Pulsed magnetic field-induced twin boundary motion on Ni-Mn-Ga

Abstract

The magnetic field-induced strain (ferromagnetic shape memory effect - FSME) in Ni-Mn-Ga was first reported in 1996 by Ullakko et al. Since then, up to 6% FSME in single-crystal tetragonal-Ni-Mn-Ga samples has been observed in static fields, and up to 3% at 500 Hz. The present work demonstrates 6% FSME of a Ni-Mn-Ga single crystal of 5 x 5 x 9.85 mm³ in 200[mu]s is by application of a magnetic field pulse. It proves the feasibility of actuators operating at frequencies above of 1 kHz at room temperature for this geometry, and that the actuation can be accomplished using compact, air-core Helmholtz coils operated in pulsed mode. The eddy-current attenuation of 620 [mu]s-long pulses in the samples tested is small, reducing the need for lamination. The field-induced extension does not begin at the same time as the field. Part of the delay is the time that the field takes to reach the threshold level for actuation. The mass-inertia of the sample results in an additional delay, which depends on the position and number of mobile twin-boundaries in the crystal. The delay is maximum for a single twin-boundary moving from the fixed to the free end of the crystal. For several twin-boundaries distributed uniformly throughout the crystal the delay is shorter. The peak acceleration observed is 50 ± 10 m/s². For typical twin-boundary energies of the order of 40 erg/cm² homogeneous nucleation of partial dislocations was found to be unlikely. Accordingly, twin-boundaries must be seeded through stress. High-speed video images and photographs have demonstrated that field-induced twin-boundary motion is not uniform along a Ni-Mn-Ga single crystal. Twin boundaries stop when they reach certain positions of the crystal, and remain pinned unless the field is increased. The observed scatter in the data of field-induced extension is related to the existence of pinning sites. The maximum rate of extension can be expressed as an exponential function of the driving force, andreaches 6 m/s for saturated driving force in the present case.