The effect of processing and microstructure development on the slip and fracture behavior of the 2.1 wt pct Li AF/C-489
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I. INTRODUCTION
IN this era of streamlined economics for the aerospace industry, weight reduction of aerospace vehicles is best achieved through decreasing the density of the aircraft’s structural materials. For aluminum alloys, lithium is added to reduce the alloy’s density with the added benefit of increasing the alloy’s modulus. In fact, the addition of 2 wt pct lithium can reduce the density of aluminum by 6 pct and increase the modulus by 12 pct,[1] thus, decreasing the system weight and operational costs while increasing the intrinsic alloy strength. Nevertheless, extensive implementation of aluminum-lithium alloys into primary aerospace structures has been hindered, in part, due to their characteristic anisotropic mechanical behavior,[2] unusual fracture behavior,[3] poor short transverse properties,[4] and higher production costs. A recent Air Force sponsored program focused on an effort to develop an aluminum-lithium alloy containing greater than 2 wt pct lithium that possessed significantly reduced anisotropic properties compared to those currently available.[5] Starke suggested that an intermediate recrystallization anneal be introduced between rolling stages to reduce the sharp textures caused by the rolling and thereby decrease the alloy’s anisotropy.[6] This added step proved highly successful as the anisotropy of both the modulus and yield strength were reduced significantly from 20 to 25 pct for current aluminum-lithium alloys to less than 10 pct for the Air Force alloy. The subsequent Air Force alloy was designated. AF/C-489 with a nominal composition of Al-2.7 wt A.A. CSONTOS, Graduate Student, and E.A. STARKE, Jr., University Professor and Oglesby Professor of Materials Science and Engineering, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. Manuscript submitted September 17, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
pct Cu-2.1 wt pct Li-0.6 wt pct Zn-0.3 wt pct Mn-0.3 wt pct Mg-0.05 wt pct Zr. Unfortunately, the elongation at peak strength was lower than the acceptable level of 5 pct for aerospace applications. As a result of this research, the Air Force and ALCOA (Alcoa Center, PA) developed a derivative of AF/C-489 with 1.8 wt pct Li and 0.09 wt pct Zn, designated AF/C-458, with the nominal composition of Al-2.7 wt pct Cu-1.8 wt pct Li-0.6 wt pct Zn-0.3 wt pct Mn-0.3 wt pct Mg-0.09 wt pct Zr. This AF/C-458 variant possessed much higher elongations, more than double the acceptable limit, while also maintaining similar isotropic mechanical properties. Therefore, the objectives of our investigations were to first identify the mechanisms for the large difference in ductility between the AF/C-489 and AF/C-458 alloys and then to develop an aging schedule to optimize the microstructure for high ductility and strength levels. II. BACKGROUND Low ductility resulting from low energy intergranular fracture in Al-Li-Cu-X alloys has been attributed to a number of factors that include tramp elements, [7,8,9] constituent particles,[10,1
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