine, Irvine, CA 92697-2575. Y. WU, formerly Postdoctoral Scientist at the Department of Chemical and Biochemical Engineering and Materials Science, University of California at Irvine, is Senior Engineer with Motorola, Phoenix, AZ. Manuscript submitted April 8, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
high solidification rates (103 to 105 K/s) associated with RS processing tend to minimize the segregation of Li, reduce grain size, and promote overall microstructural refinement. Although considerable success has, indeed, been achieved, the RS PM approach is subject to oxide contamination. The large surface areas generated by powder production techniques, together with the stability of Al2O3, render it impossible to eliminate oxides from the final PM products. In the present investigation, a spray deposition technique is used to synthesize Al-Li alloys in the high-Li (3.8 wt pct) concentration range. In this concentration range, segregation of Li becomes extensive during slow solidification. Segregation of Li should be reduced greatly during spray deposition. The microstructures and mechanical properties of the Al-Li alloys fabricated using spray
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