Microstructure of Al-Zn and Zn-Al Alloys

The change of microstructure of the title alloys with concentration, temperature and applied thermal treatment was studied in situ by XRD. The alloys, having the Zn atomic fraction, x(Zn), from 0.03 to 0.62, were subjected to: (i) rapid quenching from a temperature, T-t higher than the solid-solution temperature, T-ss, in water at RT (samples WQ); (ii) slow cooling from T-t to RT (samples SC). The WQ's were solid solutions immediately after quenching, up to x(Zn) <= 0.44. For short ageing time, the WQ's contained GP zones, rich in Zn; by a prolonged ageing the WQ's were transformed to a quasi-equilibrium state, containing beta precipitates, very rich in Zn. The SC's, also containing P precipitates, were closer to the equilibrium state than the aged WQ's, the microstructure of the latter depended on residual strains, quenched-in vacancies and a non-uniform distribution of beta precipitates. Both SC's and prolongedly aged WQ's were slowly heated from RT to T-t and cooled back to RT. Several phenomena were observed in the heating run: a decrease of diffraction line intensities due to enhanced atom vibrations, anisotropy of thermal expansion, change in the precipitate shape, partial or complete dissolution of precipitates, phase transitions, formation of solid solution. In the cooling run, the alloys exhibited a temperature hysteresis in reversal phase transitions. The temperature dependence of microstructure for the SC's was different from that of prolongedly aged WQ's. The sequence of phase transitions, found for the alloys with x(Zn) >= 0.44, was not in line with the phase diagram of the Al-Zn system, accepted in literature.