The effects of various quenching conditions on pre-aging at room temperature and the subsequent 2-step aging behavior of an Al-Mg-Si alloy were investigated mainly by using electrical resistivity and hardness measurements. Three types of quenching method (water-quenching, step-quenching, a first pre-aging treatment at 373K) were used before using a subsequent 2-step aging treatment. The results were explained in terms of the role played by quenched-in excess vacancies (Si-rich clusters, Guinier-Preston zones) in the early stages of precipitation. The results clearly indicated that the quenched-in excess vacancies were trapped within Guinier-Preston zones during the first pre-aging at 373K. This effect suppressed Si-rich cluster formation during the second pre-aging at room temperature, and resulted in a negative effect during final aging at 453K. It was therefore important to control the heat treatment sequence for Al-Mg-Si alloys so that the quenched-in excess vacancies were more associated with Guinier-Preston zones (effective nucleation sites for the β” phase) before the formation of the Si-rich clusters which were ineffective nucleation sites for the β” phase.

Role of Quenched-In Excess Vacancies in Two-Step Aging Behavior of Al-Mg-Si Alloys. K.Yamada, T.Sato: Journal of the Japan Institute of Metals, 2001, 65[10], 897-902

Figure 1

Diffusivity of 60Co in Al72.2Ni11.8Co16.0

(Filled circles: parallel to c-direction, Open circles: perpendicular to c-direction)