Microstructure of Al–Ag–Zn Alloys

The precipitation sequence in ternary aluminium rich Al–Ag–Zn alloys, after rapid quenching to RT from a temperature, Tt (820 K), higher than the solid-solution temperature, Tss, was found to be: GP zones (fcc) →´ (hcp) →(hcp). The as-quenched alloys contained GP zones having 3 to 4 nm in diameter as observed by XRD and TEM. During ageing of the quenched alloys at 420 K GP zones increased in size, remaining fcc and coherent with the alfa-phase (fcc). In parallel, metastable precipitates, ´, were formed; their unit-cell parameters depended on the solute content. Two mechanisms of ´ nucleation were suggested on the basis of composite diffraction-line profiles, discontinuous nucleation and a direct GP zones to ´ transition. The unit-cell parameters of the equilibrium phase, , observed in the alloys slowly cooled from Tt to RT, depended on the solute content. The alloys, that had been quenched from Tt to RT, aged at 420 K for 50 days and then prolongedly aged at RT, being two-phase system (+ ´), were studied in situ at high temperature. As the temperature increased, an initial increase of diffraction line intensities of both and ´ phases was observed, due to lattice strain annealing. A shift of diffraction lines due to thermal expansion took place. A small anisotropy of thermal expansion of ´ was noticed. Above ≈500 K a gradual dissolution of ´ in the matrix (M, -phase) started, as manifested in an enhanced decrease of diffraction-line intensities. Finally, solid solution was formed; Tss depended on the alloy composition. On cooling, the alloys underwent reversal changes, exhibiting a temperature hysteresis (10 to 20 K). The dependence of unit-cell parameters of ´ on temperature during cooling was little different from that on heating. At RT, after a complete heating and cooling cycle, unit-cell parameters of the precipitates were close to those of the equilibrium -phase, while diffraction-line profiles were not composite any more.