微量稀土掺杂NiMnGa合金的微观结构及物理效应研究

Ferromagnetic shape memory alloys is an important materials with potential application in actuators and sensors for the possession of large magnetostrain effect. The lattice distortion degree a/c of martensite phase is the key facter of intrinsic magnetostrain property. Currently, the ways adjusting the intrinsic magnetostrain are mainly focused on the transforming of the martensite type. Recently, we discovered that doping rare earth atoms in rapid solidified NiMnGa alloys can induce the partial solid solution of rare earth atoms and others forms nanoprecipitates. This result produces the strong distortion of martensite lattice, and suppresses the martensitic transformation, and puts forward to the formation of nano-martensite. In this way, larger intrinsic magnetostrain property and new physical effects are expected. Therefore, in this item, rare earth elements with different anisotropies are chosen to be added into NiMnGa allos and the melt spinning ribbons are produced. The influences on martensite crystal structure, lattice parameters and magnetocrystalline anisotropies by rare earth atoms addition are studied, the mechanism the martensitic transformation suppressed effect are revealed. The relationship between the microstructures and the magnetic, electrical, mechanical properties are investigated. This item may provide theoretical foundation for the development of ferromagnetic shape memory alloys.

铁磁形状记忆合金具有大磁致应变效应，在驱动器和传感器中有重要应用前景。马氏体相晶格畸变度a/c是决定其本征磁致应变的关键因素。目前在5M和7M马氏体单晶中已通过实验获得了与其本征磁致应变相当的性能并证明了其与a/c的关联性，然而如何进一步提高铁磁形状记忆合金的磁致应变值具有挑战性。申请者近期发现，在快速凝固NiMnGa合金中稀土元素部分固溶，诱导马氏体晶格畸变度显著增大，这样有望显著提升本征磁致应变值；同时，部分稀土形成纳米第二相，抑制马氏体相变，在合金中形成纳米马氏体结构，有望引发全新物理效应。因此，本项目以稀土元素掺杂NiMnGa快速凝固甩带合金为研究对象，研究稀土元素的存在形式对于合金微观结构的影响规律，揭示其对于合金内禀磁性和马氏体相变行为的作用机制，探索甩带合金大磁致应变等物理效应，期望为新性能铁磁形状记忆合金发展提供理论依据。

本项目系统研究了微量稀土掺杂Ni-Mn-Ga铁磁形状记忆合金的铸态合金、定向凝固和快速凝固甩带合金中的凝固组织和相结构演变，重点揭示了其随凝固条件变化，合金中成分分布、微观组织、晶粒尺寸、第二相尺寸与分布、基体相与第二相位相关系与交互作用等关键微观结构特征的演变。基于此，明确了随凝固增大而尺寸逐渐细化、分布逐渐弥散的多尺度富稀土第二相，其与基体相的位相关系也从非共格逐渐转变为半共格并最终为共格关系的演变关系。揭示了半共格和共格纳米相通过诱导局域晶格畸变从而对马氏体相变的抑制作用，并基于其结构分析的结果结合朗道理论进行了机理分析。同时，通过调控稀土元素成分及稀土原子在Ni-Mn-Ga合金中的取代方式，调控其马氏体相变温度和居里转变温度逐步接近并交叠，在多种成分和较宽成分窗口实现了“铁磁马氏体-顺磁奥氏体”的“磁-结构”耦合相变。基于该相变，探索了稀土掺杂Ni-Mn-Ga合金磁热效应的演变及其机理；同时结合其微观形貌变化，研究了第二相的调控对合金力学性能和形状记忆效应的优化作用及其机理。获得了兼具良好磁热效应和优异力学性能的双相铁磁形状记忆合金，对今后开展磁-力多场刺激研究打下了基础。

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