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Theses

High frequency magnetic field-induced strain of ferromagnetic shape memory alloys

Abstract : Ferromagnetic Shape Memory Alloys (FSMAs) have ability to provide large high-frequency reversible strain via magnetic field-induced martensite reorientation. But, the high-frequency frictional twin boundary motion of the martensite reorientation can induce a rapid accumulation of dissipation heat and cause a significant temperature rise in the material, which poses instability problems about the dynamic performance of FSMA. Particularly, the output strain amplitude would be reduced significantly when the temperature increases to be high enough to trigger the Martensite-Austenite phase transformation. However, such thermal effect on the dynamic responses of FSMA has not yet been investigated in literature where most existing dynamic experiments were performed only for a short-time period (a few seconds) to avoid the temperature variation. In this thesis, multi-scale experimental and theoretical analyses of the long-time performance of FSMA under high-frequency magnetic actuation are performed. Systematic experiments of the long-time magnetic actuation (> 100 seconds) on a Ni-Mn-Ga single crystal bar are conducted at various levels of magnetic field frequency, initial compressive stress and ambient airflow (ambient heat-exchange efficiency) to investigate their influences on the stable state of the high-frequency FSMA-actuator. A one-dimensional heat-transfer model is developed and the new experimental phenomena of the thermal effects are well understood. Based on the experimental results and theoretical analysis, critical conditions to achieve the large and stable output strain amplitude in the high-frequency actuation are derived. Moreover, to understand the heat-exchange dependence of the output nominal-strain from a microscopic view, the local strain distribution/evolution and the associated transformation/reorientation among the different phases/variants during the high-frequency actuation under various heat-exchange efficiencies are demonstrated via the in-situ Digital Image Correlation observations. A novel mechanism is revealed: the temperature-driven phase boundary motion (phase transformation) and the magnetic field-driven twin boundary motion (martensite reorientation) can be activated at the same time under the magneto-thermal-mechanical actuation (i.e., the high-frequency magnetic field, the mechanical spring force and the varying ambient airflow) as the material can self-organize its volume fractions of the different phases/variants to satisfy all the thermo-magneto-mechanical boundary conditions. Further, the self-organized morphology/pattern composed of various variants and phases during cyclic deformation (with the moving habit plane and twin boundaries) can be explained by microstructure compatibility analyses.
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  • HAL Id : tel-02880025, version 1

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Shaobin Zhang. High frequency magnetic field-induced strain of ferromagnetic shape memory alloys. Mechanics of the solides [physics.class-ph]. Université Paris Saclay (COmUE), 2018. English. ⟨NNT : 2018SACLY011⟩. ⟨tel-02880025⟩

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