Microstructural Changes at the Initial Stage of Precipitation in Al-1.2%Si Alloys

In Al-Si alloys, it is known that at the pre-aging stage clusters are generated which result in the growth of Si phase precipitates during the precipitation process. In this paper, we use a transmission electron microscope (TEM) to observe the microstructure of Al-Si alloys from immediately after quenching to the onset of precipitation. The results reveal the presence of streak regions immediately after quenching that are thought to be Si clusters, and these are coherent with the {111}matrix. At the initial stage of aging, plate-like Si crystals are observed. Moreover, the plate-like structure of the observed Si crystals contradicts the round shape previously described. We propose that the streak regions that appear immediately after quenching act as nuclei for the plate-like Si crystals. As the aging time increases, the plate-like Si crystals increase in length and thickness, and thus grow to become plate-like Si phases.