Design of ferromagnetic shape memory alloy composites based on TiNi for robust and fast actuators

Recently, ferromagnetic shape memory alloys (FSMAs) attracted strong attentions due to their fast actuation with relatively large strain. However, the mechanical properties of popular ferromagnetic shape memory alloys are found to be lower than TiNi alloys. A TiNi has high mechanical performances, large transformation strain and stress capability. On the other hand, the speed of TiNi alloys actuated by changing temperature is usually slow. Thus use of a ferromagnetic material is attractive in utilizing it for induction of magnetic force at high speed. If we combine both merits of TiNi and ferromagnetic material, we can design a new ferromagnetic SMA composite. The actuation modes examined here are plate bending and bar torsion, where the stress and strain across thickness direction change linearly with position. The outer layers that are subject to the larger strain are the superelastic TiNi, which sandwich the inner layer of a ferromagnetic material. The force activated in the ferromagnetic layer from the applied magnetic field causes the phase change in the superelastic TiNi layer, i.e. from austenite to martensite, thus the softening of stiffness of TiNi leads to large displacement.