Basic research of the influence of real structure on the magnetic field induced strain in NiMnGa alloys - Structure and properties of twin boundaries in NiMnGa alloys

磁场感应应变（MFIS）是基于双晶界在磁场作用下比较容易运动和高磁各向异性。为了开发新的MFIS材料，分别优化已知材料的MFIS效应，了解决定这些材料中孪晶界运动的物理过程是一个挑战。孪晶界运动与一般的晶界运动一样，受有序、偏离化学计量、调制、析出物、晶粒尺寸、织构和畴结构等参数的影响。因此，系统地研究这些参数对NiMnGa合金中MFIS效应的影响是本项目的任务。之所以选择这些合金，是因为它们显示出迄今为止最大的MFIS，而且铸造可以很容易地批量生产材料。计划的研究包括对孪晶界结构的详细描述，以及上面提到的晶体学和磁性微观结构特征。表征将在最高标准下进行，通过传统的透射电子显微镜（TEM），洛伦兹电子显微镜和电子全息摄影，在中分辨率和高分辨率。这些问题的基础研究将全面利用单晶进行。由于MFIS在技术上的预期应用，例如执行器和传感器，多晶材料似乎更合适，因此必须考虑具体的多晶方面：将分析晶粒尺寸和质地对MFIS效果的影响。此外，还将通过在大温度范围内同时施加磁场的力学试验来研究它们对孪晶边界运动的影响。

The magnetic field induced strain (MFIS) is based on the comparably easy motion of twin boundaries under magnetic fields, and a high magnetic anisotropy. To develop new MFIS materials, respectively optimize the MFIS effect of known materials, it is a challenge to understand the physical processes determining the motion of twin boundaries in these materials. Twin boundary motion, like grain boundary motion in general, is affected by parameters such as order, deviation from stoichiometry, modulation, precipitates, grain size, texture and domain structure. Therefore, it is the task of the project to systematically investigate the effect of these parameters on the MFIS effect in NiMnGa alloys. These alloys are chosen, because they are showing the biggest MFIS so far, and casting can quite easily produce materials in bulk form. The investigations planned comprise a detailed characterization of the structure of the twin boundary as well as of the crystallographic and magnetic microstructural features mentioned above. Characterization will be done at highest standards by means of conventional transmission electron microscopy (TEM), Lorentz electron microscopy, and electron holography, both at medium and high resolution. Basic research on these issues will comprehensively be performed using single crystals. Since for the intended application of MFIS in technology, e.g. for actuators and sensors, polycrystalline materials seem to be more appropriate, the specific polycrystalline aspects have to be considered: The modifications of the MFIS effect by grain size and texture will be analyzed. Furthermore their influence on twin boundary motion will be studied by mechanical tests in a large temperature range, also with simultaneously applying a magnetic field.