Time dependent electrical resistivity and magnetization of naturally aged Al-Mg-Si alloys

Early stage natural aging behaviors of Al-1.0%Mg₂Si-0.35%Cu and Al-1.0%Mg₂Si-0.35%Ag alloys were investigated by magnetization and electrical resistivity measurements at a constant temperature between 250 and 320 K. The electrical resistivity of Al-1.6%Mg₂Si was also measured for the comparison. Time dependent magnetization curves for the samples show a rapid increase within the first 24 h. The observed transition time from the clustering stage 1 to the clustering stage 2, which was estimated from the rapid increase point of magnetization and electrical resistivity, has been found to be delayed by adding Cu or Ag to Al-Mg-Si alloys. This phenomenon is explained by the cluster formation kinetics that the added Cu or Ag strongly trapped quenched-in excess vacancies and thus retarded diffusions of the solute elements of Mg and Si. Activation energies for the cluster formation were evaluated by Arrhenius plots of the transition times against measuring temperatures. The effect of Cu or Ag addition on the formation of clusters is discussed in terms of activation energies.