Influences of Morphology and Size of Mg_2Si on Microstructural Evolution of Mg-6.2Si Alloys during Partial Remelting

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After isothermally treated at 660°C for 20 min, both dendritic Mg2Si in unmodified alloys and polyhedral ones in KBF4-modified alloys transformed to nearly globular morphology, while the degree of spheroidization in KBF4-modified alloy was lower than that in unmodified alloy. This result might be attributed to both restriction effect of boron and stability of octahedral primary Mg2Si crystals. After isothermal treatment at 660°C for 20 min, however, the initial grain sizes of dendritic Mg2Si crystals in unmodified alloy or polyhedral ones in modified alloy obtained by different cooling rates (copper mold with the holes of φ20 and φ6 mm) only caused a little effect on the partial remelting microstructure. The fine primary Mg2Si particles obtained from φ6 mm hole, however, were more readily to evolve to globular crystals, which might be attributed to the more active Ostwald ripening and/or coalescence.
社団法人日本鉄鋼協会の論文
2009-12-15