Precipitation and dissolution phenomena in Al-Zn alloys

The microstructure of Al-Zn alloys, with the Zn atom fraction, x(Zn) less t han or equal to 62%, has been studied in detail by XRD in dependence on the composition, temperature and previous thermal treatment. Precipitation phe nomena in alloys, transferred to the supersaturated state by rapid quenchin g from the solid solution temperature, T-SS, in water at RT, have been foll owed in dependence on the ageing time at RT or elevated temperature. The eq uilibrium state, reached by ageing, has been compared with that obtained by slow cooling from T-SS to RT. Phase transitions, dissolution and precipita tion processes in the alloys, transferred to the equilibrium state, have be en studied in situ in dependence on temperature from RT to T-SS. A temperat ure hysteresis in reversal phase transitions has been observed on cooling f rom T-SS to RT. Repeated heating and cooling cycles have revealed different microstructure in alloys having undergone different thermal treatments bet ween RT and T-SS. A change in the phase diagram of the Al-Zn system is nece ssary in the composition region x(Zn) > 50%.