Magnetic Controlled Shape Memory Research Articles

Structure and Martensitic Transformation in Rapidly Solidified CoNiAlFe Alloy

Housler based magnetic controlled shape memory alloys are characterized by a large magnetic field induced strain. The strain was dependent on the twin martensite structure rearrangement, and the rapid solidification technology had a significant influence on the microstructure, physical, and chemical properties of the alloy. Thus, the structure and the martensitic transformation changes of Co33Ni31Al27Fe9 during the rapidly solidified process were studied. The microstructure of Co33Ni31Al27Fe9 with furnace cooled and rapid solidification (RS) constitutes a dual-phase structure, β phase and γ phase in a low cooling rate and martensite and γ phase in a high cooling rate. The γ phase at the grain boundaries reduced and became more fragile by raising the RC value. The one-step austenite-martensite phase transformation occurred during the process of heating and cooling. The phase transition temperature presents an increasing trend by rising the cooling rate, even to over the room temperature. Moreover, the martensite structure in Co33Ni31Al27Fe9 constitutes a typical L10-type twinning structure.

Hysteresis Curve Fitting Optimization of Magnetic Controlled Shape Memory Alloy Actuator

As a new actuating material, magnetic controlled shape memory alloys (MSMAs) have excellent characteristics such as a large output strain, fast response, and high energy density. These excellent characteristics are very attractive for precision positioning systems. However, the availability of MSMAs in practical precision positioning is poor, caused by weak repeatability under a certain stimulus. This problem results from the error of a large magnetic hysteresis in an external magnetic field. A suitable hysteresis modelling method can reduce the error and improve the accuracy of the MSMA actuator. After analyzing the original hysteresis modelling methods, three kinds of hysteresis modelling methods are proposed: least squares method, back propagation (BP) artificial neural network, and BP artificial neural network based on genetic algorithms. Comparing the accuracy and convergence rate of three kinds of hysteresis modelling methods, the results show that the convergence rate of least squares method is the fastest, and the convergence accuracy of BP artificial neural networks based on genetic algorithms is the highest.

Differential Drive Servo Valve Based on MSMA

Aimed at the contradiction between high frequency response and large flow of servo valve, the paper puts forward a 2D servo valve with differential drive based on magnetic controlled shape memory alloy(MSMA). To realize large flow control, the valve-spool is set of flow increasing orifice to match the flow increasing window on the valve-sleeve. The paper briefly introduces the structural features of the 2D servo valve and the working principle of the differential drive. It also has a simulation analysis on the magnetic field of the drive and the low field of the valve. Simulation results show that, the flow of the valve is linear with the input current.

Large Magnetic Entropy Change and Magnetic-Controlled Shape Memory Effect in Single Crystal Ni46Mn35Ga19

Abstract Physical properties of Ni46Mn35Ga19 single crystal with transition temperatures above room temperature were investigated by various measuring methods. The measuring results of the resistance and ac susceptibility indicate that the magnetic transition and martensitic transformation can occur simultaneously in this alloy. The measurements of the isothermal magnetization show that the increasing rate of magnetic entropy change in this alloy is 9.0 J/kg·K·T. A large magnetic entropy change of 13.8 J/kg·K is obtained under the field of 1.6 T in the single crystal. Furthermore, the strain measurement shows that accompanying with magnetic entropy change, the alloy exhibits a large spontaneous transition strain of −0.89% and a large magnetic-field-enhanced strain of −1.90%.

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.

Behaviour of Ni-Mn-Ga alloys under mechanical stress

In magnetically controlled shape memory (MSM) materials deformation can be controlled by magnetic field. It is shown that the magnetic field induced strain is directly connected with the twinning stress, i.e. mechanical stress needed to transform the sample to single variant state. In cases where the magnetically-induced stress is larger than the twinning stress, a large field-induced strain can be obtained in the magnetic field. The behaviour under mechanical stress of three single crystalline Ni-Mn-Ga alloys with different compositions was systematically investigated. The twinning stresses and deformations of these alloys and particularly their dependencies on temperatures were determined by compression tests. The present study was carried out by compression tests of the alloys in martensitic phase at different temperature. Results showed that twinning stress decreases with increasing temperature in all studied alloys.

Magnetically controlled shape memory effect in Ni 2MnGa intermetallics

Shape memory alloys develop the largest strains of the actuator materials available to date. Their strains can be several percent. The shape memory effect is based on the reorientation of the twin variants of the martensite phase by applied stress. A drawback of the shape memory actuators is, however, their slow response due to thermal control (especially in cooling). Magnetic control of the shape memory effect would lead to much more rapid response of the actuator than the thermal control. When the magnetically controlled shape memory (MSM) materials are subjected to an external magnetic field, the twins in a favorable orientation relative to the magnetic field direction grow at the expense of other twins. This results in the shape change of the material.