ON THE NATURE OF PHASE-SEPARATION IN A COMMERCIAL ALUMINUM-LITHIUM ALLOY

The formation of lithium-rich precipitate particles, known as delta' p hase, is responsible for the particularly desirable mechanical propert ies which make aluminium-lithium alloys interesting for different indu strial applications. The structure and the kinetics of the phase separ ation 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 precipi tate and during its successive growth. Modelling of the experimental f ull scattering curve has allowed derivation of several important struc tural parameters, including the composition of the separating phase. I t was found that, even during the early stages of formation, the Li co ntent of the precipitate was similar to 21% Li atoms, similar to that found by other methods at the end of precipitation, and remained const ant throughout. It was concluded that precipitation was initiated by n ucleation rather than by spinodal decomposition.