| The effects of precipitation heat treatment on the microstructure, toughness, and stress corrosion crack propagation resistance of aluminum alloy 2020 were investigated. Precipitation strengthening was found to be due to extensive co-precipitation of the (theta)' (Al(,2)Cu) and T(,1) (Al(,2)CuLi) phases, confirming the results of several previous studies. Although some evidence of (delta)' precipitation was also found, no significant contribution to strengthening was attributed to this phase.; Fracture toughness was found to decrease markedly with aging to peak strength due to an increasing tendency toward strain localization by the concentration of deformation into intense planar bands and the concomitant decrease in strain hardening capacity. The strength/toughness relationship for several high strength underaged tempers of 2020 was found to be superior to that for the peak strength temper and for standard tempers of other 2XXX alloys. One underaged temper with a yield strength 12 percent lower than the peak aged temper had 80 percent higher plane strain fracture toughness and 300 percent higher crack propagation resistance. The toughness of overaged tempers was found to be much lower than that of underaged tempers having the same yield strength.; The stress corrosion cracking (SCC) resistance of 2020 in the peak strength temper and several high strength underaged tempers was found to be excellent. Poor SCC resistance was observed for a very underaged temper. A model was proposed in which the variation in the stress corrosion resistance of 2020 with precipitation heat treatment is controlled by the electrochemical potential difference between grain boundary T(,1) precipitates and the interior of grains, which provides a driving force for the preferential dissolution of these precipitates. The SCC resistance of high strength underaged tempers is very high because this potential difference is minimized at a relatively early stage of aging. |
