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Author: Hickel, Tilmann × WGL Institute: IFWD ×

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Now showing 1 - 4 of 4
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    Thickness dependent exchange bias in martensitic epitaxial Ni-Mn-Sn thin films
    (New York : American Institute of Physics, 2013) Behler, Anna; Teichert, Niclas; Dutta, Biswanath; Waske, Anja; Hickel, Tilmann; Auge, Alexander; Hütten, Andreas; Eckert, Jürgen
    A thickness dependent exchange bias in the low temperature martensitic state of epitaxial Ni-Mn-Sn thin films is found. The effect can be retained down to very small thicknesses. For a Ni50Mn32Sn18 thin film, which does not undergo a martensitic transformation, no exchange bias is observed. Our results suggest that a significant interplay between ferromagnetic and antiferromagnetic regions, which is the origin for exchange bias, is only present in the martensite. The finding is supported by ab initio calculations showing that the antiferromagnetic order is stabilized in the phase.
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    Constrained incipient phase transformation in Ni-Mn-Ga films: A small-scale design challenge
    (Amsterdam [u.a.] : Elsevier Science, 2023) Fareed, Adnan; Rosalie, Julian M.; Kumar, Sourabh; Kar, Satyakam; Hickel, Tilmann; Fähler, Sebastian; Maaß, Robert
    Ni-Mn-Ga shape-memory alloys are promising candidates for large strain actuation and magnetocaloric cooling devices. In view of potential small-scale applications, we probe here nanomechanically the stress-induced austenite–martensite transition in single crystalline austenitic thin films as a function of temperature. In 0.5 µm thin films, a marked incipient phase transformation to martensite is observed during nanoindentation, leaving behind pockets of residual martensite after unloading. These nanomechanical instabilities occur irrespective of deformation rate and temperature, are Weibull distributed, and reveal large spatial variations in transformation stress. In contrast, at a larger film thickness of 2 μm fully reversible transformations occur, and mechanical loading remains entirely smooth. Ab-initio simulations demonstrate how an in-plane constraint can considerably increase the martensitic transformation stress, explaining the thickness-dependent nanomechanical behavior. These findings for a shape-memory Heusler alloy give insights into how reduced dimensions and constraints can lead to unexpectedly large transformation stresses that need to be considered in small-scale actuation design.
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    Structural transformations among austenite, ferrite and cementite in Fe-C alloys: A unified theory based on ab initio simulations
    (Amsterdam [u.a.] : Elsevier Science, 2015) Zhang, Xie; Hickel, Tilmann; Rogal, Jutta; Fähler, Sebastian; Drautz, Ralf; Neugebauer, Jörg
    Structural transformations in Fe–C alloys are decisive for the mechanical properties of steels, but their modeling remains a challenge due to the simultaneous changes in Fe lattice and redistribution of C. With a combination of the orientation relationships between austenite, ferrite and cementite, we identify a metastable intermediate structure (MIS), which can serve as a link between the three phases. Based on this framework, different mechanisms depending on the local conditions (C concentration, strain, magnetism) are revealed from ab initio nudged elastic band simulations, which allow us to construct a unified theory for the structural transformations among austenite, ferrite and cementite.
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    Coupling Phenomena in Magnetocaloric Materials
    (Weinheim [u.a.] : Wiley-VCH, 2018-8-5) Waske, Anja; Dutta, Biswanath; Teichert, Niclas; Weise, Bruno; Shayanfar, Navid; Becker, Andreas; Hütten, Andreas; Hickel, Tilmann
    Strong coupling effects in magnetocaloric materials are the key factor to achieve a large magnetic entropy change. Combining insights from experiments and ab initio calculations, we review relevant coupling phenomena, including atomic coupling, stress coupling, and magnetostatic coupling. For the investigations on atomic coupling, we have used Heusler compounds as a flexible model system. Stress coupling occurs in first-order magnetocaloric materials, which exhibit a structural transformation or volume change together with the magnetic transition. Magnetostatic coupling has been experimentally demonstrated in magnetocaloric particles and fragment ensembles. Based on the achieved insights, we have demonstrated that the materials properties can be tailored to achieve optimized magnetocaloric performance for cooling applications.