High fuel costs have forced the aerospace industry to seek new, lighter weight structural materials. Because aluminum alloys constitute a large portion of the weight of most aircraft, ways of increasing their strength-to-weight ratios have received considerable attention. Alloys containing Li have been of interest because they show a 6% increase in Young's modulus and a 3% decrease in density for each w/o Li added, as compared to the Li-free alloy (Sankaran and Grant, 1981). The primary strengthening mechanism in aluminum-lithium alloys is precipitation hardening, caused by the formation of the metastable delta'(Al3Li) phase during heat treatment (Williams and Howell, 1986). This phase, which has ordered L12 superlattice structure and a misfit of only -0.08% with the face-centered cubic matrix, precipitates homogeneously as coherent, spherical particles.
However, aluminum-lithium alloys exhibit poor ductility and fracture toughness (Sanders, 1981). The causes have not been clearly established (Doherty and Vasudevan, 1986, and Williams and Howell, 1986), but there appear to be three factors involved. One is that the shearable nature of the delta' precipitates leads to stress concentration effects. Also, another lithium-containing phase, delta(AlLi), forms large precipitates on the grain boundaries which are believed to lead to void nucleation during deformation. In addition, the precipitation of the delta phase causes a detrimental delta' precipitation-free zone.
Extensive efforts are presently being made to enable these materials to reach their full commercial potential. The approach that has been taken is to add additional alloying elements (Sanders and Starke, 1984). One group of alloying elements, which includes manganese and zirconium, forms dispersoids that control recrystallization. Other alloying elements, such as copper, magnesium and zirconium, promote solid solution strengthening and precipitation hardening. Because no single element appears capable of eliminating all of the problems with the mechanical properties, the effects of multiple additions are being investigated. The Al-Li-Cu-Mg-Zr system is presently receiving the most attention. Before such a complex alloy system can be fully developed, the characteristics of the binary, ternary and quaternary subsystems must be known. The present study was conducted to further this understanding by structurally characterizing one of the ternary phases in the Al-Cu-Li system, T2(Al6CuLi3).
Some texts on mechanical metallurgy and metal forming:
Dieter: Mechanical Metallurgy
Mielnik: Metalworking Science and Engineering