Work Output Enhancement of Ferromagnetic Shape Memory Micro Actuators

Ferromagnetic shape memory (FSM) alloys are a class of materials which are both ferromagnetic and capable of undergoing a structural phase transformation. FSM alloys have significant advantage over conventional shape-memory temperature-based actuators because they can be remotly actuated by fast alternating magnetic fields. Therefore, FSM alloys attract keen attention as promising candidates for a variety of MEMS applications, as they can provide large strokes using small components. The most commonly used FSM alloy is Ni2MnGa and its off-stoichiometric alloys, which are used in commercial cm-scale FSM actuator. However, at the current stage, no experiments of the magneto-mechnical behavior of micro-scale actuators were conducted. Overall, the behavior of FSM alloys involves motion of twin boundaries and is significantly influenced by its microstructure. Based on a theoretical model, we have shown that down-scale specimens have finer twin boundary microstructure that consequently may increase the blocking stress characteristic such that it will enhance the output work for actuation. In light of this, a novel experimental method was realized to establish this conjecture and to provide comprehensive information on the behavior of small actuators. A series of tests demonstrated no actuation strain reduction up to extraordinary loads of 10MPa, and thus paves the route for engineering FSM high-power micro actuators by controlling their microstructure.