A Simple Method to Characterize High Rate Twin Boundary Kinetics in Ni-Mn-Ga

Experimental characterization of twin boundary kinetics is essential to systematically test and reproduce the actuation properties of Magnetic Shape Memory (MSM) elements at high rates. Here, we present a simple, nondestructive, experimental method to quantify the dynamic response of an MSM crystal and extract the major material properties that govern its kinetics. The tested sample is subjected to a mechanical pulse that is produced by a simple off-the-shelf solenoid. The mechanical pulse leads to actuation of the tested MSM Ni–Mn–Ga single crystal within 10 ms, during which the twin boundary velocity varies between zero and 2 m/s. The displacement and force in the MSM crystal are measured simultaneously using an optical sensor and a miniature force sensor, respectively. The data captured during a single loading experiment allow plotting a dynamic stress-strain curve as well as a kinetic relation that characterizes the macroscopic response of the crystal. In particular, the obtained kinetic relation enables the extraction of the transition driving force between slow (thermally activated) and fast (athermal) twin boundary motions. This transition driving force is a key material property that governs fast actuation capabilities of MSM elements. The macroscopic behavior of the sample is correlated with the motion of individual twin boundaries within the crystal by adding high speed microscopy to the experimental setup. This allows simultaneous high-rate tracking of individual twinning interfaces in Ni–Mn–Ga.