Precipitation of (Si2-xAlx)Hf in an Al-Si-Mg-Hf Alloy.

The morphology, composition, and structure of precipitates in an Al-Si-Mg-Hf alloy after heat treatment at 560°C for 20 h were studied by means of C s -corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectrometry (EDS), high-resolution transmission electron microscopy (HRTEM), and first-principle calculations. Precipitates with three kinds of morphologies were observed. The rectangular and square precipitates were predominantly (Si2-x Al x )Hf phases, while the nanobelt-like precipitate is the Si2Hf phase. First-principle calculations were used to show that the Si6 and Si8 sites were the most favorable sites for Al incorporation in the orthorhombic Si2Hf phase.