Microstructure of Al-Si-Mg alloy with Zr/Hf additions during solidification and solution treatment

In this study, the morphology, composition and evolution of intermetallics in Al-7Si-0.3Mg, Al-7Si-0.3Mg-0.47Hf, Al-7Si-0.3Mg-0.16Zr and Al-7Si-0.3Mg-0.14Zr-0.44Hf alloys were characterized by optical microscope (OM), scanning electron microscope (SEM) with energy-dispersive spectrometer (EDS) and transmission electron microscope (TEM). The AlSiZr, AlSiHf and AlSiZrHf phases are formed with additions of Zr and/or Hf in base alloy during solidification, especially the AlZrHf primary particles. The three-dimensional morphology of the AlZrHf particle evolves from 10-polyhedron-like to compressed cube-like during solidification. The new AlZrHf phase experiences partial dissolution after solution treatment, while the others remain in the initial morphology. High Si contents lead to form thermally stable Zr/Hf-rich precipitates in Zr/Hf-containing alloys during solution treatment, especially the nanobelt-like Si2Zr, Si2Hf and Si-2(Zr,Hf) precipitates. Particular orientation relationships and the growth mechanism are identified. The finding of the work broadens the elevated temperature application of Al-Si-Mg in modern automotive aluminum engine.