This study centered on the effects of artificial aging (AA) on the precipitation kinetics and strengthening in the new-generation aluminum-lithium alloy AF/C-458. This alloy has been currently developed as a lightweight structural material for aerospace applications. Samples were solution heat treated (SHT) at 500°C / 2 hours and quenched in water, followed by aging at 150 and 200°C for times ranging from 5 minutes to 144 hours. Quenching of the solution treated samples were done in water under different conditions such as addition of 5 wt. %weight NaCl and varying temperatures, but were found to be insignificant. Strengthening effects were measured in terms of the hardness changes. For samples aged at 200oC, peak hardness of 140VHN (Vicker's Hardness Number) was observed at AA 45 minutes as compared to the original hardness of 81VHN (solution treated sample). The variation of hardness with time at aging temperature of 200oC is shown below along with the observed microstructures. Microstructural observations were made using a TEM (Transmission Electron Microscope). As solutionized sample having the minimum hardness does not show any presence of precipitates. The sample having peak hardness revealed fine circular / hexagonal-precipitates, the composition and structure of which is currently under study. Further aging of this alloy, dropped the hardness significantly indicating the dissolution of these circular / hexagonal precipitates. This is followed by the appearance of some new type of coarser precipitates, which probably do not efficiently contribute to strengthening of this alloy. Studies are underway to characterize the different kinds of precipitates observed and the effect of their distribution on the mechanical properties of this alloy.

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