Basic research of the influence of real structure on magnetic field induced strain (MFIS) in NiMnGa alloys - Magnetic field induced strain (MFIS) in textured polycrystalline ferromagnetic martensitic NiMnGa alloys

报道了NiMnGa单晶由于孪晶界运动（TBM）而表现出磁场诱导应变（MFIS）。这种行为在低温马氏体相中发现，其在冷却时由约750 °C的立方L21相形成。MFIS的前提是单轴磁各向异性和高移动的孪晶界。这两个特征不仅取决于成分，而且取决于结构和微观结构。本项目的目标是建立的条件下，通过TBM MFIS可以实现多晶NiMnGa。研究结果将用于通过有效的制备路线制备改进的磁驱动致动器。随后，本研究的结果将应用于新型合金系统，以进一步改善铁磁形状记忆元件。首先，将开发这种组合物的高度织构化的大晶粒多晶样品的制备，据报道，这些样品在单晶中通过TBM显示MFIS。研究了定向凝固和变形后再结晶两种制备方法。然后将建立处理这种多晶体以实现MFIS的规则。根据初步研究，预计需要进行多步热机械和/或热磁治疗（培训）。最后，应确定微观结构的元素，这些元素促进了响应。通过TBM防止MFIS。这一步骤需要在优先计划SPP 1239内进行扩展合作，这意味着将提供该项目的定义明确的样品，以进行详细的结构表征。

Single crystals of NiMnGa are reported to show magnetic field induced strain (MFIS) due to twin boundary motion (TBM). This behavior is found in a low temperature martensitic phase, which forms from a cubic L21 phase ordering around 750 °C upon cooling. Prerequisites for MFIS are a uniaxial magnetic anisotropy and highly mobile twin boundaries. Both features depend not only on composition but also on structure and microstructure. The goal of the present project is to establish the conditions under which MFIS by TBM can be achieved in polycrystalline NiMnGa. The results will be used to prepare improved magnetically driven actuators by efficient preparation routes. Later, the results of this study will be applied to novel alloy systems to further improve ferromagnetic shape memory elements. At the beginning, the preparation of highly textured large grained polycrystalline samples of such compositions will be developed, which are reported to show MFIS by TBM in single crystals. Two routes of preparation will be studied, directional solidification and recrystallization after deformation. Then the rules to treat such polycrystals to achieve MFIS will be established. From preliminary studies it is expected that multi-step thermo-mechanical and/or thermo-magnetic treatments (training) will be necessary. Finally, the elements of the microstructure shall be identified, which promote resp. prevent MFIS by TBM. This step requires extended cooperation within the priority program SPP1239, which means that well defined samples of this project will be provided for elaborated structural characterization.