Magnetic Field Induced Strain in Polycrystalline Magnetic Shape Memory Foam

Magnetic shape memory alloys (MSMA) are fascinating materials that show a recoverable shape change in a rotating magnetic field. Single crystalline MSMA’s display magnetic-field-induced strains (MFIS) up to 10%. However, single crystals have inherent drawbacks such as cost and chemical segregation during production. Polycrystalline materials are easier to produce and display chemical homogeneity but display a much smaller MFIS than single crystals. It has been shown recently that adding porosity to polycrystalline Ni-Mn-Ga (i.e. metal foam) can increase MFIS. Variables that affect the performance of polycrystalline Ni-Mn-Ga foam include phase transformation temperature, pore architecture, spatial distribution of pores, porosity, training, and magnetic anisotropy/texture. Samples were tested for MFIS and phase transformation temperatures to probe for a correlation. Single pore foam architecture with a mono-modal pore size distribution and dual pore foam architecture with a bi-modal pore size distribution were compared in terms of microstructure and magneto-mechanical behavior. Pore distributions were characterized with x-ray tomography and compared with the temperature dependent MFIS, to deduce the role of the large and small pores. Samples were systematically etched and tested for MFIS to investigate the effect of porosity on strain. Magneto-mechanical, thermo-magnetic, and thermo magneto-mechanical training effect on MFIS was also investigated. The results are discussed in terms of a concept of a network of struts (bridging metal) with hard and soft links. Where, hard links are struts that are unable to deform. It was found that increasing porosity increased strain, confirming the hypothesis that porosity is responsible for enhanced MFIS. The porosity strain relationship indicated strut thickness is a crucial factor in determining the strain, i.e. the thicker the strut the “harder the link.” The dual pore foam has much smaller struts and therefore has fewer hard links. Pore distribution affected the number and distribution of hard links. The metal is more compliant when the sample temperature approaches the phase transformation temperature. Therefore, samples with transformation temperatures close to the testing temperature contain softer links and produce more MFIS. Hard links can also be softened by training. For optimal MFIS and fatigue resistance foams of dual pore architecture with a spatially homogenous distribution of pores, high porosity ( 65-70%) and a martensitic phase transformation temperature close to testing temperature should be employed. Foams with such optimized microstructures and chemical homogeneity are expected to perform reproducibly and consistently

Recent Developments in Ni-Mn-Ga Foam Research

Grain boundaries hinder twin boundary motion in magnetic shape-memory alloys and suppress magnetic-field-induced deformation in randomly textured polycrystalline material. The quest for high-quality single crystals and the associated costs are a major barrier for the commercialization of magnetic shape-memory alloys. Adding porosity to polycrystalline magnetic-shape memory alloys presents solutions for (i) the elimination of grain boundaries via the separation of neighboring grains by pores, and (ii) the reduction of production cost via replacing the directional solidification crystal growth process by conventional casting. Ni-Mn-Ga foams were produced with varying pore architecture and pore fractions. Thermo-magnetic training procedures were applied to improve magnetic-field-induced strain. The cyclic strain was measured in-situ while the sample was heated and cooled through the martensitic transformation. The magnetic field-induced strain amounts to several percent in the martensite phase, decreases continuously during the transformation upon heating, and vanishes in the austenite phase. Upon cooling, cyclic strain appears below the martensite start temperature and reaches a value larger than the initial strain in the martensite phase, thereby confirming a training effect. For Ni-Mn-Ga single crystals, external constraints imposed by gripping the crystal limit lifetime and/or magnetic-field-induced deformation. These constraints are relaxed for foams

Development of an Optical Device for Magneto-Mechanical Experiments

Current magneto-mechanical experiments measure the macroscopic magnetic-field-induced strain (MFIS) of magnetic shape-memory alloys (MSMA) by measuring the deformation of a sample in one direction. The macroscopic strain is the result of reorientation of the lattice of the MSMA due to an applied magnetic field or mechanical deformation. The lattice is reoriented by moving twin boundaries (TB). To study the motion of individual twin boundaries, an optical magneto-mechanical device (OMMD) was built. Optics, lighting, and a high resolution camera (21 Mega pixels) were put into a custom built apparatus that allows the entire setup to be rotated. While the sample is aligned with the optics and camera, the apparatus is put between the pole pieces of a strong electro magnet with a magnetic field strength up to 2 T. Using the image analysis software “Machine Vision” of National Instruments, the live footage can be analyzed in real time to calculate strain values in two dimensions. Furthermore, the individual TB movements can be recorded and paired with positional data of the sample relative to the magnetic field using a rotational encoder connected in line with the OMMD. By using a modular design, the OMMD can easily be put onto a heating/cooling stage where the phase transformation, especially the martensite-austenite interface, can be studied without an applied magnetic field. Future modifications include combining the heating/cooling stage with the rotating OMMD apparatus for in situ phase transformation experiments in a magnetic field as well as the addition of optical filters and a quarter wave plate to enhance optical contrast

Effect of Pore Architecture on Magnetic-Field-Induced Strain in Polycrystalline Ni–Mn–Ga

Monocrystalline Ni–Mn–Ga alloys show magnetic-field-induced strains (MFIS) of up to 10% as a result of reversible twinning; by contrast, polycrystalline Ni–Mn–Ga shows near-zero MFIS due to strain incompatibilities at grain boundaries inhibiting twinning. Recently, we showed that porous polycrystalline Ni–Mn–Ga exhibits a small, but non-zero, MFIS value of 0.12% due to reduction of these incompatibilities by the porosity. Here, we study the effect of pore architecture on MFIS for polycrystalline Ni–Mn–Ga foams. Foams with a combination of large (550 μm) and small (80 μm) pores are fabricated by the replication method and exhibit thinner nodes and struts compared to foam containing only large (430 μm) pores. When magnetically cycled, both types of foams exhibit repeatable MFIS of 0.24–0.28% without bias stress. As the cycle number increases from a few tens to a few thousands, the MFIS drops due to damage accumulation. The rate of MFIS decrease is lower in the dual-pore foam, as expected from reduced constraints on the twin boundary motion, since twins span the whole width of the thinner nodes and struts

Effects of Surface Damage on Twinning Stress and the Stability of Twin Microstructures of Magnetic Shape Memory Alloys

Twinning is the primary deformation mechanism in magnetic shape memory alloys (MSMAs). Obstacles such as inclusions, precipitates and defects hinder or even prevent twin boundary motion in the bulk of Ni–Mn–Ga MSMA single crystals. Here, we study the effect of surface damage on the mechanical properties and twin structure of Ni–Mn–Ga single crystals. Any methods that produce defects may be considered for modifying the near-surface microstructure. In this study deformations were produced by grinding and mechanical polishing using abrasive particles. The amount of damage was characterized with X-ray diffraction: damage causes peak broadening. Deformation and damage localized near the surface increases the twinning stress. Surface damage stabilizes a densely twinned microstructure. The twins are thin but extend over the entire sample and allow a large strain to be accommodated at moderate stress. This effect is critical for preventing damage accumulation in high-cycle magnetomechanical actuation and for achieving high dynamic performance

Superelasticity and Shape Memory Effects in Polycrystalline Ni-Mn-Ga Microwires

Non-stoichiometric Ni49.9Mn28.6Ga21.5 microwires with a diameter of 30-80 μm and cellular grain size of 1-3 μm were developed by melt-extraction. The superelasticity and shape memory effect of the microwires were investigated on a Dynamic Mechanical Analyzer (DMA Q800). The onset critical stresses for austenite to martensite transformation increase linearly with temperature and can be described by Clausius-Clapeyron relationship. The martensite to austenite transformation occurs upon unloading occurs only at temperatures at least 11 °C higher than the martensite start temperature. The as-extracted microwire was loaded in its martensite state to a strain of 2.7% without breaking and 92% plastic deformation strain recovers upon heating

Giant Magnetic-Field-Induced Strains in Polycrystalline Ni–Mn–Ga Foams

he magnetic shape-memory alloy Ni–Mn–Ga shows, in monocrystalline form, a reversible magnetic-field-induced strain (MFIS) up to 10%. This strain, which is produced by twin boundaries moving solely by internal stresses generated by magnetic anisotropy energy1, 2, 3, 4, can be used in actuators, sensors and energy-harvesting devices5, 6, 7. Compared with monocrystalline Ni–Mn–Ga, fine-grained Ni–Mn–Ga is much easier to process but shows near-zero MFIS because twin boundary motion is inhibited by constraints imposed by grain boundaries8, 9, 10. Recently, we showed that partial removal of these constraints, by introducing pores with sizes similar to grains, resulted in MFIS values of 0.12% in polycrystalline Ni–Mn–Ga foams11, close to those of the best commercial magnetostrictive materials. Here, we demonstrate that introducing pores smaller than the grain size further reduces constraints and markedly increases MFIS to 2.0–8.7%. These strains, which remain stable over \u3e200,000 cycles, are much larger than those of any polycrystalline, active material

Texture and Training of Magnetic Shape Memory Foam

Magnetic shape memory alloys display magnetic-field-induced strain (MFIS) of up to 10% as single crystals. Polycrystalline materials are much easier to create but display a near-zero MFIS because twinning of neighboring grains introduces strain incompatibility, leading to high internal stresses. Pores reduce these incompatibilities between grains and thus increase the MFIS of polycrystalline Ni–Mn–Ga, which after training (thermo-magneto-mechanical cycling) exhibits MFIS as high as 8.7%. Here, we show that this training effect results from a decoupling of struts surrounding pores in polycrystalline Ni–Mn–Ga during the martensitic transformation. To show this effect in highly textured porous samples, neutron diffraction measurements were performed as a function of temperature for phase characterization and a method for structure analysis was developed. Texture measurements were conducted with a magnetic field applied at various orientations to the porous sample, demonstrating that selection of martensite variants takes place during cooling

Magnetic-Field-Induced Recovery Strain in Polycrystalline Ni–Mn–Ga Foam

Recently, we have shown that a polycrystalline Ni–Mn–Ga magnetic shape-memory alloy, when containing two populations of pore sizes, shows very high magnetic-field-induced strain of up to 8.7%. Here, this double-porosity sample is imaged by x-ray microtomography, showing a homogenous distribution of both pore populations. The orientation of six large grains—four with 10M and two with 14M structure—is identified with neutron diffraction. In situ magnetomechanical experiments with a rotating magnetic field demonstrate that strain incompatibilities between misoriented grains are effectively screened by the pores which also stop the propagation of microcracks. During uniaxial compression performed with an orthogonal magnetic bias field, a strain as high as 1% is recovered on unloading by twinning, which is much larger than the elastic value of \u3c0.1% measured without field. At the same time, repeated loading and unloading results in a reduction in the yield stress, which is a training effect similar to that in single crystals