Magnetic-field-induced bending and straining of Ni-Mn-Ga single crystal beams with high aspect ratios

Small monocrystalline beams of the magnetic shape-memory alloy (MSMA) Ni-Mn-Ga, with a square 1 x 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 degrees, 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 degrees. 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. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.