Nanocluster Formation and Two-Step Aging Behavior of Rapid Hardening Al-Mg-Cu(-Ag) Alloys

Abstract

The characteristic age-hardening response of Al-3.0Mg-1.0Cu (mass%) alloys with and without Ag addition has been investigated by the hardness measurement, differential scanning calorimetry (DSC) and electrical resistivity measurement. The alloy compositions locating in the (α+S+T) phase field of the Al-Mg-Cu phase diagram are known to be effective to harden in two stages separated by a distinct and often prolonged hardness plateau. The first stage of hardening occurs very rapidly (e.g. within 60 s at 443 K) and contributes to increase hardness as much as 50 % of the total age-hardening. In the Ag-added alloy, the hardness change during the first stage of hardening is larger and the plateau stage is shortened as a result of the fast arrival at the second stage of hardness. Small amount of Ag changes the age hardening response of the Base alloy dramatically. In the low temperature aging, the incubation stage at which no clear hardness and electrical resistivity increase appears for a long period before the first stage of hardening. After the pre-aging at this incubation period, a characteristic two-step aging response is observed. The peak hardness dramatically changes in the Al-3.0Mg-1.0Cu alloy, while no clear change in the Ag-added alloy.