Search

The magnetomechanical behavior of ferromagnetic shape memory alloys such as Ni-Mn-Ga, and hence the relationship between structure and nanoscale magnetomechanical properties, is of interest for their potential applications in actuators. Furthermore, due to its crystal structure, the behavior of Ni-Mn-Ga is anisotropic. Accordingly, nanoindentation and magnetic force microscopy were used to probe the nanoscale mechanical and magnetic properties of electropolished single crystalline 10M martensitic Ni-Mn-Ga as a function of the crystallographic c-axis (easy magnetization) direction relative to the indentation surface (i.e., c-axis in-plane versus out-of-plane). Load-displacement curves from 5–10 mN indentations on in-plane regions exhibited pop-in during loading, whereas this phenomenon was absent in out-of-plane regions. Additionally, the reduced elastic modulus measured for the c-axis out-of-plane orientation was ∼50% greater than for in-plane. Although heating above the transition temperature to the austenitic phase followed by cooling to the room temperature martensitic phase led to partial recovery of the indentation deformation, the magnitude and direction of recovery depended on the original relative orientation of the crystallographic c-axis: positive recovery for the in-plane orientation versus negative recovery (i.e., increased indent depth) for out-of-plane. Moreover, the c-axis orientation for out-of-plane regions switched to in-plane upon thermal cycling, whereas the number of twins in the in-plane regions increased. We hypothesize that dislocation plasticity contributes to the permanent deformation, while pseudoelastic twinning causes pop-in during loading and large recovery during unloading in the c-axis in-plane case. Minimization of indent strain energy accounts for the observed changes in twin orientation and number following thermal cycling. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) of magnetic shape-memory alloys (MSMAs) allows fuller use of geometry in the design of MSMA parts and avoids the segregation and high cost associated with single crystal production. While most research effort in AM of MSMAs pursues functional foams or polycrystals, epitaxial growth during liquid-phase AM may enable fully-dense single-crystalline MSMA parts, with associated availability of the full blocking stress. We melted a Ni 51 Mn 24.4 Ga 24.6 single crystal with a moving laser spot under several process parameter combinations of laser power and velocity. While tracks created with lower laser travel velocity were almost entirely epitaxial, the track created with highest velocity (10 mm/s) included non-epitaxial columnar grains and grains at the top of the track. Synchrotron-based high-energy diffraction microscopy (HEDM) experiments revealed that mosaic spread of epitaxial material was slightly higher than that of surrounding non-re-solidified material. Our results demonstrate epitaxial growth of Ni-Mn-Ga with minimal grain content using full-melting laser processing. [Display omitted] [ABSTRACT FROM AUTHOR]

Deformation rearranges the crystal lattice in magnetic shape memory alloys, which changes all anisotropic properties of the material. This study investigates leveraging the deformation-induced change of magnetic permeability for a strain measurement technique. A Ni-Mn-Ga single crystal placed inside a doubly wound coil with a primary and a secondary winding was used as a strain sensor. An AC voltage excited the primary coil and the secondary voltage was measured as the sample was strained from 0 to 5.2%. This method varies from other methods that utilize complex magnetic circuits, require high magnetic fields, or other sensing methods such as Hall probes. When the sensor element was tested statically by compressing the element manually against a bias magnetic field perpendicular to the load axis, the voltage output varied from 129.7 mV to 164.2 mV. The dynamic performance of the sensor was tested by cycling the element between 25 and 100 Hz in compression against a bias magnetic field in a displacement controlled magneto-mechanical test system. The bias magnetic field was varied from 0.2 to 0.8 T (0.16 to 0.64 MA/m) while the cyclic displacement was varied from 0.5 to 4.5% strain. The voltage amplitude of the signal in the secondary coil increased with decreasing tensile strain. The full scale RMS voltage at a 200 mm stroke increased from 53.0 mV to 78.4 mV as the bias magnetic field decreased from 0.8T to 0.2 T. As the element was compressed, there was no difference in the sensor output voltage between the static and dynamic tests. When the element expanded during unloading, the voltage output of the sensor from the static test matched the voltage output during compression. For the dynamic testing, the voltage output of the sensor exhibited a hysteresis from the loading voltage output, the hysteresis increased when the strain rate increased. [ABSTRACT FROM AUTHOR]

Small monocrystalline beams of the magnetic shape-memory alloy (MSMA) Ni–Mn–Ga, with a square 1×1 mm 2 cross section and length between 2 and 10 mm, with the 10M martensite structure and all faces parallel to {100}, were subjected to rotating magnetic fields while being held at one end. The beams deform by both magnetic-field-induced straining (MFIS) and magnetic-torque-induced bending (MTIB), in directions parallel and perpendicular to the beam’s longitudinal axis, respectively. With the field parallel to the beam axis, the beams were straight and short. Upon field rotation, the beam elongated and bent in the direction of the field. When the field reached 90°, the beam deflected rapidly and took a bent shape oriented in the opposite direction. Upon further field rotation, bending strain and axial strain decreased until the beam was short and straight again with the field at 180°. With an increase in beam aspect ratio, the bending component increases while the total axial strain remains constant. MTIB – a natural but so far neglected response of long MSMA samples exposed to a transversal magnetic field – occurs during switching of current linear (one-dimensional) actuators, thus causing friction losses, wear, and fatigue. However, MTIB provides the opportunity to actuate high aspect ratio MSMA continuously and smoothly in all directions (in three dimensions), thus mimicking slender biological actuating structures such as microorganism flagella tails and fins of fish, heart valves, leaves and petals of plants, and wings of birds or insects. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic shape memory alloys display magnetic-field-induced strain (MFIS) of up to 10% as single crystals. Polycrystalline materials are much easier to create but display a near-zero MFIS because twinning of neighboring grains introduces strain incompatibility, leading to high internal stresses. Pores reduce these incompatibilities between grains and thus increase the MFIS of polycrystalline Ni–Mn–Ga, which after training (thermo-magneto-mechanical cycling) exhibits MFIS as high as 8.7%. Here, we show that this training effect results from a decoupling of struts surrounding pores in polycrystalline Ni–Mn–Ga during the martensitic transformation. To show this effect in highly textured porous samples, neutron diffraction measurements were performed as a function of temperature for phase characterization and a method for structure analysis was developed. Texture measurements were conducted with a magnetic field applied at various orientations to the porous sample, demonstrating that selection of martensite variants takes place during cooling. [Copyright &y& Elsevier]