Giant magnetostriction of ferromagnetic shape memory alloys with the grain boundary controlled by rapidly solidified method

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The propose of this study as the developments of multi-functional sensor/actuator material, whish is useful as complex, small robots and micro-machines. Ferromagnetic shape memory alloys(FSMA), in which giant strain is caused by re-arrangements of the martensitic variants by applying magnetic field, are new type of actuator material rapidly responded to magnetic field, while, thermo-elastic shape memory alloys respond to heat slowly. In order to obtain the FMSA with good controlled texture, a rapidly solidified method was applied to Fe-Pd,Fe-Pt,Heusler-type Co_2NiGa and Fe-Ga alloys.1)Fe-29.6at%Pd,Fe-23at%Pt alloys : Although a single crystal and bulk samples have martensite phase only under room temperature, rapidly solidified ribbons exhibit giant magnetostnction of 1800(Fe-29.6at%Pd) and 420(Fe-23at%Pt> at〜373 K. The origin is due to that the ribbons are caused by strongly [100] oriented columnar microstructure with the size of grain, 1〜3μm. Especially, the grains of about 1μm size consist of fine layer-structure of 30-40nm thickness. These nano-scale layers are parallel to the columnar structure, which is stress-induced martensite twin. The nano-scale martensite twins make phase-transformation temperature increase to〜373 K.2)Co_2NiGa alloy : Heusler-type Co_2NiGa as well as Ni_2MnGa, which exhibits giant magnetostnction at low temperature, is rapidly solidified in order to develop a FSMA. The ribbon sample exhibits large magnetostnction and good shape recovery at temperature range of 350〜400K.3)Fe-17at%Ga alloy : a single crystal with large magnetostriction of 300ppm is much bnttle. The rapidly solidified ribbon has ductility and exhibits large magnetostnction of 200ppm.