Kinetics of silicon precipitation in a directionally crystallized binary aluminum-silicon alloy

The precipitation of silicon atoms in aluminum in an Al-Si alloy has been studied using differential scanning calorimetry. The alloys containing 8, 13, and 15 wt % silicon were obtained by directional solidification of a ribbon pulled from the melt through a shaper by the Stepanov method at a rate of about 10³ μm/s. From the characteristics of the exothermic effects observed in the temperature range 430–650 K, it has been found that the precipitation process leading to the formation of the Guinier-Preston zones occurs with the effective activation energy of 75 kJ/mol, and its intensity decreases with increasing silicon content in the alloy from 8 wt % to the eutectic content. The effect correlates with a decrease in the volume fraction of dendrites of the primary α-Al crystals in the alloy. It can be assumed that the precipitation occurs in the dendrite primary crystals of the solid solution. Based on this assumption, it has been concluded that, during directional solidification of an aluminum-silicon alloy at a rate of 10³ μm/s, the metastable solid solution of silicon in aluminum, in which silicon atoms of the metallic lattice are transformed into clusters with covalent bonding forces, is formed during the dendrite growth of the primary crystals.