Effect of heat treatments on the microstructure of an ultrafine-grained Al-Zn-Mg alloy produced by powder metallurgy

Previous study by Queudet et al. [1] showed that powder metallurgy allows to produce high strength ultrafine-grained (UFG) Al-Zn-Mg alloys which contains both η' and η phases. This present work investigates the effects of solid solution treatment followed by quenching (W temper) and subsequent ageing to obtain theoretical T6 tempered UFG material. TEM analyses revealed the presence of GP zones and metastable η' phase in W tempered UFG material. Compared to as-fabricated material, quenching conferred to the alloy higher maximum stress of 785 MPa and stabilized plastic flow. Further ageing resulted in over-aged alloy containing the η phase, inducing lower hardness and compressive stress. TEM observations showed that variant η2 which grows from η' is delimited by previous GP-II zones.