2010, Yeduru, S.R., Backen, A., Fahler, S., Schultz, L., Kohl, M.

The phase transformation and superplastic characteristics of free-standing epitaxial Ni-Mn-Ga stripes are reported. The stripes are prepared by micromachining a 1 μm thick Ni-Mn-Ga film sputter-deposited on a single crystalline MgO (100) substrate using optical lithography and a Chromium-based sacrificial layer technology. The stripes are oriented at angles of 0 and 45 degrees with respect to the Ni-Mn-Ga unit cell. Electrical resistance versus temperature characteristics reveal a reversible thermally induced phase transformation between 169°C and 191°C. Stress-strain measurements are performed with the stress applied along the [100]Ni-Mn-Ga as well as [110]Ni-Mn-Ga direction. Depending on the orientation, the twinning stress ranges between 25 and 30 MPa, respectively. For the [100] Ni-Mn-Ga and [110]Ni-Mn-Ga directions, superplastic behaviour with a strain plateau of 12 % and 4% are observed, respectively, indicating stress-induced reorientation of non-modulated martensite variants.

2009, Kühnemund, L., Edler, T., Kock, I., Seibt, M., Mayr, S.G.

To achieve maximum performance in microscale magnetic shape memory actuation devices epitaxial films several hundred nanometers thick are needed. Epitaxial films were grown on hot MgO substrates (500 °C and above) by e-beam evaporation. Structural properties and stress relaxation mechanisms were investigated by high-resolution transmission electron microscopy, in situ substrate curvature measurements and classical molecular dynamics (MD) simulations. The high misfit stress incorporated during Vollmer-Weber growth at the beginning was relaxed by partial or perfect dislocations depending on the substrate temperature. This relaxation allowed the avoidance of a stressinduced breakdown of epitaxy and no thickness limit for epitaxy was found. For substrate temperatures of 690 °C or above, the films grew in the fee austenite phase. Below this temperature, iron precipitates were formed. MD simulations showed how these precipitates influence the movements of partial dislocations, and can thereby explain the higher stress level observed in the experiments in the initial stage of growth for these films. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

2013, Kauffmann-Weiss, S., Kauffmann, A., Niemann, R., Freudenberger, J., Schultz, L., Fähler, S.

Twinning is a phenomenon that occurs, e.g., during deformation, martensitic transformation and film growth. The present study shows that the crystallography of twinning can be described by two twinning modes along the complete Bain transformation path and beyond connecting body-centered and face-centered cubic structures. To probe this concept, we used strained epitaxial films of the Fe-Pd magnetic shape memory system. As the substrate acts as an absolute reference frame, we could show by pole figure measurements that all observed twinning can be a body-centered and face-centered cubic twinning mode. This continuously transforms towards identity when approaching the complementary structure.