Terfenol-D shows giant magnetostriction at room temperature and relatively low magnetic field. Its maximum magnetostrictive strain is 15 times higher than that of PZT piezoelectric materials. Therefore, Terfenol's application field is so vast and it is highly required to develop the production technique. Because of high vapor pressure of Dy, crystal growth method was determined to be encapsulated Bridgman method to suppress the evaporation of Dy.
In the stage of master alloy preparation, the reaction between quartz crucible and alloy was investigated. The loss of rare earth elements during induction melting was investigated. In the stage of directional solidification, solidification microstructures along with alloy composition and growth condition was investigated. Preferred growth direction along with growth rate was investigated. Macrosegregation and microsegregation was also investigated. In the stage of magnetic property measurement, the effect of solidification microstructure and growth direction on the magnetic property was investigated. To eliminate microsegregation, high temperature annealing at 1000℃ for 40 hours was performed and the effect of annealing on the magnetic property was investigated.
It is found that the reaction between quartz crucible and alloy was oxidation reaction of rare earth elements by being reduced $SiO_2$. Reaction product formed at the interface of the crucible and the alloy causes sticking of solidified alloy to the crucible wall and this results in crystal defects such as crack, etc.. Tb and Dy loss during melting was quite large, hence it is required that initial alloy composition should contain much excess rare earth elements. Growth rate to 25 μm/s, Terfenol solidified as cellular structure, and above this tate Terfenol solidified as dendritic structure. Magnetostriction of the cystal solidified as dendrites is poorer than that of solidified as cells because of secondary dendrite arms which have different crystallographic orientation. It is found that Terfenol was grown to <112> direction at moderate growth rate, but when the growth rate was very low (12 μm/s) the growth direction changed to <110> direction. Magnetostriction of the crystal grown to <110> direction was lower than that of <112> crystal because of making larger angle between crystal axis and easy magnetization direction, <111>. Macrosegregation during directional solidification was turned to be caused by loss of rare earth elements. The smaller inner diameter of the crucible was, the higher the tendency of macrosegregation was. It is thought to be due to the increasing ratio of reaction surface area to volume of the alloy melt. High temperature annealing eliminated microsegregation of rare earth elements effectively. After annealing, stress dependence of magnetostriction curve was increased markedly. It is thought that lowering of magnetocrystalline energy, which was locally very high before the annealing, made stress anisotropy be dominant in the crystal. The wider the cell spacing is, the severer the microsegregation is. Therefore for the crystal which have wide cell spacing like in this experiments, high temperature annealing is essential for the crystal being used in practical applications.
