Dresden 2011 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 38: Topical Session Diffussionless Transformations I
MM 38.2: Talk
Thursday, March 17, 2011, 14:30–14:45, IFW D
Transition from modulated to non-modulated martensite in thin Ni-Mn-Ga films — •Anja Backen1,2, Bernhard Holzapfel1, Ludwig Schultz1,2, and Sebastian Fähler1 — 1IFW Dresden, Institute for Metallic Materials, P.O. Box 270116, 01171 Dresden, Germany — 2Dresden University of Technology, Department of Mechanical Engineering, Institute of Materials Science, 01062 Dresden, Germany
In Ni-Mn-Ga, an external magnetic field can induce strains of up to 10% by twin boundary motion. One condition for this MSM effect is a twinning stress of 2 MPa or less, which only the modulated martensite fulfills. The coexistence of 14-layer modulated (14M), non-modulated martensite (NM) and austenite in epitaxial films make them a model system to study the transition between the different martensitic phases. Kaufmann et al. [PRL 104 (2010) 145702] recently showed that 14M can be described as nanotwinned NM which forms at the austenite-martensite interface reducing elastic energy for the cost of twin boundary energy. With increasing distance to the interface, the elastic energy becomes less important and 14M transfers to NM by coarsening. To probe this, we examined Ni-Mn-Ga films of varying thickness deposited on MgO(100) substrates with a Cr buffer layer. Taking the integrated XRD-intensity ratio as a measure of phase fraction, we observe an increase of NM with increasing film thickness up to a maximum value of 63%. We conclude that for films on rigid substrates coarsening is inhibited by the elastic energy at the interface for one third of all possible 14M variants. This work is funded by DFG via SPP 1239.