Twin boundary dynamics in magnetic shape memory alloy Ni-Mn-Ga five-layered modulated martensite

Abstract

Ni-Mn-Ga magnetic shape memory (MSM) alloys are considered to be highly promising materials for actuation, damping, sensing and energy harvesting applications, due to the unique combination of a large transformation strain of ~6–12%, high mobility of twin boundaries (TBs) and fast, sub-millisecond response to magnetic fields. The investigation of TB dynamics in these alloys is of great importance, since the TB mobility determines the dynamic properties of MSM-based devices. However, there are only few studies on TB dynamics in Ni-Mn-Ga, having contradictory results. In these works, based on different displacement measurement techniques, only estimations of average twin boundary velocity were made. In addition, the previous studies dealt with uncontrollable TBs type and twin variants arrangement, or multiple TBs motion.

In the present work, for the first time, the TB dynamics in Ni50Mn28Ga22 five-layered modulated martensite was investigated on the level of single TB motion induced by a pulsed magnetic field, combined with direct velocity measurements. A unique experimental setup based on laser Doppler vibrometry technique was developed to precisely measure transient velocities of single TBs propagating through the sample, and high-speed camera imaging was used to directly observe the TB motion. The results demonstrate that primarily inertia and internal friction affect the TB motion and limit the TB velocity, whereas magnetic field-dependent resistance forces such as magneto-static forces, torques and eddy currents play only a negligible role in TB dynamics, up to applied magnetic fields of 2.5 T.

A macroscopic model was developed to describe the variable-mass problem of single TB motion. The model was applied to extract the motion parameters from the measured velocity data and for simulation of the experimental results at macro- and microscale, for bulky millimetre-sized samples and micropillars. It was found that the observed velocity limitations arise from a strong dependence of twinning stress (TS) on the TB velocity. For type 1 TB, the TS drastically increases from its quasi-static value of ~0.6 to ~3 MPa limiting the TB velocity to 3-7 m/s. For type 2 TBs, a gradual increase of the TS from ~0.1 MPa to ~2 MPa was observed with TB velocities reaching 33-39 m/s. The results obtained for Ni-Mn-Ga micropillars demonstrate a possibility of microscale MSM-based devices fabrication having working frequencies of the order of 100 kHz. The presented experimental observations and concepts differ from the known kinetic relation approach.

The results are important for the MSM-based applications development, especially for high frequency and high-speed actuation.