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Active materials couple a stimulus (electrical, magnetic, thermal) with a mechanical response. Typical materials such as piezoelectrics strain as bulk materials to the stimuli. Here we consider an undulation created by heterogeneous deformation within a magnetic shape memory alloy (MSM) transducer. We study the mechanical response of an MSM element vs. two surface treatments: a polished state with minimal surface stresses, and a micropeened state with compressive surface stress. The polished element had a sharp-featured, faceted trough shape. The micropeened element had a smooth trough shape and an additional crest. The undulation was created by a rotating localized magnetic field, which caused heterogeneous variation of the twin-microstructure. For the polished and micropeened elements, the twin-microstructures were coarse and fine, respectively. For the polished element, the undulation moved by the nucleation of a few twin boundaries, which traveled along the entire element. For the micropeened sample, the twin boundaries moved back and forth over a short distance, thereby creating a dense twin lamellar, which formed the trough. The motion of the lamellar approximated the single thick twin while allowing additional degrees of freedom due to increased mobile interface density and different initial conditions of domain volume fraction. The dense twin microstructure also smoothed the magnetic flux pattern. The undulation amplitude was about 40 μm for the sample in both treatments. 

The structure of type II twins in 10M Ni-Mn-Ga is modeled using the topological method. This method predicts the same twinning parameters as the kinematic model of Bevis and Crocker. Furthermore, topological modeling provides mechanistic insight into boundary migration rates, the twinning stresses and their temperature dependence. A type II twin is envisaged to form from a precursor, which is its type I conjugate. Disconnections on the precursor k_1 plane align into a tilt wall, which, after the relaxation of the rotational distortions, forms the type II boundary parallel on average to the k_2 plane. The component defects may align into a sharp wall or relax by kinking into a less orderly configuration. Both interfaces can host additional glissile disconnections whose motion along a boundary produces combined migration and shear. The ease of motion of these defects increases with their core width, and this, in turn, decreases with increasing sharpness of the boundary. Some experimental evidence in other materials suggests that type II twins can reduce their interfacial energy by adopting a configuration of low-index facets, which reduces twin boundary mobility. Topological modeling suggests that such a coherently faceted structure is unlikely in 10M Ni-Mn-Ga, in agreement with the high mobility of type II twin boundaries.