Effects of Sn addition on precipitation of a pre-naturally aged Al-Zn-Mg alloy during artificial aging

The effect of Sn addition and pre-natural aging on the artificial aging behavior of Al-Zn-Mg alloy was investigated using both experimental approaches and first principle study. The result shows that Sn retards hardness increase at the initial stage of artificial aging by inhibiting the formation of Guinier-Preston zones (I). Atom probe tomography and density functional theory have revealed that the high binding energy of Mg-Sn disturbs Mg-Zn clustering and delays the formation of Guinier-Preston zones (I). However, the retardation effect of Sn on the artificial aging was found to be suppressed when the pre-natural aging was applied to the Al-Zn-Mg-Sn alloy: hardness at the initial stage increased rapidly compared to the Al-Zn-Mg alloy with pre-natural aging. The reason for the rapid increase of hardness in the Al-Zn-Mg-Sn alloy with pre-natural aging is that Guinier-Preston zones (II) was directly formed from vacancy-rich clusters in the Al-Zn-Mg-Sn alloy in contrast to the Al-Zn-Mg alloy with pre-natural aging. This means Mg-Sn and Zn-rich cluster formed during pre-natural aging can accelerate precipitation kinetic by providing nucleation site for Guinier-Preston zones (II) during artificial aging.