合金磁致伸缩行为微观机制的原位实验研究

The main objective of this project is intended to explore the critical issue on the relationship between magnetostriction and microstructure unit with aim to meet the challenge in the studies of the physical mechanism of large magnetostriction in rare earth metallic compounds, FeGa solid solutions and NiMnGa ferromagnetic shape memory alloys. Based on the synchrotron radiation, neutron scattering and other advanced characterization methods, the evolution of microstructure unit will be in-situ studied in magnetostrictive materials under temperature, stress and magnetic field environments. It is predicted that some breakthroughs will be achieved in the following aspects. New experimental methods will be established for in-situ characterizing the evolution of multi-scale microstructure unit, including crystal structure, grain/variant orientation, micro- stress/stain and point defects under various environments. The intrinsic relation between multi-scale microstructure unit and macro-scale performance, i.e., stress/strain output, will be clarified. The physical mechanism of large magnetostriction will be established with the assistants of first principles calculations and molecular dynamics simulation. The successful execution of this program will provide a significant experimental and theoretical basis to optimize the microstructure unit and further to improve the functional performance in magnetostrictive materials.

针对稀土金属间化合物、FeGa固溶体、NiMnGa铁磁形状记忆合金等大磁致伸缩材料的磁致伸缩微观机制研究中遇到的挑战，本申请项目拟围绕磁致伸缩与微观结构基元的关联性这一科学问题，依托同步辐射X射线衍射和中子散射等先进表征技术，建立材料磁致伸缩过程中多尺度微结构单元原位表征方法，分析微观结构基元随温度、应力、磁场的演化信息，阐明磁致伸缩材料多尺度微结构单元（晶体结构、晶体取向、畴取向、微应力、点缺陷）与宏观应力、应变输出性能的相互关联性，结合第一性原理计算和分子动力学模拟，构建材料磁致伸缩微观机制的物理模型，为磁致伸缩材料微结构调控和性能优化提供实验和理论基础。

针对大磁致伸缩材料的磁致伸缩微观机制研究中遇到的挑战，本项目以稀土金属间化合物和FeGa固溶体等大磁致伸缩材料为研究对象，围绕合金大磁致伸缩行为与微观结构基元的关联性，开展了基于同步辐射的高能X射线原位衍射实验研究。研究发现稀土金属间化合物的磁致伸缩源于外磁场诱发的具有低对称性的磁晶变体重取向而非磁致晶格畸变，指出稀土金属间化合物的场致磁致伸缩源于磁致晶格畸变的推论或许是因X射线衍射的倒易空间分辨率所限而产生的误判。基于磁致晶体畴转动模型，给出了Terfenol-D化学成分偏离准同型相界的晶体学解释。实验证实了FeGa合金出现第一个磁致伸缩极值时为Ga元素的饱和固溶体，发现随Ga含量增加而出现的中程有序富Ga相导致了合金磁致伸缩的迅速下降。通过原位X射线观测给出了中程有序富Ga相存在的直接实验证据。最后，本项目设计了系列FeCoNiMn四元合金，在多晶材料中获得了120ppm的磁致伸缩值和抗拉强度大于1200MPa及延伸率达5%的力学性能。