Microstructure Evolution And Magnetostriction Behavior Of Rare-earth-iron Based Magnetostrictive Materials Under High Magnetic Field Conditions

The major function of magnetostrictive materials is the transition between electrical signal and mechanical signal. The first application for the rare-earth giant magnetostrictive materials of TbFe2 and Tb0.27Dy0.73Fe1.95 alloys were used in sonar system in military since these were coined in the 1970s. After several decades development, the applications of Tb0.27Dy0.73Fe1.95 alloy have been developed a lot in industry and civil fields. This material is widely used in the field of high technologies, such as magnetomechanical transducers, actuators, and adaptive vibration control systems.TbFe2 and Tb0.27Dy0.73Fe1.95 alloys have a typical MgCu2-type cubic Laves phase structure and exhibits different magnetostrictive properties along different crystal orientations. The<111> orientation of them is the easy magnetization axis, so the linear magnetostriction along<111> orientation is higher than those along other directions. Generally, directional solidification method is used to prepare TbFe2 and Tb0.27Dy0.73Fe1.95 alloys. However, the crystal oriented along<110> or <112> direction is always obtained and both of these two directions need a higher external magnetic field for better magnetostriction. By using seed crystal technology, the<111> preferred growth orientation can be obtained. However, the relatively small size and high brittleness induced by the appearance of linear ReFe3 do not suit the industrial application. Therefore, the preparation of TbFe2 and Tb0.27Dy0.73Fe1.95 products with high<111> orientation by a new method is of great importance.As an extreme condition, high magnetic field has received increasing attention from researchers for its application in materials science. The oriented structure of metal materials can be attained by rotating magnetic domain with high magnetic fields during heat treatment and solidification processes. In this paper, the effects of different high magnetic fields on the crystal orientation and the magnetostriction of TbFe2 and Tb0.27Dy0.73Fe1.95 alloys were investigated and the relation between structures and performances in these two alloys was also analysed. The main results are listed as follows:(1) The effects of high magnetic fields on the magnetostriction of TbFe2 and Tb0.27Dy0.73Fe1.95 alloys during heat treatment were studied. It was found that high magnetic field didn’t change the orientation and phase composition of the alloys. But the orientation degree along the<111> direction had enhanced. In addition, the magnetostrictive property of TbFe2 alloy increased after the heat treatment with high magnetic field. But for Tb0.27Dy0.73Fe1.95 alloy, the magnetostriction was not obviously enhanced.(2) The effects of high magnetic fields on the magnetostriction of TbFe2 alloy during semi-solid forming process were also examined. With the increase of magnetic flux density from 0 T to 11.5 T, the orientation of the TbFe2 phase underwent a transformation from <113> to<111> to<110>. The alloy treated at 8.8 T showed a magnetically anisotropic behavior because it had the preferred orientation along the easy magnetization axis. Farther more, its magnetostrictive coefficient without applied stress increased 34% to the 0 T-case sample and showed the "jump effect" remarkably.(3) The effects of high magnetic fields on the microstructural, magnetic and magnetostrictive properties of the TbFe2 alloy after solidification process were analyzed. It was found that with the magnetic flux density increasing, the microstructure of the alloy was changed from a random orientation to the <110> orientation and then the <111> orientation. The results of the magnetization measurement confirmed such change. The TbFe2 and TbFe3 phases in high magnetic fields samples had distinct grain arrangements. For the sample solidified in a static magnetic field of 4.4 T, the magnetostrictive coefficient of the alloy improves by 24% contrary to that without magnetic field.(4) The effects of different magnetic field intensity and cooling rates on the microstructural, magnetic and magnetostrictive properties of the Tb0.27Dy0.73Fe1.95 alloy after solidification process were studied. The (Tb,Dy)Fe3 phase appeared in the microstructure after solidification process. The 2.2 T and 4.4 T-case samples had the distinct grain arrangements parallel to the magnetic field direction with a cool rate of 1.5℃/min during the solidification process. When the cool rate was 1.5℃/min, the orientation of the TbFe2 phase underwent a transformation from<113> to<110> to <111> with the increase of the magnetic flux density from 0 T to 4.4 T. The magnetization measurement also confirmed this change. When the cool rate was 5℃ /min, the sample had the preferred orientation along the<111> direction when the magnetic field flux density was higher than 4.4 T. When the cool rate was 60℃/min, the orientation had no change with high magnetic field. In addition, the magnetostriction results showed that the magnetostrictive coefficient obviously enhanced after the slow cooling solidification(the cool rate was 1.5℃/min or 5℃/min) and has no clear effect during the fast cooling solidification process(the cool rate was 60℃/min).(5) The above results demonstrated that high magnetic field could induce crystal orientation along the easy magnetization axis and grains growth parallel to the magnetic field direction. Crystal orientation along the easy-magnetization axis parallel to the magnetic field direction can reduce the free energy in system. Only magnetic torque was higher than the sum of torque caused by viscous resistance and Lorentz force, high magnetic field was able to induce grains arrangements along the magnetic field direction. In addition, the free space and time were also necessary conditions for the grain rotation. It is really a new method for increasing magnetostriction performance and developing magnetostrictive materials by controlling crystal orientation and grain arrangement under high magnetic field conditions.