Ferromagnetic Shape Memory Alloys

Development of Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory alloys have been attracting much attention since the discovery of large magnetic-field-induced strain of several percent in Ni-Mn-Ga alloy[1, 2]. This strain was attributed to the motion of the martensitic twins, and is different from the conventional (magnetoelastic) magnetostriction. Thus the material is an attractive candidate for a new class of magnetic actuator materials. Since then, various ferromagnetic shape memory alloys, e. g., Fe-Pd[3], Fe-Pt[4], Co-Ni-Al[5, 6], Co-Ni-Ga, [6, 7] have been investigated. Apart from the practical interests, these materials also offer an excellent opportunity to investigate the various aspects of phase transformations and microstructural formation because magnetic and structural phase transformations can be realized in a single system.In Ni-Mn-Ga alloys, which is one of the prototypical ferromagnetic shape memory alloys, a high temperature cubic phase of this alloy has the L21 ordered structure, also known as the Heusler structure. The alloy has relatively high Curie temperature (~363 K), and martensitic transformation temperature can be controlled by changing chemical composition of the alloy. Also in this system, phonon behavior is of particular interests. The phonon softening preceding martensitic transformation has been observed for a number of shape memory alloys, such as Ni-Al[8], Au-Cd[9], and Ti-Ni[10]. In these alloys this is apparent as a dip at a particular wavelength in the TA2 branch in the phonon dispersion curves, which corresponds to the {110}<1-10> type lattice displacements. The dip becomes significant as the temperature approaches the phase transformation temperature. The wave number corresponding to the dip is not necessarily related to the modulation period in the low temperature martensite phase. In the case of Ti-Ni[10], the phonon softening is complete at the R-phase transformation temperature, while in Ni-Al, this softening is incomplete. In Ni-Mn-Ga, this softening is very different from other alloys; the phonon energy reaches maximum at the temperature 30 K above the martensitic transformation temperature and then increase as the temperature approaches Ms [11]. This is also reflected in some other physical properties, such as, the internal friction [12] and low field magnetization [13]. The state corresponding to the minimum phonon energy is referred to as the intermediate phase.We have been investigating the various aspects of phase transformation behavior, microstructures and effect of fourth elements [14-15] in Ni-Mn-Ga and Fe-Pd alloys

Surface relief of martensite phase of Ni-Mn-Ga alloy.

Reference
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