铁基磁制冷功能合金La-Fe-Si复相共生定向凝固机制解析

Room-temperature magnetic cooling technology has attracted much attention during the past two decades, and La-Fe-Si (83at.%Fe) with the advantage of high performance-price ratio is considered to be one of potential candidate for magnetocaloric application. However, the phase evolution excessively depends on the extended high-temperature annealing, and it is impossible to make a reasonable technological route for the heat treatment without sufficient understanding on the fundamental phase formation and evolution mechanism. Additionally, it seems more challenging that intrinsic brittleness largely restricts its production, fabrication and practical applications. In this proposal, the brittleness will be addressed through growing dual-phase coupled unidirectional materials by controlling the solidification process. It is expected to overcome the brittleness by taking advantage of the dual-phase composite effect, and to achieve enhanced magnetocaloric properties by exploring both texture orientation effect (closely linked to magnetocrystalline anisotropy) and melt-purification effect. WE firstly clarify the phase formation mechanism, and then prepare high-quality unidirectional materials with dual-phase coupled microstructure using a modified high-pressure zone-melting set-up. Interfacial microstructure and magnetic/mechanical interactions will be studied. Lastly, we will establish a comprehensive microstructure-processing-property model based on the solidification process and post heat treatment. The main scientific merit of this research is to develop a novel metallurgical manufacturing science on the Fe-based functional alloys.

近二十年来绿色节能的室温磁制冷科学迅猛发展，性价比优异的铁基磁制冷功能合金La-Fe-Si（83at.%Fe）被认为最具商业应用前景，然而现阶段La-Fe-Si合金的物相调控研究过度依赖长时间高温热处理，熔体凝固过程未得到应有的重视，合金的本征脆性更是严重制约着其加工制造及服役应用。本课题提出采用熔体受控凝固法，通过制备原位复相共生的定向凝固合金以解决其脆性之难题，综合发挥与凝固过程密切相关的熔体洁净化效应及晶体织构取向效应，最终实现高性能铁基磁制冷功能合金制造的新思路。本课题将在全面解析宽成分区域内La-Fe-Si合金高温物相形成的热力学及动力学过程基础上，利用高压区熔凝固装置制备复相共生定向材料，揭示定向材料微观结构特征与其磁学和力学特性间的内禀关系，构建出凝固过程与热处理阶段兼顾协调的物相调控理论。本课题的科学意义主要在于推动发展以熔体凝固过程控制为特色的铁基功能合金冶金制造科学。

本研究采用差示扫描量热仪开展原位热处理实验，解析La-Fe-Si合金中tao1相高温转变机制；引入基于电流加热的快速热处理技术，揭示闪速退火过程中合金物相组织演变规律，探索更高效的热处理方案。本项目主要发现有：1、采用离心铸造法制备出的铸态LaFe11.6Si1.4板材物相组织由黑色α(Fe)相和白色富La相组成，在升温过程中首先发生α(Fe)相的磁性转变，随后富La相开始熔化并与α(Fe)相发生包晶反应形成t1相，随后发生tao1相和α(Fe)相的熔化。2、采用差示扫描量热仪开展原位高温热处理实验研究发现，经1353K热处理60min后，铸态板材试样中tao1相体积分数为66.20%；提高热处理温度至1423K后达到96.36%；前者tao1相来源于固态扩散型相变，后者源于部分液相参与的包晶反应机制。JMAK方程计算结果表明其生长方式属于扩散控制型一维生长。在1423K热处理120min后合金的等温磁熵变达到了26.94J/(kgK)，有效制冷能力为227.10J/kg。3、采用电流加热闪速退火技术对LaFe11.6Si1.4板材进行快速热处理，在20K/s加热至1448K、1473K、1498K进行2s等温处理后分别获得38%、54.1%、67.6%的tao1相。在1498K下保温5s可使基体内的富La相完全参与反应，延长至10s后可获得约84.6%的tao1相。相关研究结果为La-Fe-Si合金高效热处理工艺探索提供了重要的实验依据。