Ambient effects on the output strain of Ni-Mn-Ga single crystal magnetic shape memory alloy

With proper magneto-mechanical driving forces (e.g., a high-frequency magnetic field plus a mechanical force), Magnetic Shape Memory Alloy (MSMA) can provide a large cyclic deformation (strain oscillation amplitude up to 6%), which makes it a good candidate for high-frequency large-stroke actuators. Moreover, as a kind of shape memory alloys, MSMA's magneto-mechanical coupling behaviors are very sensitive to temperature, which allows researchers/engineers to modify the strain oscillation amplitude for a wider range of applications by controlling the working temperature. In this paper, we report systematic experiments on the thermal-ambient-dependent oscillation amplitude of the magnetic-fieldinduced martensite reorientation-utilizing compressed air (with controlled airflow velocity) passing through the surface of the MSMA specimen (Ni-Mn-Ga single crystal) to control the heat transfer between the ambient and the MSMA specimen during the high-frequency magneto-mechanical loading. It is found that the extremely weak or extremely strong ambient heat transfer can only have small strain oscillation amplitude while the maximum strain amplitude can be achieved only at a mild heat-transfer condition (i.e., non-monotonic dependence of the strain amplitude on the ambient heat transfer). It is also demonstrated that the strain amplitude is closely related to the working temperature, satisfying the balance between the heat generation (from the dissipative strain oscillation of the martensite reorientation) and the heat transfer to ambient (due to the temperature difference between the MSMA specimen and the ambient). With such understanding and constraints, three different schemes for controlling the strain amplitude by the thermal method are proposed and tested for their robustness/ reliability. These results not only provide some guidelines/principles for designing MSMA actuators with flexible strain amplitude, but also demonstrate the delicate dynamics of the thermo-magnetomechanical coupling that demand further theoretical/modelling study. (C) 2020 Elsevier B.V. All rights reserved.