Reorientation Of Twin Variants Research Articles

Performance of magnetically controlled shape memory alloys and actuators

Magnetically controlled shape memory (MSM) alloys develop large strains when exposed to a magnetic field. Strains are based on reorientation of twin variants in the martensite phase. Magnetic-field-induced reorientation of variants is possible if magnetocrystalline anisotropy energy of the material is sufficiently high and twinning stress is low enough. Among several ferromagnetic shape memory alloys only few of them exhibit so low twinning stress that conversion of twin variants can be controlled by the magnetic field. Such alloys are, e.g., Ni-Mn-Ga, Co-Ni-Ga, Fe-Pd and Fe-Pt. MSM effect was demonstrated in a non-stoichiometric Ni 2 MnGa alloy in 1996, and still the best performance is achieved in these alloys. Magnetic-field-induced strains of 10 % are reached in an orthorhombic phase of this alloy system. In a tetragonal phase free strain is 6 % and the reversible strain against a load of 1.5 MPa is even 5.5 percent. AdaptaMat produces single crystalline Ni-Mn-Ga whose twinning stress is as low as 0.7 MPa. In this material even 70 % of magnetic field energy is converted to mechanical work. Lowest operating temperature of the MSM effect in Ni-Mn-Ga was measured 120 K and the highest temperature is 330 K so far. Several kinds of actuator and sensor devices have been made, e.g., pneumatic valves, positioning devices and vibrators. MSM actuators work at high frequencies (over 2.5 kHz). After 200 million strokes actuators do not show any decrease of the magnetic-field-induced strains. MSM materials are a new way of producing motion and force in mechanical engineering. They are faster than hydraulic and pneumatic devices and over 100 times faster than shape memory alloys. MSM actuators' power output is higher than that of piezoelectric and magnetostrictive devices, voice coil transducers and solenoids.