Browsing by Subject "magnetic shape memory alloys"
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Item Magnetic field-induced phase transformation and variant reorientation in Ni2MnGa and NiMnCoIn magnetic shape memory alloys(2009-05-15) Karaca, Haluk ErsinThe purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm?3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.Item Synthesis of NiCoMnX (X = In, Al) Heusler-type Magnetic Shape Memory Alloy Thin Films(2014-08-13) Rios, Steven EliMagnetic shape memory alloys are a class of shape memory alloys, and therefore exhibit a thermoelastic martensite phase transformation between symmetric and asymmetric crystalline states induced by appropriate temperature and/or stress changes. Shape memory alloys are able to recover strain when stress is applied, which can generate higher actuation forces and displacements compared to piezoelectrics and magnetostrictive materials when the material is constrained. While shape memory alloys have found applications in biomedical and aerospace industries, actuator applications are limited to relatively low frequencies compared to piezoelectric materials. The slow response of shape memory alloys is associated with heating or cooling the material from an external source. Compared to traditional shape memory alloys, the coupling of structural and magnetic ordering result in magnetic and structural transformations that increase the functional properties in magnetic shape memory alloys, such as magnetic field-induced rapid martensite transformation (forward and reversed), giant magnetoresistance, and the magnetocaloric effect. While bulk MSMAs can be used for structural components, in many cases MSMA thin films are preferred for device applications, such as miniaturized actuators, small scale propulsion devices, and micro-electro-mechanical systems (MEMS). This thesis focuses on the synthesis of NiCoMnX (X=In, Al) Heusler-type magnetic shape memory alloy thin films via physical vapor deposition, and details the challenges associated with controlling film composition, precipitation, microstructure, residual stress, and mechanical properties. As-deposited films were found to contain a mixture of amorphous and nanocrystalline microstructure, and thus, did not exhibit a martensitic transformation. Appropriate post-deposition heat treatments were required to crystallize the films, tailor the grain size, and reduce the formation of precipitates. Crystallized films exhibited martensitic transformations that showed a grain size dependence. An analytical model that uses a thermodynamic framework was developed to explain the suppression of the martensitic transformations for films with submicron-sized grains. Hence, in addition to chemical composition, sub-micron grain size can be used to tailor the martensitic transformation temperature of NiCoMnX (X=In, Al) thin films for device applications. Additionally, the analytical model may reduce the uncertainty associated with a direct scale-up of thin film compositions used for combinatorial investigations of magnetic shape memory alloys.Item The Effect of Crystallographic Orientation and Thermo-mechanical Loading Conditions on the Phase Transformation Characteristics of Ferromagnetic Shape Memory Alloys(2011-02-22) Zhu, RuixianThe effects of crystallographic orientation, temperature and heat treatment on superelastic response of Ni45Mn36.5Co5In13.5 single crystals were investigated. Superelastic experiments with and without various magnetic field were conducted under compression on a custom built magneto-thermo-mechanical test setup. Magnetostress, which is the difference in critical stress levels for the martensitic transformation with and without magnetic field, was determined as a function of crystallographic orientation, heat treatment and temperature parameters. Magnetostress of [111] crystals was observed to be much higher than that of [001] crystals with same heat treatment. Water quenched samples have the highest magnetostress among other samples with the same orientation that were oil quenched and furnace cooled. Crystal structure and atomic ordering of the samples were examined using Synchrotron High-Energy X-Ray Diffraction to rationalize observed differences. Magnetostress levels were also traced at various temperatures. A Quantum Design superconducting quantum interference device (SQUID) was utilized to examine the magnetic properties of the material. The difference in saturation magnetization at various temperatures was analyzed to explain the temperature effect on magnetostress. Calculations based on the energy conversion from available magnetic energy to mechanical work output were used to predict the magnetic field dependence of magnetostress, which provides a guideline in material selection for the reversible magnetic field induced martensitic phase transformation. Isothermal superelastic response and load-biased shape memory response of Co48Ni33Al29 single crystals were determined as a function of temperature and stress, respectively. The aim of the work is to provide a new direction to understand the anomaly of transformation strain and hysteresis for ferromagnetic shape memory alloys. Thermo-mechanical behavior of Co48Ni33Al29 single crystal was determined by a custom built thermo-mechanical compression setup based on an electromechanical test frame made by MTS. Transformation strain was observed to decrease with increasing applied stress in isothermal tests or increasing temperature in superelastic experiments. The variation in the lattice constant in martensite and austenite was verified to account for such a trend. It was also discovered that both thermal and stress hysteresis decreased with increasing applied stress and temperature, respectively. Multiple factors may be responsible for the phenomenon, including the increase of dislocation, the compatibility between martensite and austenite phase.