Performing mechanical testing experiments in all the different martensitic phases of Ni-Mn-Ga under the constant magnetic field applied perpendicular to the load direction we show that such a magnetic field can dramatically modify standard zero field strain stress relationships of MSMA's like Ni-Mn-Ga. In a particular case of 5M and 7M martensites we observe a so-called pseudo-elastic or rubber-like behavior during the standard compression-decompression cycling under the field at about 1T. This effect is finally discussed from the point of the general thermodynamic background and some particular modeling concepts.
Magnetic shape memory materials are expected to have a high potential in practical applications. Several ferromagnetic materials exhibiting the large magnetic-field-induced strain have been found in recent years. The largest field-induced strain is observed in Ni-Mn-Ga system. The most important experimental results on crystal structure, magnetic anisotropy and twinning stress of martensitic phases in Ni-Mn-Ga having tetragonal five-layered, orthorhombic seven-layered and tetragonal non-layered crystal structures are reported. Depending on the martensite crystal structure Ni-Mn-Ga alloys are able to show a really giant strain response (approximately 6% in tetragonal five-layered or 10% in orthorhombic seven-layered martensitic phase) in a magnetic field less than 1 T. Contrary to these two phases, a detectable field-induced strain is not observed in non-layered tetragonal martensitic phase in Ni-Mn-Ga system. Effect of crystal structure is in a good agreement with calculation of the magnetic-field-induced strain based on the model developed by authors. The effect of composition on appearance of undesirable non-layered tetragonal martensitic phase in Ni-Mn-Ga system is discussed based on the new experimental results.
Magnetic-field-induced strain of about 10 percent is reported in Ni48.8Mn29.7Ga21.5 alloy at ambient temperature in a magnetic field order of 1 T. It was confirmed by different experimental methods that the strain is contributed by twin boundary motion. The crystal structure of thermally-induced martensitic phase in this alloy was found to be close to orthorhombic one in temperature range from 245 K to 333 K with lattice parameters a equals 0.619 nm, b equals 0.580 nm, c equals 0.553 nm (relating to the cubic parent phase coordinates) at ambient temperature. More detailed x-ray studies revealed seven-layer shuffling-type modulation along and directions. High magnetic anisotropy properties were found for this phase. The magnetic measurements revealed that the shortest axis (c-axis) is the axis of easiest magnetization, the longest (a-axis) is the axis of hard magnetization, and b-axis is the intermediate one. The orthorhombic phase has low twinning stresses. The compressive stress applied along a-axis of single-variant sample at most 2 MPa is enough to produce approximately 10 percent strain realized by twin boundary motion. The necessary conditions for observation a giant magnetic-field-induced strain in non-stoichiometric Ni2MnGa alloys based on the new experimental data are discussed.
Present article briefly summarizes the mechanism of magnetic shape memory, main modeling principles and most important information about the main structural, magnetic and mechanical properties related to a family of non- stoichiometric Ni-Mn-Ga alloys. We also first consider in details the problems of energy balance, energy losses, optimization of work output and estimation of thermodynamic efficiency for Ni-Mn-Ga based MSMAs.
Magnetically controlled shape memory (MSM) materials are considered now as a potential candidate for a new class of actuators and sensors. Magnetic and mechanical properties of two NiMnGa alloys with different thermally induced martensitic phases have been studied. Five-layered tetragonal martensite (c/a=0.94) in the first alloy is ferromagnetic with easy axis of magnetization. This martensite has low twinning stresses (approximately 2 MPa) and shows a giant magnetic field-induced strain. Non-modulated tetragonal martensite (c/a=1.20) in the second alloy is ferromagnetic with easy plain of magnetization. The absolute value of the magnetic anisotropy constant is approximately twice time higher in the second alloy compare to the first one. After mechanical training of the second alloy, stress at most 15 MPa is enough to produce approximately 19 % strain realized by twin boundary movement. Experimental data and theoretical considerations show, however, that in order to observe a giant magnetic field-induced strain in the second alloy the twinning stresses should be lower.
Present publication gives a detailed report about the experimental results obtained concerning the effect of external constant stress on the magnetic field controlled strain response during the cyclic change of the magnetic field. Simultaneously we represent a brief overview of the most important structural and magneto-mechanical properties of Ni48Mn30Ga22 - family magnetic shape memory alloys. We also discuss the physical mechanism of this effect using our last model developments.
Materials that develop large shape changes in the magnetic field at a short response time provide a new method of producing motion and force in electromechanical devices. The strains are based on the magnetic-field-induced reorientation of the twin variants of the material. This effect is called magnetic shape memory (MSM) effect, because the shape of the material can be controlled by the magnetic field. MSM effect can operate in the martensite phase, and it does not require temperature changes to occur unlike regular shape memory effect (SME). Ni-Mn-Ga alloys and several iron-based alloys are currently being developed by many research groups. In this report, magnetic-field-induced strain of two polycrystalline non-stoichiometric Ni2MnGa alloys with oriented crystal structure were studied during cooling through the martensitic transformation temperature (Ms) and at a constant temperature below Ms. X-ray diffraction measurements confirmed that the origin of the induced strains was the change of the proportions of different twin orientations. Frequency response of the MSM strains in the alloy was shown to be over 5 kHz. One of the present alloys was deposited by laser ablation on a silicon substrate. The thin film exhibits tetragonal lattice structure.
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