The effect of processing and microstructure development on the slip and fracture behavior of the 2.1 wt pct Li AF/C-489 and 1.8 wt pct Li AF/C-458 Al-Li-Cu-X alloys

Although Al-Li-Cu alloys showed initial promise as lightweight structural m aterials, implementation into primary aerospace applications has been hinde red due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air Force recently developed two isotropic Al-Li-Cu-X alloy s with 2.1 wt pct Li and 1.8 wt pct Li designated AF/C-489 and AF/C-458, re spectively. The elongation at peak strength was less than the required 5 pc t for the 2.1 wt pct Li variant but greater than 10 pct for the 1.8 wt pct Li alloy. The objectives of our investigations were to first identify the m echanisms 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 microst ructure for high ductility and strength levels. Duplex and triple aging pra ctices were designed to minimize grain boundary precipitation while encoura ging matrix precipitation of the T-1 (Al2CuLi) strengthening phase. Certain duplex aged conditions for the AF/C-489 alloy showed significant increases in ductility by as much as 85 pct with a small decrease of only 6.5 and 2. 5 pct in yield and ultimate tensile strength, respectively. However, no sig nificant variations were found through either duplex or triple aging practi ces for the AF/C-458 alloy, thus, indicating a very large processing window . Grain size and delta' (Al3Li) volume fraction were determined to be the m ajor cause for the differences in the mechanical properties of the two allo ys.