A review of shape memory alloy research, applications and opportunities

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Simple rules for the understanding of Heusler compounds

Large magnetic-field-induced strains1 have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field1,2,3. These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy4. We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni45Co5Mn36.7In13.3 single crystal.

Magnetic-field-induced shape recovery by reverse phase transformation.

Giant magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase. The strain proved to be caused by magnetic-field-controlled twin boundary motion. According to an analysis of x-ray diffraction data, the crystal structure of this phase is nearly orthorhombic, having lattice parameters a=0.619 nm, b=0.580 nm, and c=0.553 nm (in cubic parent phase coordinates) at ambient temperature. Seven-layer shuffling-type modulation along the (110)[110]p system was recorded. The results of mechanical tests and magnetic anisotropy property measurements are also reported.

Giant magnetic-field-induced strain in NiMnGa seven-layered martensitic phase

Strains of nearly 0.2% have been induced along [001] in unstressed crystals of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K. These stains are associated with the superelastic motion of twin boundaries in the martensitic phase that is stable below about 274 K.

Large magnetic‐field‐induced strains in Ni2MnGa single crystals

A room temperature extensional strain of 5.1% was observed in martensitic Ni/sub 48/Mn/sub 31/Ga/sub 21/ alloy in the magnetic field of 480 kA/m. The magnitude of field-induced strain decreases with increasing external compressive stress applied in the direction of expansion. The compressive stress of about 3 MPa prevents the development of the substantial field-induced strain. Magnetization curves obtained by VSM exhibit an abrupt magnetization change and a transient hysteresis in the first quadrant. Large reversible field-induced strain and the abrupt magnetization change are due to the rearrangement or redistribution of martensitic twin variants by the applied magnetic field. It was confirmed by optical observation of movement and nucleation of martensitic twin boundaries.

Giant field-induced reversible strain in magnetic shape memory NiMnGa alloy

The crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered (5M) and seven-layered (7M) martensitic phases, exhibit the martensitic transformation temperature (TM) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, TM and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or TM has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni–Mn–Ga system has been confirmed.

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K. Ullakko

Papers

Large magnetic‐field‐induced strains in Ni2MnGa single crystals

Giant magnetic-field-induced strain in NiMnGa seven-layered martensitic phase

Giant field-induced reversible strain in magnetic shape memory NiMnGa alloy

Composition and temperature dependence of the crystal structure of Ni–Mn–Ga alloys

Magnetic-field-controlled twin boundaries motion and giant magneto-mechanical effects in Ni–Mn–Ga shape memory alloy