CRYSTALLOGRAPHIC STRUCTURE OF S' PRECIPITATES IN AL-LI-CU-MG ALLOYS

The crystallographic structures of some phases present as precipitates in Al-Li-Cu-Mg alloys are not perfectly resolved. In particular, seve ral structural models have been proposed for the S' phase, and this co ntroversy is not yet resolved. New x-ray powder diffraction measuremen ts have been performed to compare the theoretical powder diffraction p atterns associated with each proposed model with the measured spectra obtained from two different kinds of alloys, The structural model of t he S' phase described by Mondolfo [L. F. Mondolfo, Aluminum Alloys Str ucture and Properties (Butterworths, London, 1976)] agrees with the ex perimental results and allows us to reject the two other proposed crys tallographic models.