On the role of Ag additions on the initial solute hardening and competitive precipitation of Al-Cu-Mg alloys

Abstract

The initial solute clustering is known to significantly affect the subsequent precipitation of various strengthening precipitates that determine the mechanical properties of Al-Cu-Mg-Ag alloys. In this work, the influence of Ag variations on the initial cluster hardening and microstructure evolution of Al-Cu-Mg-Ag alloys is investigated by quantitative transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and atom probe tomography (APT) analysis. Our statistical data and calculation results indicate that the overwhelming Mg-Ag co-clustering by increasing Ag from 0.41 to 0.97 indeed enables a higher nucleation rate of Ω phase and contributes to a denser Ω precipitation generating a stronger age-hardening response at 180 ℃. Analysis of the contribution of individual precipitates to the yield strength highlights the positive role of increasing Ag on improving the cluster hardening capacity of Mg-Ag co-clusters by increasing its order hardening level. Compared with Cu-Mg co-clusters and GP zones, Mg-Ag co-clusters exhibit a greatest strengthening contribution to yield strength of the material as the bulk Ag content increases. Except for GP zones, Mg-Ag and Cu-Mg co-clusters prefer to exhibit a strong order hardening rather than modulus hardening in initially aged alloys. In addition, the average grain sizes, as well as the proportions of low angle grain boundaries (LAGBs) and high angle grain boundaries (HAGBs), reveal no distinct differences in both alloys, suggesting the independence of grain structure on the bulk Ag content.