The effect of Si upon the precipitation of the θ′ phase (metastable Al2Cu) during the isothermal aging of Al-2Cu-1wt%Si was studied. The binary alloys Al-2Cu and Al-1Si were studied for comparison. Only two precipitate phases were detected: essentially pure Si in Al-1Si and Al-2Cu-1Si, and θ′ (metastable Al2Cu) in Al-2Cu and Al-2Cu-1Si. Upon aging the ternary alloy at 225C, Si precipitated first and catalyzed the θ′ phase. The precipitates in the ternary alloy were smaller, were more densely distributed, had lower aspect ratios and coarsened more slowly than those in binary Al-2Cu aged at the same temperature. The overall evolution of particle shape with size obeyed the predictions of Khachaturyan-Hairapetyan thermo-elastic theory. This theory provided an estimate of the interfacial tension of the broad Al/θ′ interface of 85 to 96mJ/m2. This was to near the values for other low-energy interfaces in Al, such as the twin boundary energy (100mJ/m2) and the antiphase boundary energy in δ′ Al3Li (70mJ/m2).

Catalyzed Precipitation in Al-Cu-Si. D.Mitlin, V.Radmilovic, J.W.Morris: Metallurgical and Materials Transactions A, 2000, 31[11], 2697-711