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Low twinning stress (TS) is a prerequisite for magnetic shape memory functionality in ferromagnetic martensites. We compare Ni50Mn28Ga22 (nominal at.%) single crystals from four different producers to reveal the effect of crystal quality on the TS. Near the reverse martensite transformation, the TS is generally low, about 1 MPa, regardless of mosaicity of up to 1.7° and chemical composition deviations of up to 2 at.% of Mn. Pure type I and type II twin boundaries occur in crystals with smooth chemical composition gradients. The corresponding temperature dependences of TS follow universal linear trends with the slopes of −0.045 MPa/K for type I twins and −0.001 MPa/K for type II twins, enabling a very low TS down to 2 K for the latter. An intermediate slope of −0.023 MPa/K appears for mixed type I/type II twin boundaries in a crystal with sharp local chemical deviations. We conclude that the crystal quality affects the magnitude of the TS indirectly, through its impact on the character of the twin boundaries. The effect is weak near the martensite transformation temperature and strong at low temperatures.

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.

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-%.

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.

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.