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Temperature dependence of microstructure of Al-Ag-Zn alloys

Al-rich Al-Ag-Zn alloys, quenched to RT from a temperature, Tt, higher than the solid-solution temperature, Tss, contained GP zones, as proved by XRD and TEM. GP zones grew during ageing, remaining fcc and coherent with the Al-rich α -phase (fcc). In parallel, metastable precipitates, ε ´ (hcp), were formed by discontinuous nucleation and by a direct transition from GP zones. The unit-cell parameters of ε ´ and of the equilibrium phase, ε (hcp), formed in the alloys slowly cooled from Tt, to RT, depended on the solute content. The quenched and prolongedly aged alloys, being two-phase systems, α + ε ´, were studied in situ at high temperature. The following was observed: an initial lattice strain annealing, an anisotropy of thermal expansion of ε ´, a gradual dissolution of ε ´ in the α -phase, above 500 K, the formation of solid solution. Tss depended on the alloy composition. On cooling, a temperature hysteresis was noticed in reversal changes. The dependence of unit-cell parameters of ε ´ on temperature during cooling was different from that during heating. After the heating and cooling cycle, unit- cell parameters of the precipitates were closer to those of the equilibrium phase, ε .

aluminium-based ternary alloys; X-ray powder diffraction; microstructure; phase transitions

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