Thermodynamic description of the Al–Li–Zn system

Abstract

The Al–Li–Zn system was critically assessed using the CALPHAD technique. The solution phases (liquid, bcc, fcc and hcp) were described by the substitutional solution model. The compounds Al 2Li 3 and Al 4Li 9 in the Al–Li system had homogeneity ranges of Zn and were treated as (Al,Zn) 2Li 3 and (Al,Zn) 4Li 9 in the Al–Li–Zn system, respectively. The compounds αLi 2Zn 3, βLi 2Zn 3, αLi 2Zn 5, βLi 2Zn 5 and αLiZn 4 in the Li–Zn system had no solubility of the third component Al in the Al–Li–Zn system. A two-sublattice model (Al,Li,Zn) 0.2(Al,Li,Zn) 0.8 was applied to describe the compound βLiZn 4 in the Al–Li–Zn system in order to cope with the order–disorder transition between hexagonal close-packed solution (hcp-A3) and βLiZn 4 with the Mg-type structure. The ternary compound τ 2 with a NaTl-type structure (B32) had the same structure with the compounds AlLi in the binary Al–Li system and LiZn in the binary Li–Zn system. In the present work, the three compounds AlLi, LiZn and τ 2 were treated as one phase by a two-sublattice model (Al,Li,Zn) 0.5(Al,Li,Zn) 0.5 in order to cope with the order–disorder transition between B32(AlLi, LiZn and τ 2 ) and body-centered cubic solid solution (bcc-A2). The ternary intermetallic compounds τ 1 and τ 3 in the Al–Li–Zn system were treated as the formula Li(Al,Zn) 2 and (AlLi,Zn)Zn 3, respectively. A set of self-consistent thermodynamic parameters describing the Gibbs energy of each individual phase as a function of composition and temperature in the Al–Li–Zn system was obtained.