Fe-Ga合金脱溶转变机制与中间亚稳相增强磁致伸缩效应的研究

Fe-Ga alloys with low-field large magnetostriction and good mechanical properties are promising for applications in aero-crafts, precise machining and petroleum extraction. Although their large magnetostriction has been reported for nearly 20 years, how soluting nonmagnetic Ga into α-Fe can enhance its magnetostriction over ten-fold remains a fundamental question. The magnetostrictive performance of Fe-Ga alloys is closely related to the nanoscale structural heterogeneities formed during the metastable exsolution transition. However, both cubic and tetragonal nano-heterogeneities coexist, posing a big challenge to distinguish their individual contributions to the large magnetostriction. Consequently, it is necessary to investigate the formation and evolution of these nanoscale metastable intermediate phases (IPs), especially the tetragonal ones, as well as their response mechanisms to the external fields. Such investigations will not only advance the in-depth understanding of the relationship between the microstructure and the magnetostrictive property of Fe-Ga alloys but also provide a clear clue to further enhance their performance.. The present project intends to investigate the formation sequence and evolution mechanism of the metastable IPs during exsolution transition of the BCC supersaturated solid solutions and how such metastable IPs influence the magnetostrictive properties. Through revealing their formation sequence and evolution process, the microstructure difference between the cubic and the tetragonal IPs will be magnified by heat-treating. The reorientations of their magnetization and strain order parameters and possible lattice distortion under external magnetic/stress fields, as well as the magnetoelastic coupling against the surrounding matrix phase will be investigated using in-situ small angle neutron scattering (SANS) and synchrotron X-ray diffraction (XRD) techniques. In combination with the measurements of magnetic, magnetostrictive, damping and mechanical properties, it is inspiring to reveal how nano-heterogeneities play the essential role on the low-field large magnetostriction. The results may also provide a feasible approach to enhance the magnetostrictive properties of Fe-Ga alloys and help to design novel magnetostrictive materials through engineering nano-heterogeneities.

Fe-Ga合金因其低场大磁致伸缩和良好的力学性能，在航空航天、精密机械和石油开采等高技术领域具有重要应用前景，成为磁致伸缩材料研究的热点。但是，非磁性Ga元素固溶于α-Fe显著提升磁致伸缩性能(10倍以上)的物理机制仍不清楚。研究表明，Fe-Ga的磁致伸缩性能与非平衡脱溶析出的纳米亚稳相高度相关。由于立方与四方纳米相共存，其作用机制难以辨析。因此，深入研究纳米亚稳相的形成、演变及在外场中的响应机理，对理解Fe-Ga的磁致伸缩机制和进一步提高磁致伸缩性能都具有重要意义。本项目拟研究Fe-Ga亚稳相的脱溶析出贯序及演变规律，进而通过热处理放大立方和四方纳米相的微结构差异。在此基础上，采用小角中子散射和同步辐射XRD等多种手段，研究并揭示它们在原位磁化或加载条件下的取向过程与结构演化；结合宏观性能测试，阐明纳米亚稳相增强磁致伸缩效应的微观机制，从而为高性能磁致伸缩材料的研制提供理论和技术支持。

具有低场大磁致伸缩效应和良好塑韧性的Fe-Ga合金成为高精度磁智能器件的重要候选。但是，非磁性Ga元素固溶于α-Fe显著提升磁致伸缩性能的物理机制仍不清楚，导致其高性能化缺乏足够的基础理论指导。本项目从合金磁致伸缩性能与脱溶亚稳相的密切相关性出发，围绕“四方纳米析出相的演变过程及外场响应机制”这一关键科学问题开展研究。(1)发现Fe-Ga合金BCC过饱和固溶体在析出FCC平衡相之前，存在由原子扩散不充分和晶格畸变受阻所形成的四方、正交和六角等多种构型中间相，尤其是对磁致伸缩性能起关键作用的四方中间相的四方度具有强烈的尺寸依赖性，丰富和发展了固态相变理论。(2)阐明了纳米亚稳相对Fe-Ga合金磁致伸缩性能的作用机理：共格四方纳米相迫使低弹模BCC基体相局域马氏体相变，显著增强磁致伸缩效应，其尺寸小于两相交换耦合长度时，还可降低磁各向异性，从而诱发低场大磁致伸缩效应。(3)成功建立了Fe-Ga合金非平衡时间-温度-转变(TTT)和时间-温度-磁致伸缩性能(TTP)相图，通过优化热处理工艺，使BCC相基体中析出细小、弥散且共格的四方纳米相，将多晶材料的磁致伸缩性能提高了3倍，与单晶材料相当；同时大幅降低了驱动场，使灵敏度提高了5倍，为克服不同于传统单相材料“大磁应变与低驱动场难兼得”的原理性瓶颈和研制高灵敏磁致伸缩材料提供了有效途径。(4)通过引入非平衡态的结构缺陷，使Fe-Ga合金阻尼系数提高了3倍，有望为设计高阻尼铁磁合金提供新方向。发表标注资助论文28篇，其中在金属材料领域旗舰刊物Acta Mater.上发表论文5篇；申报国家发明专利1项；在ICOMAT等本领域重要国际会议上做邀请报告3次，在IFAM等国内会议上做邀请报告2次；培养博士后1人、博士/硕士毕业研究生3人，其中2人被评为西安交通大学优秀毕业研究生。