FIRST-PRINCIPLES CALCULATION OF THE PRESSURE-DEPENDENCE OF PHASE-EQUILIBRIA IN THE AL-LI SYSTEM

The solid-phase portion of the AI-Li phase diagram has been computed f rom first principles both at zero pressure and at a hydrostatic compre ssion of 5.4 GPa. Computation of the pressure dependence of the AI-Li phase equilibria answers two questions: (1) how important is the effec t of the atomic size difference, and (2) is the stability of the Al3Li precipitates influenced by high hydrostatic stress. The zero-pressure first-principles phase diagram exhibits excellent qualitative agreeme nt with experimental data. The presence or absence of solid solutions (SS), of stable and metastable intermetallic phases, and their degree of order are computed correctly. Compression is predicted to affect th e phase equilibria in Al-Li as follows: (1) the solubility of Li in fe e Al-rich SS is decreased, (2) the solubility of Al in Li is increased . However, the low melting point of Li limits the range of SS, and (3) the metastable Al3Li Al-rich fcc SS phase equilibrium is unaffected a nd the stability of the precipitates is unchanged, (4) the ordering te ndencies at Li-rich compositions are slightly enhanced. Although high pressure eliminates the difference in atomic volume of the pure consti tuents, it has almost no effect on the solid-solid phase equilibria in this alloy system. A simple method for verifying the accuracy of the cluster expansion for the configurational internal energy is presented and applied. Moreover, it has been shown that with a convenient choic e of the occupation numbers, one can define correlation functions whic h greatly facilitate the determination of new ground state structures.