Effect of the Growth Velocity on Microstructure, Orientation, and Magnetostriction in Fe 81 Ga 19 Alloy
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INTRODUCTION
FEGA magnetostrictive alloys have attracted considerable interests for their potential applications as sensor or actuator materials[1,2] due to large magnetostriction, high strength, and low cost.[3–7] Strongly magnetostrictive anisotropy exhibits in FeGa alloys, and the maximum magnetostriction along the easy axis h001i direction can reach about 320 ppm.[8,9] Any deviation of the crystallographic orientation from the h001i direction will decrease the magnetostriction.[10] Directional solidification is an effective approach for preparing anisotropic materials. During the directional solidification, the crystallographic orientation evolution[11–13] and solidification morphology transition[14–16] lead to the differences in the magnetostrictive properties. As for the Terfenol-D magnetostrictive alloy, with the growth velocity increasing, the preferred orientation along the growth direction varied from h110i to h112i.[11,12] Furthermore, planar, cellular, and dendritic morphologies were observed at different growth velocities.[17] Both the preferred orientation and solidification morphology influence the magnetostriction of the Terfenol-D alloy. Oriented FeGa alloys have been prepared by the Bridgman method[6,7] and the zone melting method.[18–20] But the influence of the growth velocity on the preferred orientation and solidification morphology in FeGa alloys has not been reported yet.[21–23] To clarify this issue is crucial for understanding the growth behavior of FeGa alloys and preparing h001i orientated FeGa crystals with large magnetostriction. CHUAN LI, Doctoral Student, JINGHUA LIU, Researcher, and CHENGBAO JIANG, Professor, are with the School of Material Science and Engineering, Beihang University, Beijing 100191, P.R. China. Contact e-mail: [email protected] Manuscript submitted December 16, 2011. Article published online July 10, 2012 4514—VOLUME 43A, DECEMBER 2012
In this work, an induction heating zone melting furnace was adopted to prepare oriented Fe81Ga19 crystals. The influence of the growth velocity on solidification morphology, preferred orientation, and the magnetostriction were systematically investigated. The growth velocity was experimentally determined to prepare h001i oriented Fe81Ga19 alloy with high magnetostriction.
II.
EXPERIMENTAL PROCEDURES
Ingots of Fe81Ga19 alloy were prepared by arc melting under high-purity vacuum, with high-purity elements Fe (99.99 pct) and Ga (99.99 pct). The raw materials were melted four times and then drop cast into a water-cooled copper mold to obtain rods with the size of F7 9 100 mm. Fe81Ga19 rods were directionally solidified in alumina crucibles using a self-made zone-melting crystal growth furnace. A h001i oriented Fe81Ga19 crystal was used as a seed. The growth velocities of 10 mm/h, 120 mm/h, 240 mm/h, 480 mm/h, and 720 mm/h were chosen to prepare oriented Fe81Ga19 crystals. The grown rods were quenched into the cooling liquid metal by pulling down the control rod instantly to keep the solid/ liquid interface. Figure 1 shows a schematic diagram of
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