Influence of barium addition on the formation of primary Mg₂Si crystals from Al-Mg-Si melts

In this study, the influence of different contents of Ba additions on the microstructure evolution, phase reaction characteristic, and mechanical property of Al-Mg-Si alloy was investigated. Microstructural characterization was conducted by means of scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) facility, X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The mechanical property was examined using hardness test. The results revealed that cubic primary Mg2Si crystals shaped by {100} facets with an average particle size of 35 mm were successfully fabricated through the addition of 0.2 wt. % Ba element to Al-Mg-Si melts. For the first time, it is revealed that the Al4Ba compound can act as the nuclei for the primary Mg2Si during solidification, which leads to the refining of the primary Mg2Si particle size extensively. Furthermore, the formation of Al2Si2Ba and AlSiBa intermetallic compounds (IMCs) are liable for the Mg2Si particle refinement; hence, the hardness of the alloy increased from 60.21 to 67.83 Hv. Besides, thermal analysis showed that the nucleation temperatures of the primary Mg2Si phase increased with the addition of Ba. Ba additions perform a substantial role in determining the shapes of primary Mg2Si crystals, which can be altered from coarse dendritic structure (0 wt. %) to crystals with a combination of eight {111} and four {100} facets (0.08 wt. %), then to truncated cube (0.1 wt. %), and finally to a cube fully bounded by {100} facets (0.2 wt. %) with increasing Ba concentrations. This study revealed that the growth process of the cubic primary Mg2Si is due to the absorption and poisoning effect of Ba atoms, which leads to the fading of the growth rates of {100} faces of primary Mg2Si and as a result the {100} faces are exposed. Furthermore, in the modified alloy, the skeleton-type growth process of the cubic primary Mg2Si was found, in which growth steps with some hillocks were detected. (C)2021 The Author(s). Published by Elsevier B.V.