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Here we report a systematic research on effects of Fe and Cu upon properties relevant for the magnetic shape memory effect of Ni–Mn–Ga ferromagnetic shape memory alloys. Fe and Cu were identified as elements with potential synergism to increase the martensite transformation temperature of Ni–Mn–Ga magnetic shape memory (MSM) alloys. Eighteen Ni–Mn–Ga–Fe–Cu alloys with different systematic trends in substituting the ternary elements with Cu and Fe have been investigated. We found a method to describe the effectiveness of Ni, Mn, and Cu upon raising the martensitic transformation temperature, lowering the saturation magnetization, and varying the Curie temperature. We find the martensite transformation temperature most influenced by the Ni content, followed by Mn, with a smaller effect of Cu. The saturation magnetization decreases with similar coefficients for Mn and Cu alloying. The Curie temperature monotonously decreases with Mn, but not Cu. The 10M martensite structure is stable for the composition Ni46.5Mn25+XGa25−X−YFe3.5CuY with X and Y range of 0–5.7, and 0.8–3.0. Used in combination with the total e/a, the elemental e/a-ratio gives some insight into the complex behavior of quinary MSM alloys and is a useful method of analyzing MSM alloys for improved functional properties.

Funding Information: We gratefully acknowledge the support of the Aalto University seed funding for the DEMINFUN-project ( 974110 ). Frans Nilsén also acknowledges (in part) support by Operational Program Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 - CZ.02.1.01/0.0/0.0/16_019/0000760 ). Publisher Copyright: © 2021 The Authors Laser powder bed fusion (L-PBF) is used to produce foam-like Ni-Mn-Ga with tailored microscale and mesoscale features. Ni50-Mn28.2-Ga21.8 (at%) powder was gas atomised and processed in an L-PBF system with a range of energy density from 26.24 and 44.90 J/mm3. We characterised microscale and mesoscale properties, such as the chemical composition, crystal structure, magnetisation measurements, density, and porosity measurements as a function of process parameters, in a systematic design of experiment. Preliminary research on macroscale properties included tensile testing and magnetic field induced strain (MFIS) measurements. Results show how controlling process parameters allows tailoring the Ni-Mn-Ga polycrystalline microstructure. Hence, obtaining twinned martensitic structures with a predominant orientation going across the visible grain boundaries. All the processed samples showed a 56 Am2/kg magnetisation level, close to Ni-Mn-Ga 10 M single crystals. Mesoscale results show a distinctive porosity pattern that is tailored by the process parameters and the laser scanning strategy. In contrast, macroscale mechanical tensile test results show a brittle fracture of Ni-Mn-Ga due to the high porosity with yield stress 2–3 times higher than shown in single crystals. In sum, we built geometrically complex demonstrators with (i) microscale twinned martensitic structures with a predominant orientation going across the visible grain boundaries and (ii) mesoscale tailored periodic porosity patterns created by modifying power, scanning speed, and scanning strategy systematically. L-PBF demonstrates great potential to produce foam-like polycrystalline Ni-Mn-Ga, reducing grain boundary constraints and thus the magnetic force needed for MFIS.

We investigate experimentally the universality of the temperature dependences of twinning stress of type Ⅰ and type Ⅱ twins in Ni50Mn28Ga22 five-layered modulated martensite. Single crystals from different producers are compared to distinguish the universal behaviour and the effects of crystal quality. The twinning stress is both measured directly and calculated from magnetisation loops. In the vicinity of martensite transformation, the twinning stress is about the same for both types of twins. For type Ⅰ twins, it increases linearly with decreasing temperature with the slope of -0.045 MPa/K. For type Ⅱ twins, the twinning stress is almost temperature independent, but it increases rapidly near the intermartensite transformation temperature. In the crystal, which does not undergo an intermartensitic transformation and exhibits type Ⅱ twins, the magnetically induced reorientation of martensite is observed down to 2 K. Chemical inhomogeneity and mosaicity of the crystal results in mixed type twin boundaries with temperature dependences deviating from the universal behaviour of type Ⅰ and type Ⅱ twins.

A new approach for the processing of porous crystalline Ni-Mn-Ga compacts by spark plasma sintering (SPS) from a mixture of gas atomized Ni-Mn-Ga and NaCl powders (25/75 vol%) is reported. The NaCl was removed to reveal a highly porous skeleton of Ni-Mn-Ga. Solid skeletons were heat-treated in vacuum to grow the grains to a size of 22–23 μm, from the 1.7 μm found in the powder. The sample with the highest porosity showed a magnetic-field-induced strain of 1.24%. The MFIS is attributed to magnetically induced twinning due to the reduction of grain boundary constraints and the formation of preferred orientation.

Increasingly, metal parts made by additive manufacturing are produced using powder bed fusion (PBF). In this paper we report upon the combined effects of PBF parameters, including power and scan speed, in layer-by-layer manufacturing of gas atomized non-modulated (NM) Ni-Mn-Ga alloy. The effects of process parameters upon PBF is studied by applying nine different parameter sets in the as-printed state and after homogenization and ordering. The chemical composition of the samples is analyzed using EDX attached to an SEM, and the crystal structures are determined by X-ray diffraction. The phase transformation temperatures are measured using a low-field ac susceptibility measurement system and the magnetic properties are measured with a vibrating sample magnetometer (VSM). Before the heat-treatment, all as-printed samples showed paramagnetic behavior with low magnetization and no phase transformations could be observed in the susceptibility measurements. After annealing, the samples recovered the ferromagnetic behavior with comparable magnetization to annealed gas atomized powder. The as-printed samples were composed of a mixture of different crystal structures. However, after annealing the original NM structure with a = b = 5.47 Å and c = 6.66 Å with a c/a -ratio of 1.22 was recovered and crystallographic twins could be observed in an SEM.

Anatase TiO2 nanoparticles were synthesized from sol–gel processing, and they were used as a precursor for titanate nanotubes (TNT) formation. TNT were synthesized under reflux heating of anatase TiO2 in concentrated NaOH solution followed by repeated washing with distilled water and 0.1 M HCl. The nanotubular structure was preserved till 450 °C, above which nanorod formation started. The as-synthesized nanotubes were found to have mixed crystal structure of anatase and Na x H2−x Ti3O7·nH2O (where 0

Two double dispersion composites were prepared by pulsed electric current sintering (PECS) from two powders on Ni–Mn–Ga–Co/WC–Co system. Composite 1 consists of 90 vol% WC–12Co and 10 vol% of non-modulated Ni–Mn–Ga–Co martensite and Composite 2 of 18 vol% of WC–12Co and 82 vol% of 10 M Ni–Mn–Ga–Co martensite. Damping capabilities of both composites were studied with DMA and cavitation erosion resistance tests. Both composites show promise as candidates as damping element materials when high hardness and wear resistance is required but Composite 2 had a higher damping capability. The high damping ability of composites are proposed to be derived from the combined effect of energy dissipation by twin boundary movement in the Ni–Mn–Ga matrix combined with the high hardness and wear resistance arising from the tungsten carbides.

Magnetically controlled hybrid Ni-Mn-Ga composites are potential candidates for actuation and damping applications. The combination of ductile polymer and gas atomized large grained Ni-Mn-Ga powder has many advantages compared to bulk single crystals. These advantages include ease of manufacturing and freedom of shape, while still being magnetically controllable. In this report, Ni-Mn-Ga-epoxy hybrid composite structures are manufactured at three different filling ratios 25, 30 and 35 vol-% and damping properties of the composites are compared to those of 5M Ni-Mn-Ga single crystal. The damping properties are characterized using a laboratory made high-frequency dynamic mechanical testing instrument and a dynamic mechanic analyzer (DMA) in single cantilever mode. The mechanical cycling experiments revealed that the damping ability of the Ni-Mn-Ga composites depends on the filling ratio. The magnetic field induced stiffening observed in the mechanical cycling experiments of the single crystal sample at 100 Hz correlated roughly with that of the composite sample having filling ratio of 35 vol-%.

Gas atomization is a well-known process for obtaining high quality metallic powders. It produces spherical high-density particles with a controllable particle size distribution. We used gas atomization to obtain two different Ni-Mn-Ga powders and investigated their microstructure, transformation temperatures and magnetic properties as well as the effects of heat treatment on those. The phase and magnetic transformation temperatures of the powders were considerably lower as compared to similar conventionally prepared alloys. Nevertheless, heat treatment of the atomized powder was found to recover the magnetic properties of the materials, probably by releasing the residual strains generated by the rapid solidification during the gas atomization process. Sintering of powders seemed to start at a significantly lower temperature when compared to the powder prepared by ball milling of bulk Ni-Mn-Ga-alloys.

Martensitic Ni-Mn-Ga based alloys are known for the Magnetic Shape Memory (MSM) effect, which upon application of an external magnetic field can generate a strain up to 12 % depending on the microstructure of the martensite. The MSM effect occurs by rearrangement of the martensite variants, which is most advantageous in single crystals. Single crystals are, however, rather tedious to produce and there has been attempts to achieve MSM effect in polycrystals. However, in polycrystals the magnetic field induced shape change remains low as compared to single crystals. As an alternative to the former, hybrid MSM materials offer several advantages. When compared to single crystals, hybrids have extended freedom of shaping, lower raw material price, relatively large MSM strain and easier manufacturability. Embedding MSM particles into a suitable polymer matrix results in actuation function or good vibration damping performance. In the present study we report on the mechanical, structural and magnetic properties of MSM polymer hybrids, which are prepared by mixing gas-atomized Ni-Mn-Ga MSM powder into epoxy matrix and aligning the magnetic particles in a magnetic field.