Characterization of NiMnGa magnetic shape memory alloys

Abstract

Magnetic shape memory (MSM) alloys are a new class of actuator materials withhigh actuation frequency, energy density and strain and they can be used in themanufacturing of actuators, smart structures, sensors and transducers. NiMnGa alloys experience a reversible martensitic transformation, which is a temperature-dependent phasetransformation from a highly symmetric crystallographic structure (austenite) to lowsymmetry (martensite). These materials are ferromagnetic. Ferromagnetism is aphenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetization. Ferromagnetic metal alloys whose constituents arenot ferromagnetic in their pure forms are called Heusler alloys, named after Friedrich Heusler. Applying a strong magnetic field to some of the Heusler alloys may inducereorientation of martensite variants with high magneto-crystalline anisotropy energy, which leads to a net shape change of the material. In this study, the effect of alloycomposition, cutting direction and heat treatment on the microstructure, local composition,and thermal and dilatometric properties of NiMnGa alloys were investigated.Characterization tests involved various crystals, with and without post-crystal growth heat treatment, by chemical analysis, differential scanning calorimetry (DSC), dilatometry,optical microscopy, scanning electron microscopy and radiography. Metallographic studies showed that as solidified, off-stoichiometric alloys had three distinct microstructural features-a Heusler phase, a Mn rich phase and a eutectic or eutectoid region. Various heattreatment procedures were applied to successfully remove the last phase and produce MSM effect. Heat treatment was also essential for the production of a distinct martensitetransformation in DSC and dilatometry traces and a martensitic transformation to occurover a narrow temperature range. Bulk and microanalysis showed that there are significant concentration variations in the boules grown by the Bridgman method, that lead to changesin phase transformation behavior which were observed by DSC. The presence of composition variations in the boules is a major issue because of its effect on the martensitetransformation temperature. For boules with composition variations, both transformed anduntransformed regions will exist over some temperature range, degrading the performanceof any actuator made from them. Clearly, further effort on the improvement of the crystalgrowth technique is needed to remove the composition gradient and variations and toobtain a fine dendritic structure, which would be much easier to homogenize. For thecurrent growth conditions, coarse cellular structures have been obtained which showsignificant solute segregation. An increase in the thermal gradient during the directionalsolidification process resulted in a finer cellular structure.