THE METALLURGY OF INDUSTRIAL Al-Li ALLOYS

Abstract
All industrial unrecrystallized Al-Li-X alloys exhibit a strong tendency to intergranular failure leading to a slaty fracture of the specimens, associating transgranular shear and intergranular delamination in L-LT planes. The level of alkali impurities measured in industrial metal quality cannot explain this fracture behaviour. In the opposite, the studies conducted on bending specimens indicate that the slip heterogeneity within the grains induce the delamination through a severe and detrimental shear localization at grain boundaries. This behaviour is still possible in strongly aged tempers because of the heterogeneity of the precipitation. In order to avoid or reduce the stress localization at grain boundaries, increasing the homogeneity of deformation has become the guideline for the metallurgy of industrial Al-Li-X alloys, illustrated by the following three examples. An efficient means to reduce or even suppress slatiness consists in underageing alloys like 2091 or CP 276 which offer a high potential of matrix coprecipitation. These underaged tempers maximize short transverse properties as well as the reslstance against exfoliation corrosion. In addition, toughness and crack growth resistance reach higher levels. Recrystallization is another way to induce better microscopic slip homogenization. In that case, the slaty fracture is no more possible. The best combination of properties for damage tolerant alloys (2024 T3 replacement) is obtained when associating the recrystallized struçture and underageing. This applies well to 2091 sheets where the choice of the T8X temper ensures a high and stable level of toughness, On profiles, which are characterized by an unrecrystallized structure, the sensitivity to texture of underaged or peakaged tempers may be too high for some alloys. A slight overageing may reduce the microscopic tendency to slip localization. Though the delamination is not totally suppressed, this kind of treatment applied to 8090 extrusions leads to more homogeneous properties which can be combined with improved resistance towards exfoliation corrosion for medium strength alloys replacement (2214 T6 - 7XXX T73).