Microstructure and mechanical behavior of Al-Li-Cu-Mg alloy 8090 microalloyed with V and Be
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I.
INTRODUCTION
T H E A1-Li alloys offer promise of substantial weight savings in aerospace applications by virtue of their reduced density and increased elastic modulus compared with conventional aluminum alloys. I~1 The increased strength of A1-Li alloys is attributed to precipitation of 8' (A13Li); however, 8' precipitation also lowers ductility by strain localization and precipitate free zone (PFZ) formation. In recent years, research on A1-Li alloys has concentrated on improving ductility and toughness in two alloy systems: the ternary A1-Li-Cu alloy (2090 type) in which 8' precipitation is supplemented by the formation of T~ (A12CuLi) and 0' (CuA12) and quaternary A1-LiCu-Mg alloys (8090 type) in which the precipitation of S' (A12CuMg) occurs, t2] The presence of a second precipitating phase can alter the deformation mode.J3-6] In the case of 8090 alloy containing S' phase, extensive cross slip occurs to give irregular slip lines, indicating S' phase is not sheared by glissile dislocations. The/'1 phase was found less effective in dispersing slip than S'. [3] Also, precipitation of S' does not result in PFZs along low-angle grain boundaries. I7~Both 7"1 and S' nucleate heterogeneously on dislocations and other defect sites and, hence, the practice of prior-aging deformation (3 to 5 pct stretch) to ensure widespread nucleation in the matrix. ~8) The prior-aging deformation step is not feasible for some alloy products, and so the enhancement of nucleation by other means must be explored. Pickens and co-workers t9,~~have shown that relatively small amounts of Ag and Mg are extremely effective in stimulating Tt precipitation in A1-Li-Cu alloy, which led to the patent of WELDALITE* 049. It has been shown that small ad*WELDALITE is a trademark of Martin Marietta Laboratories, Baltimore, MD.
ditions of Ag, t~l) Si, t12] and Be t~3) in A1-Cu-Mg alloys AIHUA LUO, Ph.D. Candidate, and WILLIAM V. YOUDELIS, Professor, are with the Engineering Materials Group, Department of Mechanical Engineering, University of Windsor, Windsor, ON, Canada N9B 31>4. Manuscript submitted January 24, 1992. METALLURGICAL TRANSACTIONS A
stimulate the nucleation and refinement of S' precipitate, with an attendant increase in the peak hardnesses, for the aged alloys. This suggests a similar study in search of suitable microalloying elements to stimulate S' phase precipitation in 8090 alloy. The selection of specific elements for study is based on two principal factors: the potential for microalloying elements to concentrate in the precipitating phase and/ or to significantly restrict the solubility of the precipitate components in the matrix. Both factors increase the entropy and corresponding nucleation rate of the precipitating phase. [14,15A6] The object of the present research is to study the effects on age-hardening, microstructure, mechanical properties, deformation, and fracture behavior of A1-Li alloy 8090 by microalloying with V and Be. II.
EXPERIMENTAL
A. Materials
The as-cast A1-Li-Cu-Mg alloy 8090 (base alloy) was provided by
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