Effect of high magnetic fields on the microstructural, magnetic and magnetostrictive properties of TbFe<inf>2</inf> alloy during solidification process

Abstract

Giant magnetostrictive material is a kind of new function materials, which have the features of big magnetostriction strain, rapid responsibility and low frequency. So they are widely used in the fields of high technologies, such as sonar system, underwater acoustic transducers, electroacoustic transducer, etc [1, 2]. One of the typical representations of this kind of materials is TbFe 2 , which exhibits the strongest anisotropy known for a cubic system. The magnetostrictive property of TbFe 2 differs significantly depending on crystal orientation. The orientation is the easy magnetization axis of TbFe 2 , and therefore linear magnetostriction along the orientation is higher than that along other directions. Therefore, to obtain a better magnetostrictive property, the control of crystal orientation for the TbFe 2 phase in TbFe 2 based materials is of great importance. Recently, depending on the magnetic anisotropy of crystals, which can induce a difference in magnetization energy in various crystallographic directions if they are subjected to a magnetic field, oriented or textured structures have been prepared using a high magnetic field during solidification process [3-5]. There then arises the possibility of a direct processing route in the in situ control of the crystal orientation and corresponding magnetostriction of RFe 2 compounds. In this work, TbFe 2 was solidified in various high magnetic fields. The effect of the high magnetic fields on the crystal orientation, magnetization behavior, and magnetostriction was investigated. The relation between the magnetic properties and the crystal orientation evolution in the TbFe 2 alloy induced by the magnetic fields was examined. The TbFe 2 master alloy was prepared from Fe (purity 99.9%) and Tb (purity 99.9%) by arc melting in a cold copper crucible under an argon atmosphere. The obtained alloy was cut into cylindrical specimens 10mm in diameter and 15mm in length. The specimens were placed in quartz crucibles and heated in an argon atmosphere to 1623 K at a rate of 5 K/min and held at that temperature for 5 min. The specimens were cooled to 973K at a rate of approximately 1.5 K/ min, and then cooled to room temperature by turning off the DC power source. The magnetic field conditions were 0, 1, 2.2 and 4.4 T. Further details of the experimental apparatus are given elsewhere [6]. Specimen surfaces were prepared using a standard metallographic technique and etched with the natal solution (HNO3 (5 vol. %)/C2H5OH). The microstructures of the specimens were observed with an optical microscope and SEM. The compositions were characterized by chemical analysis and EDX analysis. The crystal orientation of phases was measured by x-ray diffraction (XRD, Cu Ka radiation) analysis on the transverse section. Magnetization was measured with a vibrating sample magnetometer at room temperature. The magnetostrictive strains were measured with a standard resistant strain gauge at room temperature. Fig. 1 shows XRD patterns of the solidified alloys from transverse sections (i.e. perpendicular to the direction of the high magnetic field) at room temperature. The XRD patterns show that with an increase in the magnetic flux density the orientation of the TbFe 2 phase transforms from to , then to . With a 4.4 T magnetic field, a highly -oriented TbFe 2 compound was produced. Figure 2 shows magnetostriction curves for the alloys solidified in various high magnetic fields without applying stress. The magnetostriction in these curves is the difference between the magnetostriction measured parallel and perpendicular to the magnetic fields, as applied during solidification. The application of high magnetic fields is found to increase the magnetostriction of the alloys, especially for 4.4 T. The maximum magnetostriction increases from about 1360 ppm at 0 T to about 1700 ppm at 4.4 T. Furthermore, the magnetostriction for the low-field intensity is strongly enhanced by the application of the magnetic fields during solidification process. The change in the magnetostrictive property can be attributed to the change in the crystal orientation behavior of TbFe 2 and microstructure evolution induced by the high magnetic fields.