Catalyzed precipitation in Al-Cu-Si

The work reported here concerns the effect of Si on the precipitation of theta' phase (metastable Al2Cu) during the isothermal aging of Al-2Cu-1Si (wt pet). 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 theta' (metastable Al2Cu) in Al-2Cu and Al-2Cu-1Si. On aging the ternary alloy at 225 degreesC, Si precipitates first and catalyzes the theta' phase. The precipitates in the ternary alloy are smaller, are more densely distributed, have lower aspect ratios, and coarsen more slowly than those in the binary Al2Cu aged at the same temperature. While the shapes of individual theta' precipitates in binary Al-2Cu are strongly affected by the kinetic problem of nucleating growth ledges, which produces a significant scatter in the aspect ratio for samples of given thickness, the overall evolution of particle shape with size follows the predictions of the Khachaturyan-Hairapetyan (KH) thermoelastic theory, which reduces to kappa = L/d proportional to rootL at large sizes. The KH theory provides an estimate for the interfacial tension of the broad Al-theta' interface of 85 to 96 mJ/m(2), which is near the values for other low-energy interfaces in Al, such as the twin boundary energy (100 mJ/m(2)) and the antiphase boundary energy in delta' Al3Li (70 mJ/m(2)). Si and theta' precipitates in Al-2Cu-1Si have a strong elastic interaction because of their compensating strain fields. This elastic interaction promotes the nucleation of theta' precipitates on Si, decreases the expected aspect ratio of theta', and inhibits coarsening. Finally, Si precipitation in ternary Al-2Cu-1Si differs from that in binary Al-1Si in that the Si precipitates are coarser, more equiaxed, and more extensively twinned. These changes appear to be effects of Cu, which increases the solubility of Si in Al and adsorbs on the Si-Al interface, promoting twinning by a "step-poisoning" effect at the interface.