The influence of various Ag additions on the nucleation and thermal stability of Ω phase in Al-Cu-Mg alloys

The effect of the bulk Ag content ranging from 0.14 to 0.57 wt% on the aging hardening response and microstructures of Al-Cu-Mg alloys was investigated in the present study. The strong dependence of the hardening response on the bulk Ag content was obtained as the hardening response at 165 degrees C was accelerated by increasing Ag content. The highest hardening response was observed in the alloy containing 0.57 wt% Ag, whereas the least one was obtained in the alloy containing 0.14 wt% Ag. The tensile testing at various temperatures also revealed a similar trend. Based on the TEM study on peak aging conditions at 165 degrees C, it was found that the competitive precipitation kinetics between Omega and theta' phases was sensitive to the variation of Ag content. The dense precipitation of the Omega plates promoted by increasing Ag content was responsible for the increasing hardening response. Meanwhile, the formation of theta' phase was greatly suppressed. Subsequent long term aging at 200 degrees C for 500 h following the peak aging treatment further suggested that the thermal stability of the Omega plates was enhanced with increasing Ag as the presence of the Omega plates could be clearly confirmed in 0.57 wt% Ag alloy but was hard to distinguish in the alloy containing 0.14 wt% Ag. (C) 2012 Elsevier B.V. All rights reserved.