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Author: Lünser, Klara × Author: Nielsch, Kornelius × End date: 2024 × End date: 2023 × Start date: 2020 × DDC: 600 × Has files: Yes ×

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    Self‐Patterning of Multifunctional Heusler Membranes by Dewetting
    (Weinheim : Wiley-VCH, 2021) Lünser, Klara; Diestel, Anett; Nielsch, Kornelius; Fähler, Sebastian
    Ni-Mn-based Heusler alloys are an emerging class of materials which enable actuation by (magnetic) shape memory effects, magnetocaloric cooling, and thermomagnetic energy harvesting. Multifunctional materials have a particular advantage for miniaturization since their functionality is already built within the material. However, often complex microtechnological processing is required to bring these materials into shape. Here, self-organized formation of single crystalline membranes having arrays of rectangular holes with high aspect ratio is demonstrated. Dewetting avoids the need for complicated processing and allows to prepare freestanding Ni–Mn–Ga–Co membranes. These membranes are martensitic and magnetic, and their functional properties are not disturbed by self-patterning. Feature sizes of these membranes can be tailored by film thickness and heat treatment, and the tendencies can be explained with dewetting. As an outlook, the advantages of these multifunctional membranes for magnetocaloric and thermomagnetic microsystems are sketched. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH
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    Solving the puzzle of hierarchical martensitic microstructures in NiTi by (111)-oriented epitaxial films
    (Amsterdam : Elsevier, 2023) Lünser, Klara; Undisz, Andreas; Wagner, Martin F.-X.; Nielsch, Kornelius; Fähler, Sebastian
    The martensitic microstructure decides on the functional properties of shape memory alloys. However, for the most commonly used alloy, NiTi, it is still unclear how its microstructure is built up because the analysis is hampered by grain boundaries of polycrystalline samples. Here, we eliminate grain boundaries by using epitaxially grown films in (111)B2 orientation. By combining scale-bridging microscopy with integral inverse pole figures, we solve the puzzle of the hierarchical martensitic microstructure. We identify two martensite clusters as building blocks and three kinds of twin boundaries. Nesting them at different length scales explains why habit plane variants with ⟨011⟩B19' twin boundaries and {942} habit planes are dominant; but also some incompatible interfaces occur. Though the observed hierarchical microstructure agrees with the phenomenological theory of martensite, the transformation path decides which microstructure forms. The combination of local and global measurements with theory allows solving the scale bridging 3D puzzle of the martensitic microstructure in NiTi exemplarily for epitaxial films.