Magnetic shape memory alloys, such as the Ni-Mn-Ga Heusler alloy, have been studied intensively for the last twelve years due to the high and reversible magnetic-field-induced strains (MFIS) exhibited by their twinned martensitic structure at room temperature. The large elongation and high cycling speed due to twin variant boundary movement, induced either by magnetic field or by mechanical stress, makes such materials an ideal choice for fast actuators and sensors in applications ranging from active damping elements to medical micropumps. However, the properties of MSM alloys are highly dependent on the chemical composition. The highest reversible MFIS have so far been found only in single crystals, which are troublesome and expensive to manufacture. Polycrystalline structures are easy to manufacture, but they usually have low or non-existent MFIS due to grain boundary constraints and are brittle. Researchers have tried to solve these problems by developing the single crystal manufacturing process so that it is more suitable for industrial scale production, and by reducing the grain boundary constraints by manufacturing magnetic shape memory foams with high porosity and texture. In this work, industrially viable Ni-Mn-Ga hybrid composites are manufactured in laboratory scale and their properties as actuators and vibration damping elements are studied. The first hybrid structure presented is a Ni-Mn-Ga-Co/WC-Co double dispersion metal matrix composite (MMC), with high cavitation resistance and damping properties. Though previous research on Ni-Mn-Ga composite structures has focused primarily on polymer composites, the results from the Ni-Mn-Ga/WC-Co MMC show that it is possible to produce a material with high damping and wear resistance by adding a metal dispersion such as WC-Co into a Ni-Mn-Ga matrix. The next objective was to develop a heat-treatment process with minimal chemical composition change for gas atomized Ni-Mn-Ga powder. This powder was further used to manufacture highly porous spark plasma sintered structures with pronounced MFIS comparable to previously manufactured textured polycrystals. Lastly, the powder was also used to produce cast Ni-Mn-Ga/epoxy composites with magnetically controllable vibration damping properties and pronounced MFIS. The vibration damping properties of these hybrid Ni-Mn-Ga/epoxy composites were found to be higher than previously reported structures, even though pronounced damping is usually associated with disorganized Ni-Mn-Ga/polymer composites. In comparison to other Ni-Mn-Ga powder manufacturing methods, the results from this thesis work show that the heat-treated gas atomized powder method can be used to produce hybrid composite structures with comparable and at times even better properties than with other methods.
|Translated title of the contribution||Magneettisten muistimetalli-pohjaisten hybridimateriaalien käsittely ja ominaisuudet|
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- Ni-Mn-Ga, ferromagnetic shape memory alloys, hybrid composites structures, damping, magnetic-field-induced strain