Atomic scale analysis of Hf-containing precipitates in an Al-Si-Mg-Hf alloy

Abstract The purpose of this work is to explore new possible precipitates, unveil their crystal structures as well as the precipitation sequence in an Al-Si-Mg-Hf heat treated alloy. Precipitates in the Al-Si-Mg-Hf alloy after heat treated at 560 °C for various holding times are investigated by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive x-ray spectroscopy (EDS). Three kinds of precipitate phases and their crystal structures, i.e., Si 3 Hf (metastable phase, cubic L1 2 structure), Si 2 Hf 2 (metastable phase, tetragonal structure) and Si 2 Hf (stable phase, orthorhombic structure), are identified based on the atomic resolution HAADF-STEM micrographs. The precipitation sequence in the alloy is first proposed as: Si-Hf clusters→Si 3 Hf→Si 2 Hf 2 →Si 2 Hf. This precipitation sequence is further confirmed by the first principle calculation. These results are probably useful for developing new aluminum alloys with high temperature stable strengthening phases in automotive applications.