Directional Solidification Of Tb-Dy-Fe Magnetostrictive Alloy Induced By A Static Magnetic Field

It is the main function of magnetostrictive materials to achieve electromechanical convertibility. Giant magnetostrictive material Tb-Dy-Fe found in 1970s. One of its earliest applications was as hydroacoustic transducers for military purpose. With the increasing use in civil industry from 1990s, the increasing market is about 5 hundred millions dollar.Magnetostrictive properties of magnetostriction materials depend greatly on the grain orientation and microstructure for highly anisotropic magnetostriction. The best magnetostrictive properties can be obtained along the <111>direction of Tb0.3Dy0.7Fe1.9 alloy. However, <112> or <110> is the preferred growth direction in conventional directional solidification technique. The <111> oriented crystal of Tb0.3Dy0.7Fe1.9 alloy can only be grown by seeding technique, but this process is complicated and chemical composition along the axial direction of rod is heterogeneous. It is of significant that searching an industrial method to prepare Tb0.3Dy0.7Fe1.9 with <111> orientation.Studies in recent twenty years show that Solidification in a static magnetic field is a novel process, which can produce oriented magnetic materials with either their easy or their hard magnetization axes along the field. There are different orientation mechanism for conventional directional solidification and that in static magnetic field. Heat is transferred along a direction in conventional directional solidification; the crystal faces with minimum interface energy after competitive growth extend continually into liquid along the direction of heat, supercooling factors in interface will modify the direction of crystal growth eventually. In solidification with non-directional heat conduction under a static magnetic field, the rivalry between magnetic energy of single nuclei and disorder disturbance energy in environment will determine the crystal orientation. In a liquid-solid coexisting area with a majority of liquid, disorder disturbance factors include mainly thermal disorder effect and turbulence in melt. Static magnetic field will suppress the turbulence in melt, meanwhile, induce magnetic energy of single nuclei along the field to overcome melt viscosity and thermal disorder effect. Consequently, the orientation of crystals will be achieved during solidification course. For magnetic materials possessing magnetic anisotropy,crystal structure will be oriented along hard-magnetization axes in diamagnetic materials, and crystal orientation along the easy magnetization axes will be obtained in ferromagnetic materials etc..Due to absence of experimental equipment, no quantitative researches were done for essential conditions of orientation for magnetic materials solidified at high temperatures far above their Curie point. Therefore there is no industrial application of solidification in a magnetic field to achieve crystal orientation presently.We made a simple magnetic apparatus, which can be installed in a vacuum container of a directional solidification equipment with super high temperature gradient. The magnetic field of 140 mT can be maintained while solidification at slow cooling rate. Adaptation of high-frequency was to heat sample in experiment. Textures begin to appear along the field while sample solidified at slow cooling rate in a magnetic field of 140 mT. For Tb0.3Dy0.7Fe1.9 alloy with sizeφ16mm×18mm, the crystal structure with <111> orientation along the field is dominant in middle part of sample while at cooling rate of 0.8℃/min, crystal grains along the easy axes are about of 70% in specimen. Plate-like crystal grains of Laves phase in oriented specimen are along the field, size of crystal grains is coarser, which is beneficial to magentostrictive properties. The orientation of crystal grains are in random while at the same cooing rate in zero magnetic field.As a magnetostrictive material of cubic Laves phase in Re-Fe system, the TbFe2 and DyFe2 compounds possess the large magnetocrystalline anisotropy at room temperature. Due to the combination of the TbFe2 compound and DyFe2, the magnetocrystalline anisotropy of Tb0.3Dy0.7Fe1.9 alloy is reduced greatly. Crystal orientation along the easy axis for TbFe1.9 and DyFe1.9 also can be obtained by the similar condition of orientating Tb0.3Dy0.7Fe1.9 alloy. TbFe1.9 alloy is oriented along <111> direction while DyFe1.9 alloy along <100> direction. The TbFe2 compound and Tb0.3Dy0.7Fe1.9 alloy possess the minus magnetocrystalline anisotropy constant K1, and DyFe2 exhibits a large positive K1. Therefore, their directions of easy magnetic axis are along different crystal orientations. However, for those compounds, the great difference in magnetocrystalline anisotropy at room temperature seems to have no notable influence on the texture formation during their solidification course at high temperature respectively. It may be explained by that the values ofΔχ(the anisotropy of the paramagnetic susceptibility of crystal) for TbFe2,DyFe2 and Tb0.3Dy0.7Fe1.9 alloy tend to be close to each other at high temperatures.Other factors influencing the crystals orientation are also studied in experiment. The composition is found to affect the crystal orientation during solidification. For RFey alloys, excessive RFe3 phases will precipitate in liquid-solid phase when Fe content tends to y≥2. Furthermore, the temperature interval of liquid-solid phase is greatly reduced, then solid phase increase greatly as well as viscosity of liquid as temperature of melt decreases, which are unfavorable for crystal orientation. It is necessary to control the composition of RFex alloys for a better crystal orientation.The magnetostriction of the oriented Tb0.3Dy0.7Fe1.9 alloy along the field reaches 1700ppm under 18.7MPa and 6900 Oe. Such a value is higher than that in the grain-aligned sintered compact prepared by powder metallurgy technique, and lower than that in <111>-oriented single crystal. However, the higher orientation degree of Tb0.3Dy0.7Fe1.9 alloy should be achieved in practical application by slow cooling in a relatively strong magnetic field, the magnetostrictive properties would be enhanced further.Experiments show that using a low cooling rate can greatly weaken the disturbance degree inside melt, and a weak disturbance can be restrained in a relatively weak magnetic field. It necessary to make clear that there are differences for suppressed turbulence and restrained motion in melt. The former means that laminar flow replaces turbulence but the motion in melt still exists. Restraining the motion requires very strong magnetic field. The even temperature field can be fit for crystal orientation by suppressing turbulence in melt. Summarizing those investigation and application, a model of controlling crystal orientation by solidification in magnetic field is proposed. Many ferromagnetic materials may be readily oriented during solidification course in field due to their relatively large residual |Δχ| at high temperatures near melting point.