Precipitation and dissolution phenomena in Al-Zn alloys

The microstructure of Al-Zn alloys, with the Zn atom fraction, x(Zn) less than or equal to 62%, has been studied in detail by XRD in dependence on the composition, temperature and previous thermal treatment. Precipitation phenomena in alloys, transferred to the supersaturated state by rapid quenching from the solid solution temperature, T-SS, in water at RT, have been followed in dependence on the ageing time at RT or elevated temperature. The equilibrium state, reached by ageing, has been compared with that obtained by slow cooling from T-SS to RT. Phase transitions, dissolution and precipitation processes in the alloys, transferred to the equilibrium state, have been studied in situ in dependence on temperature from RT to T-SS. A temperature hysteresis in reversal phase transitions has been observed on cooling from T-SS to RT. Repeated heating and cooling cycles have revealed different microstructure in alloys having undergone different thermal treatments between RT and T-SS. A change in the phase diagram of the Al-Zn system is necessary in the composition region x(Zn) > 50%.