Effect of small Sn addition on the initial strengthening and microstructural evolution of an Al-Cu-Mg-Ag alloy

Abstract

The influence of small Sn addition on the mechanical properties and microstructure of an Al-Cu-Mg-Ag alloy is systematically investigated in this work. The age-hardening level of the material at 180 ℃, as well as the strength properties during exposing at 200 ℃, is found to decrease with small Sn addition. With Sn, the morphology of the tensile fracture surface gradually evolves from a transgranular mode to an intergranular one during the prolonged exposing at 200 ℃. Quantitative transmission electron microscopy (TEM) analysis proposes the negative role of Sn on the dense precipitation of Ω phase by accelerating the θ' formation, thereby leading to a loss of strength properties and the lower thermal stability of Sn-added alloy. This detrimental effect by Sn, as revealed by atom probe tomography (APT), is primarily ascribed to the suppressed Mg-Ag co-clustering, which reduces the nucleation rate of Ω phase. Our calculation also indicates that a reduction of the cluster hardening by Sn is mainly responsible for the observed loss in the strength of underaged alloy. The contributions of modulus hardening and order hardening to the cluster hardening are also quantified for each cluster. A similar recrystallized grain size in Sn-free and Sn-added alloys eliminates the grain refinement effect by Sn.