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, several structural models have been proposed for the S′ phase, and this controversy is not yet resolved. New x-ray powder diffraction measurements have been performed to compare the theoretical powder diffraction patterns associated with each proposed model with the measured spectra obtained from two different kinds of alloys. The structural model of the S′ phase described by Mondolfo [L. F. Mondolfo, Aluminum Alloys Structure and Properties (Butterworths, London, 1976)] agrees with the experimental results and allows us to reject the two other proposed crystallographic models.