Advances in Science and Technology Vol. 59

Abstract: In the shape memory alloys (SMAs) the thermal triggering induces reversible dimensional change by the phase transformation – these materials may also be ferrior ferromagnetic, however, here only the ferromagnetic SMAs are discussed. In certain SMAs the austenitemartensite phase transformation is influenced by the magnetic field as either austenite or martensite is promoted by the field and this is exploited for the dimensional changes. However, in the magnetic shape memory (MSM) alloys no phase transformation occurs as the remarkable dimensional changes take place by the twin variant changes in the martensitic phase activated by the external magnetic field at constant temperature. In addition to the phase transformation or magnetic shape memory effect, the applied magnetic field may also result in the conventional magnetostriction (MS), enhance the superelasticity (magneticfieldassisted superelasticity MFAS) or induce the giant magnetocaloric effect (GMCE). Certain alloys such as NiMnGa may even be multifunctional showing more than one of these effects. The present paper gives an overview of the different types of the magnetically activated SMA alloys, their properties as well as their potentials for applications in the frameworks of the recent studies.

Abstract: Introduction Ni-Mn-Ga films were grown on (110) MgO substrates by r.f. sputtering and pulsed laser deposition techniques. Texture analyses, in combination with electron microscopy measurements, reveal that the films from the two deposition techniques both have a cubic L21 structure, but differ in terms of their texture and grain morphology. The sputtered films grow along the <110> direction with completely random in-plane orientation. The PLD films, on the other hand, grow in the <422> direction such that there exists two distinct grain-types, where both types share an epitaxial relationship with the substrate. In general, the overall quality of the PLD films is better than the sputtered films. The growth conditions that influence film morphology and texture are discussed.

Abstract: In this work a 50Ni50Ti at % powder mixture, commercially pure, prepared by mechanical alloying in an attritor with the following conditions: the milling speed and the ball charge were 1500 rpm and 15:1 respectively. The milling time was 1h under a nitrogen atmosphere at room temperature. After milling it was determined the particles size distribution, the phases by Xray diffractions (XRD) and the powder morphology by scanning electron microscopy (SEM). The powders after milling were compacted and hot extruded at 600 °C with an extrusion ratio of 3 to 1 and characterized by evaluation the density and microstructural determination by optical microscopy. The obtained results are discussed to show that this route could be an alternative route to prepare the shape memory alloy.

Abstract: It is well known that FePd alloys are effective as a magneto-thermoelastic actuator material, because they have large magnetostriction and shape memory effect. In order to use the alloys for a micro-actuator, magnetic properties and microstructures have been examined as for rapidly solidified Fe-29.6 at% Pd alloy ribbons. The ribbons exhibit a large magnetostriction at room temperature and good shape memory effect. Magnetostriction and coercive force of the ribbons markedly depend on the direction of the applied magnetic field. Maximum values of magnetostriction and coercive force are obtained at θ = 85 degree (θ is the angle between the magnetic field and the ribbon plane). Relief effects corresponding to the formation of FCT martensite variants are observed on the grains. X-ray diffraction profile at room temperature shows that FCT martensitic phase and FCC parent phase coexist in the ribbon. Dense striations are observed in the TEM bright field images of FCT martensite plates. Selected area electron diffraction patterns revealed the striations to be thin twins.

Abstract: In order to fabricate two-dimensional micro actuators with shape memory alloy films, it is especially important to evaluate the anisotropy of transformation strain that is caused by texture. In this paper, microstructures of sputter-deposited TiNi films are examined. The films of 1 μm in thickness are sputter-deposited on Si(001) substrates by RF magnetron multi-sputtering system equipped with four separate confocal sources as well as with substrate heating. Pure Ti and Ni targets of 50 mm in diameter are used for the sources. The films deposited at ambient temperature have been generally amorphous. However, we find that some films which are deposited at 773K of substrate temperature are crystalline, when we appropriately choose sputtering parameters such as source voltage and the distance between a target and the substrate. X-ray powder diffraction and pole figure measurements reveal that these films are oriented with {110}B2 parallel or inclined at 45 degree to the substrate. Furthermore, we also find that crystallized film is deposited even at 673K of substrate temperature by applying pulse bias voltage to the substrate.

Abstract: Power supplies are often the limiting factor for operation of many portable electronic devices. Batteries contribute significantly to their weight and impose limitations on operational longevity. Harvesting vibratory energy from the environment for conversion to electrical energy has been proposed as a means to address these problems. Previously, DRDC Atlantic has shown that single crystals of nickel-manganese-gallium (NiMnGa) can produce large reversible stressinduced and magnetic field-induced strains of up to 10%. It has been proposed that NiMnGa magnetic shape memory alloys may be an ideal alloy to harvest mechanical energy. The drawback to monolithic NiMnGa crystals is that they are susceptible to intergranular fracture. To address this issue, a composite of the alloy in a polymer matrix has been examined in the hopes of improving toughness and formability. Good bonding between the polymer and the powder is needed to facilitate optimum transfer of force between the two components. The objective of this study was to understand and optimize the polymer-alloy interactions. The polymer matrix chosen was Dow Corning Sylgard 186. It was determined that the Sylgard 186 prepolymer base interacts with the oxidized surface of the NiMnGa particles. Silane coupling agents were also investigated to examine their effect on the interfacial interactions. No change in properties were observed.

Abstract: Shape memory alloys (SMA) exhibit functional properties associated with the shape memory effect, responsible of the SMA shape recovery after a cycle of deforming-heating and of a simultaneous generation of mechanical work. Composite systems incorporating SMA wires have the ability to actively change shape and other structural characteristics. The functional properties of such adaptive composites are related to the martensitic transformation in the SMA elements and to the constraining behaviour that the composite matrix has on the SMA wires. In this work the behaviour of a shape memory alloy hybrid composite (SMAHC) is numerically and experimentally investigated. A plate was fabricated using prestrained SMA wires embedded in an epoxy resin pre preg glass fibres composite system. Upon calorimetric and mechanical material characterization, a finite element model was used in order to predict the structural behaviour of the SMAHC. In the experimental tests, the plate was clamped at one side and actuated via electrical heating. Temperature and displacement data were collected and compared with the prediction of the finite element model. The results show that the model is able to capture the shape change in the actuation region, although a thorough description of the SMAHC behaviour requires further modelling work, including the simulation of the SMA loading history during composite manufacturing.

Abstract: In order to better understand the unique functional responses of shape memory alloys, improve the currently existing SMA modeling tools and used them beneficially in smart structure applications, it is desirable to investigate the deformation/transformation processes in these materials in action – i.e. under stress and temperature variation. In this work, an overview is presented on the applications of various recently developed or originally employed in-situ experimental methods and approaches to martensitic transformations in SMAs.