  The phase diagram of the Al-Li system was determined by means of first
principles calculations in combination with the cluster expansion formalism and
statistical mechanics. The ground state phases were determined from first
principles calculations of fcc and bcc configurations in the whole
compositional range while the phase transitions as a function of temperature
were ascertained from the thermodynamic grand potential and the Gibbs free
energies of the phases. Overall, the calculated phase diagram was in good
agreement with the currently accepted experimental phase diagram but the
simulations provided new insights that are important to optimize microstructure
of these alloys by means of heat treatments. In particular, the structure of
the potential GP zones, made up of Al0.5Li0.5 (001) monolayers embedded in Al
matrix, was identified. It was found that Al3Li is a stable phase although the
energy barrier for the transformation of Al3Li into AlLi is very small (a few
meV) and can be overcome by thermal vibrations. Moreover, bcc AlLi was found to
be formed by martensitic transformation of fcc configurations and Al3Li
precipitates stand for favorable sites for the nucleation of AlLi because they
contain the basic blocks of such fcc ordering. Finally, polynomial expressions
of the Gibbs free energies of the different phases as a function of temperature
and composition were given, so they can be used in mesoscale simulations of
precipitation in Al-Li alloys.
