The alloy $Tb_xDy_{1-x}Fe_y(0.27≤x≤0.30, 1.8＜y≤2)$, which is called Terfenol-D, is known to have giant magnetostriction and comparatively low magnetocrystalline anisotropy energy. This alloy has outstanding magnetostrictive properties combining a low crystalline anisotropy at room temperature. And it also has superior properties in energy density, efficiency, response time and Young`s modulus, compared with PZT ceramics.
In manufacturing giant magnetostrictive alloys, rare earth metals are very reactive with oxygen. So these rare earth metals easily form oxides. And during the melting process, these metals also easily react with crucible. For this reason, this giant magnetostrictive alloy is easily contaminated by impurities, especially by oxygen contents.
To investigate the effect of impurities on the properties of Terfenol-D, several specimens were manufactured by vacuum suction casting and zone melting method. Magnetostrictive properties were assessed by measuring magnetostrictive strain and magnetostrictive coefficient, and these properties were compared with optical microstructure.
The effect of microstructure was investigated with varying oxygen partial pressure, varying purity, and varying initial oxygen composition. As the oxygen content was higher, dendritic inclusions were formed in the specimen, and the magnetostrictive properties decreased.
And the relations between the formation of dendritic inclusions and the processing conditions were investigated. Dendritic inclusions decreased as alumina crucible was used. Dendritic inclusions also decreased as the melt temperature was lower and the growth velocity was higher.
