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The Role of Adaptive Martensite in Magnetic Shape Memory Alloys

Authors


  • The authors like to thank S. Kaufmann, P. Entel, A. T. Zayak, A. Hucht, M. Siewert, A. Diestel, L. Straka, and M. Wuttig for helpful discussions. Supercomputing resources and computational support were kindly provided by the John von Neumann Institute for Computing (NIC) at Jülich Research Center (IBM Blue Gene/P) and the Center for Computational Sciences and Simulation CCSS of the University of Duisburg-Essen (Cray XT6m). This work receives financial support from the Priority program SPP 1239 (fund number FA 453/7, RO 2238/8, and GR 3498/1) (www.MagneticShape.de), the project FA 453/8 from the German Research Society and from the Academy of Sciences of Czech Republic (M100100913).

Abstract

Magnetic shape memory materials require a high twin boundary mobility and low hysteresis for applications mainly as actuators, sensors, and magnetocaloric cooling elements. Usually, outstanding properties are found only in samples with a modulated martensitic structure. Here, we analyze the question why a modulated structure is beneficial and show evidence that the modulated martensite is not an equilibrium phase but a nanoscale microstructure of non-modulated (NM) martensite. In this review, we combine results from continuum and atomistic theory, as well as local and integral measurements on the model system Ni–Mn–Ga. Following the concept of adaptive martensite the modulated phase forms to minimize elastic energy near the phase boundary by introducing low-energy twin boundaries between lamellae of the NM martensite that have widths of a few unit cells.

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