Effect of prior cold working on microstructural evolution of precipitation in Zn-22Al-2du alloy during aging

The microstructural evolution of precipitation was characterised by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and microhardness measurements during two different types of isothermal aging in a Zn-22Al-2Cu alloy. Solution treated samples were quenched and rolled at room temperature and then aged at 250°C for different times. The X-ray results showed that the decomposition of the β_sss phase and the formation τ′ phase, during the aging treatment occurred by the following reactions β_sss→α+η+ε and α+ε→η+τ′ respectively. Similar phase transformations were observed in the sample solution treated, quenched and then cold worked before aging condition, with a faster kinetics. Besides, the microstructure of the latter samples consists of equiaxial grains of the τ′, α and η phases. In contrast, the former sample showed a lamellar structure of the α and η phases. This difference of microstructure caused a slightly higher hardness in the samples without cold working. Furthermore, the aging process caused a decrease in hardness with aging time, which seems to be related to the coarsening process of the equilibrium phases for both heat treatments.