Mechanical and magnetic behavior of oligocrystalline Ni-Mn-Ga microwires

Ni-Mn-Ga wires with sub-millimeter diameter, either as individual wires or as part of 2D or 3D wire assemblies, are promising candidates for actuators, sensors, magnetic cooling systems and energy harvesting devices. Here, we report on Ni-Mn-Ga wires with 200-400 μm diameter fabricated by the Taylor method, followed by annealing at 1050 °C for 1 h to achieve grain growth. The mechanical behavior of these oligocrystalline wires with bamboo grains was studied by tensile tests at room temperature. Wires with martensitic structure exhibited a very low Young's modulus of 5-8 GPa and superelastic behavior with twinning stresses of 22-30 MPa and maximum recoverable strain of 3.5%, and accumulated a plastic strain of ∼0.6% over 8 consecutive 0-50 MPa mechanical loading cycles. A 1 T rotating magnetic field caused the wire to bend to a curvature corresponding to a surface strain of 1.5%. During a full-field revolution, the wire deflected back and forth twice when the field direction was approximately perpendicular to the average wire direction. This effect was attributed to magnetic-torque-induced bending (MTIB) enabled by twinning.