Regensburg 2016 – wissenschaftliches Programm

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MA: Fachverband Magnetismus

MA 10: Thyssen-Krupp Electrical Steel Ph. D. Thesis Award (Dissertationspreis)

MA 10.3: Hauptvortrag

Montag, 7. März 2016, 15:50–16:15, H32

Nucleation and Growth of adaptive martensite in epitaxial magnetic shape memory films — •Robert Niemann — IFW Dresden, P.O. Box 270116, 01171 Dresden and TU Dresden, Dept, of Physics, Institute for Solid State Physics, 01062 Dresden, Germany

Magnetic Shape Memory alloys exhibit a reversible phase transformation : a high temperature cubic phase (austenite) turns into a low temperature tetragonal phase (martensite). The Austenite is usual ferromagnetic while the martensite has a complex magnetic order. These materials can be used for large-strain actuator materials and in magnetocaloric regenerators of solid-state cooling devices. The energy efficiency of both applications depends strongly on the microstructure developing during the transformation. A martensitic microstructure that forms with as low elastic energy as possible and which contains only weakly pinned twin boundaries is optimal for application, since it requires only low magnetic fields. We use the model system Ni-Mn-Ga in thin film geometry to study the nucleation and growth of martensite in-situ in the scanning electron microscope and by in-situ-X-ray-diffraction. We compare the results to a kinematic diffraction simulation and continuum theory and explain why this multifunctional material shows such a unique and beautiful microstructure which is essential for its application. Finally, the nucleation barrier is estimated and it is shown that nanoindentation can effectively decrease this barrier. [1]

Supported by DFG FA 453/8 , SPP1239 and SPP1599

[1] R. Niemann, Dissertation, TU Dresden (2015)