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

Magnetically controlled shape memory effect in Ni2MnGa intermetallics

Materials that develop large strokes under precise and rapid control exhibit a great potential in mechanical engineering. Actuators made from those kinds of materials could replace hydraulic, pneumatic, and electromagnetic drives in many applications. However, no such materials are available to date. Piezoelectric and magnetostrictive materials exhibit rapid response, but their strokes are small. In shape memory alloys, strokes are large, but their control is slow due to thermomechanical control. Magnetic control of the shape memory effect was recently suggested by the present author for a principle of new kinds of actuator materials. These materials can develop strains of several percent, and their control is rapid and precise. Actuation of these materials is based on the reorienting of the twin structure of martensite or the motion of austenite-martensite interfaces by applied magnetic field. In the present report, magnetically induced motion of the austenite-martensite interfaces is demonstrated in an Fe-33.5Ni alloy.

Magnetically controlled shape memory alloys: A new class of actuator materials

Magnetically driven actuator materials, such as the ternary and intermetallic Heusler alloys with composition , are studied within the density-functional theory (DFT) with the generalized gradient approximation (GGA) for the electronic exchange and correlation. The geometrical and electronic structures for the magnetic structure are calculated. The structures and magnetic moments at equilibrium are in good agreement with the experimental values. The structural trends with varying X and Y are explained by a d-occupation model, while a rigid-band model can account for the trends with changing M.

/pdf/structural-properties-of-magnetic-heusler-alloys-38j87unhno.pdf

Structural properties of magnetic Heusler alloys

Summary form only given. Magnetic shape memory materials are expected to have potential for a variety of actuating devices and sensors. Magnetic-field-induced rearrangement of the crystallographic domains (twin variants) can produce a large strain similar to a stress-induced one. We have found a giant magnetic field-induced strain approximately 10% at ambient temperature in a magnetic field less then 1 T in NiMnGa seven-layered martensitic phase. The strain is contributed by twin boundary motion which was confirmed by different experimental methods. From the analysis of X-ray diffraction data it was found that crystal structure of this phase is nearly orthorhombic having lattice parameters at ambient temperature a=0.619 nm, b=0.580 nm and c=0.553 nm (in cubic parent phase coordinates). The magnetic anisotropy properties of this phase were determined on the single-variant constrained samples using the magnetization curves M(H) recorded along [100], [010] and [001] directions. We demonstrate that low twinning stresses and a high level of magnetic anisotropy energy are the critical factors for the observation of a giant magnetic field induced strain.

Crystal structures and magnetic anisotropy properties of Ni-Mn-Ga martensitic phases with giant magnetic-field-induced strain

Magnetically controlled shape memory materials are a new w ay to p roduce motion and force. The material changes s hape in a magnetic field. Shape c hanges are e ven l0 p ercent and response time is less than a millisecond. Usual applications of the material are ac tuators that produce linear motion. In this article several magnetic shape memory actuators are presented and their properties and advantages are discussed.

K. Ullakko

Papers

Magnetically controlled shape memory effect in Ni2MnGa intermetallics

Magnetically controlled shape memory alloys: A new class of actuator materials

Structural properties of magnetic Heusler alloys

Crystal structures and magnetic anisotropy properties of Ni-Mn-Ga martensitic phases with giant magnetic-field-induced strain

Basic properties of magnetic shape memory actuators