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)  → ε′ (hcp) → ε (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 α-phase (fcc). In parallel, metastable precipitates, ε′, were formed; their unit-cell parameters depended on the solute content. Two mechanisms of ε′ nucleation were suggested on the basis of composite diffraction-line profiles, namely, discontinuous nucleation and a direct GP zones to ε′ transition. The unit-cell parameters of the equilibrium phase, ε, 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 (α +ε′ ), were studied in situ  at high temperature. As the temperature increased, an initial increase of diffraction-line intensities of both α and ε′ 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 ε′ was noticed. Above 500 K a gradual dissolution of ε′ in the  α-phase  started, as manifested in an enchanced 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 the unit-cell parameters of ε′ 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 ε-phase, while diffraction-line profiles were not composite any more.  

S. Popovic and D.E. Passoja, J. Appl. Cryst. 4 (1971) 427-434; S. Popovic, Cryst. Res. Technol. 19 (1984) 1351-1358; S. Popovic, Z. Skoko and G. Stefanic, Acta Chim. Slovenica ( submitted).

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