NiMnGa-based magnetic shape memory (MSM) alloys have attained magnetic-field-induced strains up to approximately 10%, making them very attractive for a variety of applications. However, for applications that require the use of an alternating magnetic field, eddy current losses can be significant. Also, NiMnGa-based MSM alloys' fracture toughness is relatively low. Using these materials in the form of particles embedded in a polymer matrix composite could mitigate these limitations. Since the MSM effect is anisotropic, the crystallographic texture of the particles in the composites is of great interest. In this work, a procedure for fabricating NiMnGa-based MSMA/elastomer composites is described. Processing routes for optimizing the crystallographic texture in the composites are considered.
In the current work, repeated mechanical and magnetic forces have been applied to Ni-Mn-Ga samples with different compositions and different thermomechanical histories in order to determine the combined effects of these parameters on the magnetic shape memory effects, especially the magneto-mechanical properties, of these alloys. The results demonstrate that prior history has strong influence on the twinning start stress and twinning strain. In addition, heat treatment of the materials seems to increase the amount of strain that can be obtained (up to the theoretical limit). Moreover, there is indication that prior heat treatment may also affect the martensite crystal structure that is formed during cooling. In addition, the dependence of martensitic transformation on composition and prior thermomechanical treatments was also studied by differential scanning calorimetry (DSC) analysis.
In order to understand the solidification behavior of Ni-Mn-Ga alloys, ingots with different compositions were prepared by arc melting. Two series of compositions were investigated: Ni100-2xMnxGax (15≤x ≤30) and Ni50Mn50-yGay (0≤y≤50). The microstructures obtained were observed and the compositions of the phases occurring in the ingots were identified by energy dispersive spectroscopy in the scanning electron microscope. Based on these observations, three solidification paths were identified: direct solidification of γ-Ni from the liquid, direct solidification of β-NiMnGa from the liquid, and solidification of β-NiMnGa phase via a peritectic reaction. It was found that the γ-Ni liquidus surface covers a large area of the ternary phase diagram. The γ-Ni liquidus boundary is located between Ni50Mn25Ga25 and Ni45Mn27.5Ga27.5 in the equal Mn and Ga alloy series, and between Ni50Mn5Ga45 and Ni50Mn10Ga40 in the 50 at.% Ni alloy series. The alloys with compositions close to the stoichiometric Ni2MnGa composition that show the magnetic shape memory effect are all covered by the γ-Ni liquidus surface. The β-NiMnGa liquidus surface covers the remaining alloy compositions.
The magnetic shape memory (MSM) effect occurs in some ferromagnetic martensitic alloys at temperatures below the martensite finish temperature and involves the re-orientation of martensite variants by twin boundary motion, in response to an applied stress and/or magnetic field. The driving force for twin boundary motion is the magnetic anisotropy. In this study, magnetization measurements as a function of magnetic field were made on several oriented single crystals of Ni-Mn-Ga alloys using a vibrating sample magnetometer. The magnetization versus magnetic field curves were characteristic of magnetically soft materials with magnetic anisotropy consistent with literature estimates for the different martensite structures observed in Ni-Mn-Ga alloys. Differences in the slope of the curves were due to the martensite structure, the relative proportion of martensite variants present, and their respective easy and hard axis orientations. Thermo-magneto-mechanical training was applied in an attempt to transform multi-variant specimens to single variant martensite. Training of the orthorhombic 7M martensites was sufficient to produce a near single variant of martensite, while the tetragonal 5M martensite responded well to training and produced a single-variant state. The strength of the uniaxial magnetic anisotropy constant for single-variant tetragonal 5M martensite, Ni52.9Mn27.3Ga19.8, was calculated to be Ku=1.8 x 105 J/m3, consistent with literature values. To obtain single-variant martensites, heat-treatment of the specimens prior to thermo-magneto-mechanical training is necessary.
Magnetostrictive materials such as Terfenol-D are increasingly being considered for demanding applications such as active noise damping, sonar devices and reactive structures, due to their large strain capability. A limiting factor for the use of magnetostrictive material slies in their inherent susceptibility to brittle fracture. The present study applies finite element technology in support of experimental investigations to assess the mode II fracure toughness of magnetostrictive materials. In this exploratory effort, the fully coupled non-linear behavior of the material is not considered. Rather, a methodology for converting the applied magnetic field to an equivalent mechanical load, based on the material's magnetostrictive properties, is devised and applied. The DSA-VAST finite elemtn software is emlpoyed to model the cylindrical, pre-cracked test specimen using both conventional solid elements and enriched twenty-noded solid fracture elements. Two load cases are investigated, namely one in which a mechanical load is applied to the specimen in the absence of a magnetic field, and a second case in which both a magnetic field and a mechanical load are applied to the specimen. In the absence of an applied magnetic field, the mode II fracture toughenss is found to be approximately 4.497 MPa√m, a value comparable to that reported for ceramic-like materials. On the other hand, in the presence of an applied magnetic field (simulated by an equivalent compressive prestress), the mode II fracture toughness is reduced to 2.768 MPa√m, a significant reduction from the 'zero-field' value. FE results indicate significant specimen bending and an appreciable mode III component to the fracture behavior, both of which are consistent with observed crack growth patterns in laboratory specimens.
Magnetic shape memory (MSM) alloys give recoverable strain when subjected to an applied magnetic field. The strongest MSM effect has been observed in single crystals. The magnitude of the effect and the consistency of behavior over the entire volume of a sample is strongly dependent on the solute and phase distributions in crystals. Samples of stoichiometric and off-stoichiometric Ni2MnGa magnetic shape memory alloys were directionally solidified by a seedless Bridgman method using different rates of growth. The growth conditions used resulted in oriented polycrystals exhibiting a coarse cellular structure. Significant macro-segregation was observed, with the top of the ingot enriched in Mn and the bottom enriched in Ga. Micro-segregation also occurred, resulting in Mn-rich intercellular eutectic or eutectoid structures, and coarse intra- and inter-cellular Mn-rich particles. An increase in the pulling rate during the directional solidification process resulted in finer cellular and eutectic / eutectoid structures, as well as finer particles.
The martensite transformation temperatures of both as-grown and heat-treated specimens removed from a Bridgman grown boule of off-stoichiometric Ni2MnGa were determined by differential scanning calorimetry (DSC) and hot/cold stage microscopy. The work showed that martensite start and austenite finish transformation temperatures determined by the hot/cold stage microscope technique were in agreement with those determined by the DSC method. The hot/cold stage microscope technique was shown to be useful for characterizing variations of transformation temperature across a specimen. The results revealed that residual stress, deformation and boule composition variations produce artefacts in DSC traces which need to be identified, understood and controlled. Transmission electron microscope results suggest that the possible contribution of a premartensitic transformation to the high temperature edge of the martensite peak on DSC scans needs further investigation.
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