Natural Aging Effect of Al-20Zn-3Cu Alloy on Mechanical Properties and Its Relation to Microstructural Change

We investigate the effect of the natural age-hardening response of the Al-20Zn-3Cu alloy with natural aging times up to 12 months. The ultimate tensile strength of the Al-20Zn-3Cu alloy is drastically enhanced from 308 to 320 MPa after 2 months and from 320 to 346 MPa after 9 months. Then, natural age hardening becomes saturated after 9 months. A microstructural investigation reveals that the natural age-hardening mechanism is mainly induced by the diffusion of the Zn element. First, a rapid decrease in the volume fraction of the eutectoid lamellae (α-Al+η-Zn) is observed at the early stage of natural aging, leading to an increase in the tensile strength. This originates from the relatively high diffusivity of Zn due to its low melting temperature. Then, the diffusion of Zn into the Al matrix induces clusters of solute atoms that enhance the growth rate of the nanoprecipitates formed in the Al matrix. As a consequence, the tensile strength of the natural-aged Al-20Zn-3Cu alloy increases drastically after 9 months, whereas the ductility is significantly degraded.