Aluminum–lithium alloys

8.7.3 Aluminum–lithium alloys

The advantages of aluminum–lithium alloys have been known for a long time, but lower density and increased elastic stiffness were offset by poor ductility and fracture performance. Basic Al–Li alloys

4 Alloy treatment is usually described by a suffix letter and digit system (e.g. F – as fabricated, O – annealed, H – work-hardening and T – heat treated). Digits following H specify the work-hardened condition, and that fol-

lowing T the type of ageing treatment (e.g. T6 is solution heat treated and artificially aged, T4 solution heat treated and naturally aged, T3 solution heat treated and cold worked).

5 Launched in the USA in 1992.

476 Physical Metallurgy and Advanced Materials Table 8.9 Composition of commercial aluminum–lithium alloys.

Alloy

Fe Si Zr (8090) Lital A

0.12 (9091) Lital B

precipitate the (Al 3 Li)δ ′ , a spherical ordered precipitate. Precipitation hardening leads, however, to localized deformation with limited cross-slip and poor fracture behavior. Additions of copper to the alloy so that the Li/Cu ratio is high leads to the formation of both δ ′ and a T 1 -phase (Al 2 CuLi). This gives some improvement in fracture toughness by independent control of the two precipitates. In the quaternary system Al–Li–Cu–Mg the S-phase precipitates in addition to the δ ′ and T 1 . The S-phase is better at dispersing slip than T 1 and with adjustment of composition can be made to dominate the structure. Both S and T 1 are nucleated heterogeneously on dislocations and the best results are obtained by cold working the alloy after solution treatment. Commercial alloys based on this background are Lital A, B and C, which have been developed to match the (1) conventional medium-strength 2014-T6, (2) high-strength 7075-T6 and (3) damage- tolerant 2024-T3 alloys, with a 10% reduction in density and 10% improvement in stiffness (see

Table 8.9). Lital A in T6 sheet form typically has 365 MN m −2 0.2% proof stress, 465 MN m −2 TS, 6% elongation, 66 MN m −3/2 fracture toughness, an elastic modulus of 80 GN m −2 and density of 2550 kg m −3 . Lital B has roughly 10% improvement in strength. Lital C is a variant of the 8090 alloy and is heat treated to increase toughness ( ∼76 MN m −3/2 ) at the expense of strength (TS ∼440 MN m −2 ).

Lithium additions are also being made to conventional aluminum alloys. The addition of lithium has

a major influence since Li possesses a significant vacancy binding energy of about 0.25 eV. Lithium atoms therefore trap vacancies and form Li–V aggregates. This decreases the concentration of mobile vacancies available for the transport of zone-forming atoms, and therefore inhibits the diffusion of Zn and Mg in 7075, and Si and Mg in 6061, into zones. Second, the Li–V aggregates, very probably present during quenching and immediately after ageing, act as heterogeneous sites for subsequent clustering of zone-forming atoms during ageing.

Additions of Li into either Al–2Mg–0.6Si–0.3Cu–0.3Cr (6061) or Al–5.9Zn–2.4Mg–1.7Cu (7075) modify the precipitation scheme and age-hardening behavior of the original alloys. The precipitates which form in the base alloys are inhibited or even suppressed. For the 6061 the addition of 0.7% Li retards the precipitation of needle-shaped GP zones and produces a ternary compound AlLiSi, whereas the addition of 2.0% Li results in the dominant precipitation of δ ′ and extremely delayed and limited formation of needle-shaped GP zones and AlLiSi. For 7075 the addition of 0.7% Li alters the

conventional precipitation scheme from solute-rich GP zone →η ′ → ηMgZn 2 into vacancy-rich GP zone →T ′ → T(AlZn) 49 Mg 32 , whereas the addition of 2.0% Li produces the dominant δ ′ precipitate and limited and delayed formation of T-phase. As a result, the age-hardening response relating to these major hardening phases in both base alloys is delayed or decreased. Such additions can produce narrower PFZs and give improved fracture properties.

A further commercial alloy is UL40, which is essentially a binary alloy containing 4% Li. The alloy is cast using a spray-deposition process resulting in a fine-grained microstructure, with uni- form distribution of second phase, free from oxide. The high Li content alloy has a very low density (2400 kg m −3 ) and is almost a third lighter than conventional aluminum and magnesium alloys. It extrudes well and can be welded with Al–Mg–Zr filler, producing components for air- craft and helicopters, such as pump housings and valves, and for yachting with good corrosion resistance.

Advanced alloys 477