Magnetostriction Increase of Polycrystalline Fe-Al-B Thin Sheets after Thermomechanical Process
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INTRODUCTION
MAGNETOSTRICTIVE materials are widely used to build sensors, actuators, and energy harvesting devices.[1–3] Particularly, in applications in which the actuator uses alternating current, thin magnetostrictive materials are desired to reduce the eddy current losses.[4–6] Although single crystals of Fe-Al have higher values of magnetostriction, ~150 ppm,[7] their manufacturing is slow, expensive, and not an easy issue.[8] For these reasons, the low cost and high manufacturing speed of polycrystalline samples could be attractive for usual applications.[9] Recently, several studies reported that the addition of boron in the Fe-Al alloys improves the total magnetostriction.[10,11] For example, an isotropic polycrystalline Fe0.8Al0.2 alloy doped with 2 and 3 pct of boron reached total magnetostrictions values of 77.5 and 97.6 ppm, respectively. These values are 103 and 155.5 pct higher than the Fe0.8Al0.2 undoped polycrystalline alloy.[10] Boron atoms combine with Fe atoms to form the Fe2B phase, which segregate at the grain boundaries. At M.B.S. DIAS, G.O. FULOP, C.A. BALDAN, and C. BORMIONUNES are with the Escola de Engenharia de Lorena, Depto. de Engenharia de Materiais, Universidade de Sa˜o Paulo, Lorena, SP, 12602-810, Brazil. Contact e-mail: [email protected] Manuscript submitted April 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the low magnetization level, the magnetostriction differences between the matrix (Fe(Al)-a and/or Fe3Al) and Fe2B phase create a stress field, which causes the improvement of the magnetostriction of Fe-Al-B alloys.[12] Besides that, the Fe2B improves the grain boundary resistance, changing the fracture from intergranular to transgranular cleavage, i.e., increasing the ductility of the Fe-Al alloys.[13] Therefore, the appropriate magnetostrictive properties of Fe-Al alloy could be combined with an improvement of the mechanical deformation behavior. In the present work, (FexAl100 x)98.4B1.6 (x = 86.6, 82 and 79.4)* alloys were rolled down to 0.7 mm of *The (Fe86.6Al13.4)98.4B1.6 outcomes was first published in Reference 14.
thickness and annealed for 2 hours at 1473 K (1200 C) to produce thin samples without deteriorating the magnetic properties. The actuation sensitivities of the alloys with different aluminum contents and after the thermomechanical process were compared using the piezomagnetic coefficient d33 = dk/dH. The application of a material as actuator requires d33 value to be higher than 1 nm/A.[15] The hysteresis loops, the magnetostriction values, and the piezomagnetic coefficient d33 were assessed to evaluate the potential of these alloys to be used for the construction of actuators which requires the magnetostrictive material in the form of thin sheets. This is the
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Alloy B
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Fe(Al)-α Fe3 Al Fe2 B
Intensity (a.u.)
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AC Fig. 1—Picture of the home-build magneto device showing: the specimen, the ferrite cores, and the excitation coil.
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2θ(°) Fig. 3—X-ray diffraction of alloy B at AC, R, and AN condit
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