On the nature of phase separation in a commercial aluminium-lithium alloy

The formation of lithium-rich precipitate particles, known as delta' phase, is responsible for the particularly desirable mechanical properties which make aluminium-lithium alloys interesting for different industrial applications. The structure and the kinetics of the phase separation process are conveniently studied by small-angle X-ray scattering, though uncertainties remain on the actual shape of the phase diagram of the system, particularly in the region of interest. In this paper are reported small-angle X-ray scattering measurements on a commercial AI-Li (8.49% Li atoms) both in the region of formation of the precipitate and during its successive growth. Modelling of the experimental full scattering curve has allowed derivation of several important structural parameters, including the composition of the separating phase. It was found that, even during the early stages of formation, the Li content of the precipitate was similar to 21% Li atoms, similar to that found by other methods at the end of precipitation, and remained constant throughout. It was concluded that precipitation was initiated by nucleation rather than by spinodal decomposition.