Control of Microstructure Driven by Magnetic Field in Ferromagnetic Intermetallics
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0980-II03-02
Control of Microstructure Driven by Magnetic Field in Ferromagnetic Intermetallics Tomoyuki Kakeshita and Takashi Fukuda Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, 565-0871, Japan
ABSTRACT Rearrangement of martensite variants (RMV) by magnetic field in ferromagnetic intermetallic compounds of Ni2MnGa and Fe3Pt (degree of order is about 0.8) has been examined. For both the compounds the RMV by magnetic fields is realized below the martensitic transformation temperature down to 4.2 K, and the variant with c axis parallel to the field direction grows, consuming the others. Although a single variant state is realized by magnetic field application in Ni2MnGa, it is not realized in Fe3Pt. A fraction of the field-induced strain (about 1% at 20 K) recovers in Fe3Pt while this is not observed in Ni2MnGa. Conditions for the RMV by magnetic fields have been identified. Also, the ordering process of L10-type CoPt from the disordered matrix under a magnetic field has been studied. It is shown that a single variant state of ordered CoPt is realized by applying magnetic field only in the early stage of ordering near the Curie temperature of the ordered phase.
INTRODUCTION An external magnetic field can be used effectively for controlling the phase transformations in magnetic materials although it is not widely known compared with stress and temperature. One of the reasons promoting the use of magnetic fields to control transformation in solid state is the recent progress on magnet technology. Thus many studies have been undertaken recently and a magnetic field is found to influence not only transformation temperature [1,2] but also the crystallographic domains (variants) in the martensite phase of ferromagnetic shape memory alloys. In the present paper, we will focus on the effect of magnetic field application on variant selection formed by solid-solid phase transformations. One is the rearrangement of martensite variants (RMV) by magnetic field in the martensite phases of Ni2MnGa and Fe3Pt, and the other is selected growth of variant during a disorder-order transformation in CoPt. The former is a typical example of a diffusionless process, while the other is a typical example of diffusive nucleation and growth. We will show the characteristic features of variant selection in these intermetallic compounds and briefly discuss the condition for the variant selection. EXPERIMENTAL Ingots of Ni2MnGa, Fe3Pt, and CoPt were prepared by arc melting. Single crystal of each alloy was grown by a floating zone method or a Bridgeman method and then heat-treated for
homoginization. Lattice parameters of the alloys were determined by an x-ray diffraction analysis, magnetic field-induced strain was measured by three terminal capacitance method, magnetization was measured by SQUID magnetometer and stress-strain curves were obtained by Instron-type universal mechanical testing machine. Additional details of the procedures are described elsewhere [3-5].
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