Etude de la lacune de miscibilité métastable et de la structure cristallographique des zones G.P. dans les alliages aluminum–argent

Measurements made by small-angle X-ray scattering on Al–Ag alloys aged at various temperatures have shown that the asymptotic Porod's law was obeyed except for a constant term. This additional term represents a Laue monotonic intensity, due mainly to atomic concentration fluctuations inside G.P. zones. The integrated intensity Q₀ of the G.P. zones themselves can then be calculated, and the boundaries of the metastable miscibility gap estimated with more precision than previously. Furthermore, the integrated intensities in the first reciprocal-space shell make it possible to discuss the crystallographic structure of the G.P. zones. After quenching from 550 to −80°C, the measured intensities agree with the theoretical values, which implies that small G.P. zones ′ are disordered. However, for aging above room temperature, there is no agreement between measured and theoretical values. When the aging temperature is increased from 140 to 190°C, an evolution towards a complex structure of G.P. zones is shown.