Effects of Si additions on the precipitation hardening response in Al-Cu-Mg(-Ag) alloys

The effects of Si additions on the precipitation hardening response of Al-4Cu-0.3Mg(-0.4Ag) (wt%) alloys have been examined systematically using transmission electron microscopy. The addition of Si to the Al-4Cu-0.3Mg alloy produces an increase of up to 30 VHN in the maximum hardness obtainable during ageing at 200 degrees C and, for Si contents in the range 0.3-0.7 wt%, the maximum hardness exceeds that achievable in the Al-4Cu-0.3Mg-0.4Ag alloy (similar to 140 VHN) under similar ageing conditions. Maximum hardness of the Al-Cu-Mg-Si alloys is associated with coexistence of uniform, fine-scale distributions of {100}(alpha) plates of theta' and <100>(alpha) laths of Q phase (Al4Cu2Mg8Si7). The addition of greater than or equal to 0.2 wt% Si to the Al-4Cu-0.3Mg-0.4Ag alloy results in complete suppression of the Omega phase that dominates the microstructure of the quaternary alloy. However, the hardness of the Si-modified alloys may still be enhanced with appropriate Si additions, even in the absence of the Omega phase.