NiMnGa Magnetic Shape Memory Alloys Research Articles

Determination of ordinary magnetostriction in Ni–Mn–Ga magnetic shape memory alloy

A saturation magnetostriction of five-layered modulated martensite (5 M) was determined from magnetization curves and by using direct measurements of the field-induced strain of strongly constrained single crystalline sample. The saturation magnetostriction of 5 M martensite was determined to be about 50 ppm. This is about five times smaller than previously reported. The error in determination of magnetostriction due to magnetic shape memory effect and due to the presence of residual martensitic variants with different crystalographic orientation is discussed.

Characterization of NiMnGa magnetic shape memory alloys

Magnetic shape memory (MSM) effect is a new invention in actuator materials field, allowing even 50 times greater strains than in previous magnetically controlled materials (magnetostrictive). In MSM materials the magnetic field moves microscopic parts of the material (so-called twins) that leads to a net shape change of the material. In this study, the effect of alloy composition and heat treatment on the microstructure, local composition, and thermal and dilatometric properties of Ni2MnGa alloys were investigated. The results showed that as solidified, off-stoichiometric, alloys had three distinct microstructural features—a Heusler phase, a Mn-rich phase and a eutectic or eutectoid region. Various heat treatment procedures were applied to successfully remove the last phase. Heat treatment was also essential for the production of a distinct martensite transformation in DSC and dilatometry traces and a magnetic shape memory effect. Composition variations from Bridgman growth were large enough that a shift in martensite start temperature might occur in some parts of the alloy, based on literature data for the dependence of martensite start temperature on composition.

Acoustic emission of Ni–Mn–Ga magnetic shape memory alloy in different straining modes

Abstract We investigated a Ni 49.7 Mn 29.1 Ga 21.2 single crystalline sample exhibiting 5.7% magnetic-field-induced strain (magnetic shape memory effect) by means of acoustic emission (AE) experiments. The AE was monitored during thermally- and stress-induced martensitic transformations and during compressive stress-induced martensite reorientation. Partial cycles were measured for the stress-induced transformation. All processes are acoustically active. The highest AE activity was detected during stress-induced transformation from martensite to austenite upon unloading. Observed difference between AE activity during stress-induced forward and reverse transformations might be attributed to different dynamics of transformation processes. Austenite remains acoustically inactive during loading and unloading as well as during thermal cycling. Martensite exhibits acoustic activity during stress-induced reorientation, during loading and unloading, and lower activity during thermal cycling. This acoustic activity can be tentatively ascribed to twinning processes.

Magnetic properties of stress-induced martensite and martensitic transformation in Ni–Mn–Ga magnetic shape memory alloy

Thermoelastic and strain-induced martensitic transformations of single crystalline Ni 49.7 Mn 29.1 Ga 21.2 magnetic shape memory alloys were investigated by simultaneous detection of the strain and magnetization. The transformation temperature to austenite was T A =314 K. The thermoelastic transformation was shifted to higher temperature with increasing compressive stress with rate about 4 MPa/K. The effect of the moderate magnetic field up to 1 T on transformation was negligible. Very similar magnetic properties of thermally and stress-induced martensites measured in the field up to 1.2 T suggest an equivalence of the phases. Direct comparison of the magnetic properties of austenite and stress-induced martensite at constant temperature above the thermal martensitic transformation is made. At 317 K, the stress-induced martensite had a large uniaxial magnetic anisotropy, K u =1.2×10 5 J/m 3 and the saturation magnetization increased, about 14%, compared with the parental austenite.

Magnetic field-induced stress in the Ni–Mn–Ga magnetic shape memory alloy

Magnetic Shape Memory (MSM) materials generate stress in a magnetic field, which can change the shape of the material. Up to now, little work has been carried out in modeling and measuring the magnetic field-induced (MFI) stress in MSM materials. In the present study, direct MFI stress measurements in a Ni–Mn–Ga MSM material, giving full 6% MFI strain, were performed at different field strengths. The MFI stress was also calculated from the magnetic anisotropy of the material, based on the magnetization curves along the easy and the hard magnetization directions. Both measurement methods are described in detail. The measurement results demonstrated good agreement with the calculated ones. The dependence of the MFI stress on the MFI strain and the magnetic field strength was also revealed. Potential errors during MFI stress measurements in MSM materials are discussed, and avoided by properly selecting the measurement conditions during the study.

Compositional dependence of structure, magnetization and magnetic anisotropy in Ni–Mn–Ga magnetic shape memory alloys

Abstract Magnetic and structural properties of Ni–Mn–Ga polycrystalline and single crystalline alloys were investigated in wide composition range. Depending on the average electron concentration, the structure of room temperature martensite can be five-layered, seven-layered and nonmodulated. Low-temperature saturation magnetization decreased linearly from the peak at stoichiometry due to antiferromagnetic arrangement of excess Mn atoms regardless of the martensitic structure. Magnetic anisotropy of five-layered martensite increases with increasing Mn content.

Temperature variation of structure and magnetic properties of Ni–Mn–Ga magnetic shape memory alloys

When cooling from high temperature, the material of composition Ni 48.5Mn 30.3Ga 21.2 undergoes several magnetic and structural transitions. The phases are, at high temperature austenitic cubic L2 1 phase; at 297 K 5-layered modulated tetragonal martensite; and below 210 K non-modulated tetragonal martensitic phase with c/ a larger than unity. Five-layered martensite exhibits giant field-induced strain of about 6% (magnetic shape memory (MSM) effect). The mechanism of the MSM effect was substantiated by simultaneous measurement of the strain and magnetization.