Effect of μG Quality on Unidirectional Solidification of Iron-Rich Sm-Fe Magnetostrictive Alloys
Takeshi Okutani, Hiromichi Ono, Hideaki Nagai and Mikito Mamiya
Sm-Fe magnetostrictive material was produced by unidirectional solidification of Sm-Fe alloy with atomic ratio from 1/2 to 2/17 in microgravity (μg) within ±4 × 10-3g for 1.43 s obtained using a 10-m drop tower and with fluctuating gravity between 0.1 and 0.02 g obtained by parabolic flight. SmFe2 and a small amount of Sm2Fe17 as well as Fe were formed from unidirectional solidification in μg of Sm-7Fe alloy, obtained using the drop tower and parabolic flight. The structure consisted of sheet dendrites of SmFe2 and Fe-rich Sm-Fe layers between the sheet dendrites having no gaps with an orientation along the solidification direction. The surface of the sheet dendrites of SmFe2 formed in μg obtained by parabolic flight was rougher than that formed in μg obtained using the 10-m drop tower. A crystalline orientation of of SmFe2 along the solidification direction was found in the products formed in μg using the drop tower, but not in those using parabolic flight. The formation mechanisms of SmFe2 sheet dendrites can be explained by microsegregation caused by the lack of convection in melt in μg. In contrast, Sm2Fe17 and a small amount of Fe were formed from unidirectional solidification of Sm-7Fe alloy in 1g, and the resulting structure consisted of sheet dendrites with no orientation. Magnetostriction of -3328 ppm at the outer magnetic fields of 0.12 T was achieved on a sample synthesized by unidirectional solidification of Sm-7Fe in ±4 × 10-3 g obtained using the drop tower. The value was higher than that synthesized in fluctuating gravity between 0.1 and -0.02 g obtained by parabolic flight, -1776 ppm, because of the crystallographic orientation and the lack of a rough surface of the SmFe2 sheet dendrites.
Keywords: Magnetostrictive materials, Microgravity, Unidirectional solidification, SmFe2, Sheet dendrites, Drop tower, Parabolic flight, Crystalline orientation, Magnetostriction.