Rapid-Solidification Effect on Magnetostriction in Iron-based Ferromagnetic Shape Memory Alloy
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Rapid-Solidification Effect on Magnetostriction in Iron-based Ferromagnetic Shape Memory Alloy YASUBUMI FURUYA*, TAKESHI KUBOTA*, TEIKO OKAZAKI*, MITSUTAKA SATO* and MANFRED WUTTIG** * Fac. of Science and Technology, Hirosaki Univ., Hirosaki 036-8561, Japan ** Dept. of Materials Science and Engineering, Maryland Univ., College Park, MD 20742-2115, USA ABSTRACT Fe-29.6at%Pd ferromagnetic shape memory alloy (FSMA) ribbon formed by rapidly solidified, melt-spinning methods is expected to be useful as a new type of material which shows giant magnetostriction as well as quick response. The giant magnetostriction in the rolling direction depends strongly on applied magnetic-field direction and has a maximum value of 8×10-4 when the field is normal to the surface. This phenomenon is caused by the rearrangements of activated martensitic twin variants. The inverse phase transformation temperatures (As) obtained from Laser micrographs and magnetization vs. temperature curve are ~307 K and 400~440 K, respectively. We analyze magnetostriction, magnetic property and crystal structure of Fe-29.6at%Pd bulk sample before rapid solidification and the ribbon sample. From these results, it can be concluded that remarkable anisotropy of giant magnetostriction of ribbon sample is caused by the fine structure formed by the melt-spinning method. It may be possible to apply this method successfully to other FSMA and Ni2MnGa, which is difficult to manufacture owing to its brittleness. INTRODUCTION Ferromagnetic Fe-Pd shape memory alloy is useful as a micromachine and intelligent / smart material system controlled by a magnetic field. Martensitic twin's initiations and the following its movements depending on magnetic field are thought to be closely related with a new type of magnetostriction[1]. The previous studies [2,3] showed that the rapidly solidified Fe-29.6 at%Pd alloy ribbon has stronger crystal anisotropy, giant magnetostriction as well as shape memory effect. Magnetostriction is changed with temperature and has a maximum of 18 ×10-4. However, the mechanism of magnetically induced strain has not yet been discussed. We think that a directional dependence of magnetostriction is probably caused by fine columnar microstructure formed by rapid solidification methods. To confirm this hypothesis, in the present study, at first, we analyze magnetostriction, magnetic property and crystal structure of Fe-29.6at%Pd bulk sample before rapid solidification and compare these properties with those of the ribbon sample. Next, we investigate the V6.23.1
relation between a phase transformation from martensite ( fct ) to austenite (fcc) to martensitic twin's mobility during magnetization process. EXPERIMENTAL DETAILS The rapidly solidified Fe-29.6at%Pd 60 μm-thin ribbon samples were prepared by originally designed electro-magnetic melt-spinning single- or twin-roll method from bulk alloy [3]. Samples were annealed at 1173 K for 0 and 0.5 h in vacuum to study the effect of heat treatments on magnetostriction. The magnetization, M vs. applied magneti
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