Thermal stability of nanocrystalline surface layer in an aged Al–Zn–Mg–Cu alloy induced by ultrasonic surface rolling processing

This paper investigates the thermal stability of nanocrystalline surface layer in an aged Al–Zn–Mg–Cu alloy fabricated by ultrasonic surface rolling process (USRP). The microstructure of the nanocrystalline surface layer and the microstructure changes of grains and precipitates in the nanocrystalline surface layer after annealing at various temperatures were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The microhardness after annealing was also analyzed. The results show that a nanostructured lamella with a thickness of about 40 nm was prepared in the surface layer of the aged Al–Zn–Mg–Cu alloy after USRP. The granulation of the nanostructured lamellae occurs during subsequent annealing due to recovery and becomes more significant with the increase of annealing temperature. Besides, the coarsening η phases occur at the lamellae boundaries by heterogeneous nucleation. Meanwhile, the precipitate inside lamellae gradually changes from GP zone and η' phase to η phase. The thermal stability of the nanostructured lamellae is excellent below 160 °C, but it deteriorates significantly when the temperature exceeds 200 °C. The hardness variation of the nanostructured lamellae is mainly determined by the variation of lamellae thickness and the precipitation behavior plays an auxiliary role.