Temperature Dependence of Magneto-Mechanical Response in Ni-Mn-Ga Magnetic Shape Memory Alloys
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Temperature Dependence of Magneto-Mechanical Response in Ni-Mn-Ga Magnetic Shape Memory Alloys Leon M. Cheng, Garrett Landry, Shannon P. Farrell, Rosaura Ham-Su and Calvin V. Hyatt Defence R&D Canada – Atlantic, Emerging Materials Section 9 Grove Street, P.O. Box 1012, Dartmouth, NS, Canada B2Y 3Z7 ABSTRACT In this work, a systematic investigation is being carried out on single crystals of Ni47.8Mn27.5Ga24.7 alloy to determine the effect of temperature on the magneto-mechanical behaviour of the Ni-Mn-Ga alloys. Repeated mechanical and magnetic forces have been applied at various temperatures below the martensite finish (MF) temperature. It has been observed that twinning start and finish stresses, critical magnetic field and maximum magnetic-field-induced strain all remain almost constant within about 20K below MF and then change substantially at lower temperatures. Eventually no magnetic-field-induced strain can be observed at temperatures below 262K. It is proposed that although magnetic anisotropy constant increases with decreasing temperature, it is not sufficient to overcome the increasing twinning stresses required for twin boundary motion at lower temperatures. INTRODUCTION Ferromagnetic shape memory alloys have received much attention in recent years since Ullakko et al. [1] observed a large reversible, magnetic-field-induced strain in Ni-Mn-Ga alloys. So far, up to 10% reversible magnetic-field-induced strains have been observed in Ni-Mn-Ga alloys with an orthorhombic seven-layered (7M) martensite. Although strains offered by magnetic shape memory (MSM) alloys are similar in magnitude to conventional shape memory alloys, the faster response due to magnetic control makes them more promising for a variety of applications. Unlike the conventional shape memory effect in which the reversible strain is related to the diffusionless structural phase transformation from a high-temperature highsymmetry phase to a low-temperature low-symmetry martensitic phase, it is generally accepted that the magnetic shape memory effect is due to the rearrangement of twin variants in the martensite by an applied magnetic field leading to an overall change of shape [1-8]. The main thermodynamic driving force for twin boundary motion in the presence of a magnetic field is the high magneto-crystalline anisotropy of the low-symmetry martensitic phase. Low twin boundary energy, high magnetocrystalline anisotropy energy and saturation magnetization are some key factors for large magnetic field induced strain. In this work, a systematic investigation is being carried out on single crystals of Ni-Mn-Ga alloy to determine the effect of temperature on the magneto-mechanical behaviour of the Ni-Mn-Ga alloys. Repeated mechanical and magnetic forces have been applied at various temperatures below the martensite finish (MF) temperature.
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EXPERIMENTAL Single crystals of Ni-Mn-Ga were extracted from ingots that were prepared by directional solidification using a seedless Bridgman crystal growth method. The alloys were grown in a h
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