Chapter 3 - Phase Diagrams and Phase Reactions in Al–Li Alloys

Abstract

The formation of metastable and equilibrium phases in binary Al–Li, ternary Al–Li–Mg and Al–Li–Cu, and quaternary Al–Cu–Li–Mg alloys has been studied by using a variety of experimental techniques including differential scanning calorimetry, electrical resistivity, X-ray diffraction, conventional and high-resolution electron microscopy and 3D atom probe measurements. Al3Li (δ′) is the strengthening phase in binary Al–Li and ternary Al–Li–Mg alloys. Mg reduces the solubility of Li in Al and also substitutes for Li in δ′. The characteristics of θ′ (and θ) and T1 phases in Al–Li–Cu alloys and the composition limits where these phases are formed are well understood. For low Li contents (<0.6%), θ′(θ) is formed, while for medium Li content (<1.4–1.5%) the main strengthening phase is T1. δ′ precipitates during artificial aging in high Li alloys (>1.4–1.5%). Formation of T1 is promoted by small additions of Ag and Mg and by cold work prior to artificial aging. Zr forms the metastable β′ (Al3Zr) phase, which has an appreciable effect on retarding recrystallization besides providing nucleation sites for composite δ′ particles. Sc and Yb additions behave in a similar way; the added advantage is improved creep strength. The available information from phase equilibria studies of Al–Li–Cu–Mg alloys is somewhat limited, but sufficient to give an indication of the desirable solution treatment and aging temperatures and the phases formed at these temperatures. 3D atom probe studies suggest the involvement of Mg atoms in the formation of clusters which lead to the formation of the T1 phase, during artificial ageing of aging of quenched Al–Cu–Mg–Ag alloys. All these aspects are covered in detail, with specific reference to different commercial and semi-commercial Al–Li alloys, wherever possible.