Stress Wave Propagation in Ni–Mn–Ga Ferromagnetic Shape Memory Alloys

Abstract

Ni–Mn–Ga ferromagnetic shape memory alloys (FSMAs) are a prominent candidate for actuation application due to their large magnetic field induced strain (MFIS). MFIS in these alloys are mainly due to the twin variant reorientation. This can be achieved only above the magnetic threshold, which requires a large electromagnet. The higher threshold magnetic field for magnetic actuation can be reduced by the assistance of stress waves. The present work is focused on the stress wave propagation in Ni–Mn–Ga alloys. It has been observed that the stress amplitude is high in the frequency range 2 to 5 kHz and is found to be dependent on the input voltage supplied to the actuator which gives out stress wave. The advantage is that, in this frequency range, the threshold field for magnetic actuation can be reduced. It is noted that the stress amplitude is higher in the Ni–Mn–Ga polycrystal (456 mV) than the Ni–Mn–Ga single crystal (385 mV) for the 30 V input peak to peak voltage (Vpp). The finding of this work supports the Ni–Mn–Ga alloy polycrystals for actuation applications with the assistance of stress waves.