磁场下定向凝固Ni-Mn-Ga合金的成分控制与奥氏体取向优化研究

Ni-Mn-Ga alloys, as typical ferromagnetic shape memory alloys (FSMAs), have potential for sensor and actuator applications. It has been found that the magnetic-controllable property of Ni-Mn-Ga alloys is strongly dependent on the composition and crystal orientation. In this project, we introduce the magnetic field into the directional solidification of Ni-Mn-Ga alloy, focusing on the solution distribution and controllable crystal growth by adjusting magnetic field, temperature gradient and the coupling of the above both factors. Firstly, the evolutions of composition segregation in the macro-scale (axial) and micro-scale (inter-dendrite) during directional solidification under magnetic field are investigated by adjusting magnetic field, solidification growth speed and temperature gradient. The effect of magnetic field on the composition distribution will be studied, and the corresponding model will be established. The result will give a guideline for high-throughput preparation of functional materials. Secondly, the crystal orientation during directional solidification under a magnetic field will be controlled by considering the preferential growth direction and easy magnetization axis, aiming to propose a new method to control crystal growth by coupling magnetic field and temperature field, resulting in single crystalline Ni-Mn-Ga alloys with controllable three-dimension orientation. This will pave a way for magnetic field-assisted directional solidification of functional materials with controllable composition distribution and crystal orientation.

Ni-Mn-Ga铁磁形状记忆合金是一种极具潜力的新型感应和驱动材料。该合金相关磁控性能具有强烈的成分和取向依赖性。申请人拟将磁场引入到Ni-Mn-Ga合金定向凝固过程中，利用磁场、定向温度场，及其耦合场影响和控制合金中成分与其母相奥氏体取向分布，主要开展以下两方面研究：首先，通过改变磁场强度、凝固速度和温度梯度等参数，研究磁场下定向凝固中宏观（轴向）和微观（枝晶）尺度的偏析行为，结合数值模拟阐明磁-热耦合场对凝固中熔体流动的机理，探索磁场对成分分布的影响机制，建立磁场下Ni-Mn-Ga合金中宏观和微观成分控制模型，为高通量功能材料的制备提供新的思路；其次，利用材料的择优生长取向和磁性取向（易磁化轴）特性，通过耦合调控磁场矢量和温度场矢量控制晶体生长，实现Ni-Mn-Ga合金奥氏体由多晶到取向三维可控单晶的转变。本项目的研究将为磁场辅助定向凝固制备成分与取向可控功能材料提供新的思路与基础。

本项目旨在探明磁场下定向凝固Ni-Mn-Ga合金的成分与取向演化，以及探究磁场-热场耦合下Ni-Mn-Ga合金组织变化与机械性能演化规律。搭建了磁场下金属功能材料定向凝固设备；揭示了在定向凝固Ni-Mn-Ga合金组织中成分分布变化规律，尤其是富镍Ni-Mn-Ga合金的不同相的成分偏析规律，探明了合金中成分变化与晶体取向、界面结构、微观组织演变机理之间联系；通过在不同成分的Ni-Mn-Ga合金定向凝固过程中施加磁场，研究了磁场对定向凝固组织的影响，发现在Ni-Mn-Ga合金定向凝固过程中施加强磁场可以诱发微观上尺度上的枝晶破碎规律；研究磁场对定向凝固组织机械行为的影响，通过断口形貌分析性能差异的原因，同时通过EBSD技术进一步研究了单轴拉伸/压缩应力下两种典型组织的Ni-Mn-Ga合金的微观断裂特征，建立了磁场-热场协同影响机械性能模型。在此基础上，进一步构建了为磁场辅助定向凝固制备柱状晶-等轴晶功能材料提供了新的思路，为多场耦合条件下智能复合材料优化设计理论奠定基础。