Influence of precipitates on the mechanical response and substructure evolution of shock-loaded and quasi-statically deformed Al-Li-Cu alloys

The mechanical response and substructure evolution of two Al-Li-Cu alloys (Al-2.90 wt pct Li-1.00 pct Cu-0.12 pct Zr and Al-2.30 pct Li-2.85 pct Cu-0.12 pct Zr) subjected to shock-loading (strain rate έ> 10⁶ s^-1), Split-Hopkinson-Pressure-Bar compression (έ ~ 5 × 10³ s^-1), and quasi-static compression (έ ~ 1.5 × 10^-3 s^-1) were examined. The strain levels achieved in these three deformation paths were desined to be comparable, i.e., all ∼15 pct. Both alloys were either naturally or artificially aged to yield an underaged or overaged condition. Various precipitates, such as the δ' and T ₁ phase, of different sizes and volume fractions were dispersed in the matrix and at the grain boundaries. The substructure in all of the shock-loaded, Split-Hopkinson-Pressure-Bar, and quasi-static compression samples was characterized by localized slip bands and microbands with the exception of the overaged alloys. The density of dislocations and dislocation loops was higher, independent of the aging condition, in the shock-loaded specimens. Well-defined cell structures were not observed in any of the samples, independent of strain rate. The influence of precipitates, shearable or not, on the substructure development in Al-Li-Cu alloys during shock-loading was seen to be pronounced, even though the size and volume fraction of precipitates was small and low, respectively. Flow stress measurements showed that the shock-loaded samples have flow strengths 3 to 8 pct higher than the quasi-statically deformed samples. This small, but reproducible, strength increment, for alloys deformed to equivalent strains at low and high rates, indicates that the Al-Li alloys studied have a small rate sensitivity. Based upon comparison of the results of the shock-loaded and quasi-static samples, it is concluded that the fundamental deformation mechanisms and substructure evolution in all three loading paths are not drastically different, corroborating previous investigations.