Effect of predeformation and preaging at room temperature in Al–Zn–Mg–(Cu,Zr) alloys

The effect on yield stress of predeformation and natural aging prior to a two-stage artificial hardening treatment has been investigated for the 7xxx series alloys AA7108 and AA7030. It was found that 10% predeformation in tension reduced the yield stress measured in the T6-state by 7–10%. About half of this reduction could be regained when a preaging period was inserted between the deformation and the artificial aging treatment. The natural aging response was followed also by measurement of electrical conductivity and hardness. Precipitate phases and GP (Guinier–Preston)-zones occurring during the aging treatments were investigated by transmission electron microscopy. The lower yield stress in predeformed samples is explained by early nucleation of the equilibrium phase η on the dislocation network, leaving less solute for formation of the main hardening phase η′. The reduction in yield stress was partly offset by the preaging, due to recovery processes in the dislocation network.