Abstract
The current study investigated the effects of gadolinium (Gd) addition (0, 0.3, 0.5, 1.0, and 2.0wt%) on microstructural development and solidification characteristics of hypereutectic Al-15%Mg2Si in-situ composite. Microstructural examinations were carried out via optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The results showed that an addition of 1.0wt% Gd, changed the morphology of primary Mg2Si from coarse dendritic to fine truncated octahedral shape. Furthermore, the average size of primary Mg2Si particles decreased from 40μm to 20μm and density increased from 495 to 1167particles/mm2. Besides that, cooling curve thermal analysis (CCTA) technique revealed that Gd addition increased nucleation and growth temperatures of primary Mg2Si phase. It is proposed that modification and refinement mechanism of Gd addition are mainly attributed to formation of tiny Al3Gd particles as the heterogeneous nucleation substrates for primary Mg2Si, which results in a refined distribution of these particles and poisoning effect by adsorbing Gd atoms, preferentially on {100} facets of Mg2Si crystals. Moreover, it is believed that formation of Gd intermetallic compounds is responsible for refinement of primary Mg2Si particles.
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