Enhanced and accelerated age hardening response of Al-5.2Mg-0.45Cu (wt%) alloy with Zn addition

The age hardening responses of the Al-5.2Mg-0.45Cu alloy with Zn addition have been investigated in this study. It is found that the age hardening process of the Zn-containing Al-5.2Mg-0.45Cu alloys can be divided into four stages, namely the initial jump(Ⅰ), the linear increase (Ⅱ), the accelerated increase to the peak (Ⅲ) and the overage (Ⅳ). The addition of Zn to the Al-5.2Mg-0.45Cu alloy leads to an enhanced and accelerated age-hardening response in two aspects: firstly introducing the T-[Mg32(Al, Zn)49] phase resulting from the high Mg/Zn ratio and relatively high aging temperature; secondly stimulating the precipitation of the Guinier-Preston-Bagaryatsky (GPB) zones by increasing the supersaturation upon quenching, and accelerating the precipitation of S-Al2MgCu phase by increasing Cu/Mg ratio in the matrix due to the formation of the T phase. The peak hardness is from the synergetic effects of the S and T phases. The clustering and partitioning behaviors of solutes after 1 h aging treatment have been investigated in detail using atom probe tomography (APT) and high resolution transmission electron microscopy (HRTEM). It is shown that the formation of Mg-Cu clusters (precursor of the S phase) precedes that of Mg-Zn clusters (precursor of T phase), and the Mg-Cu clusters are responsible for the age hardening during the first two age hardening stages in the Cu-bearing alloys.