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, T(t) (820 K), higher than the solid-solution temperature, T(ss), was found to be: GP zones (fcc) -> epsilon' (hcp) -> epsilon (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 alpha-phase (fcc). In parallel, metastable precipitates, epsilon', were formed; their unit-cell parameters depended on the solute content. Two mechanisms of epsilon' nucleation were suggested on the basis of composite diffraction-line profiles, discontinuous nucleation and a direct GP zones to epsilon' transition. The unit-cell parameters of the equilibrium phase, epsilon, observed in the alloys slowly cooled from T(t) to RT, depended on the solute content. The alloys, that had been quenched from T(t) to RT, aged at 420 K for 50 days and then prolongedly aged at RT, being two-phase system (alpha+epsilon'), were studied in situ at high temperature. As the temperature increased, an initial increase of diffraction line intensities of both alpha and epsilon' 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 epsilon' was noticed. Above approximate to 500 K a gradual dissolution of epsilon' in the matrix (M, alpha-phase) started, as manifested in an enhanced decrease of diffraction-line intensities. Finally, solid solution was formed; T(ss) 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 epsilon' 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 epsilon-phase, while diffraction-line profiles were not composite any more.