高温和强磁场协同作用下Fe基材料中合金碳化物析出的热力学机制

Alloy carbide is the important phase of heat resistant steels used in the fusion reactor, nuclear fission reactor and power plant. The precipitation, coarsening and evolution play an important role in the property, especially the creep property and service life. The reduced activation steels used in the ring which magnetically confines plasma in the nuclear fusion reaction is operated in the condition of high temperature and high magnetic field. Therefore it is of scientific and industrial importance to investigate the precipitation and evolution behavior under the coupling of high temperature and high magnetic field. This proposal will take full consideration of the effects of the free energy change induced by high magnetic field and the chemical free energy change induced by high temperature and chemical composition on the precipitation and evolution of alloy carbides. The main contents of this project are as follows: the diffusion behaviors of solute atoms in the different phases region under strong magnetic field using diffusion couple combined with the EPMA (Electron Probe Microanalysis); the magnetic structure investigation of the precipitates by means of transmission electron microscopy and related approaches；the variation of magnetic parameters with the external field by the first-principle calculation which is associated with the Weiss molecular field theory; the calculation of the magnetic free energy with magnetic field and temperature coupling by modifying the magnetic heat capacity, and finally the establishment of experimental observations and physical model of the precipitation and evolution for alloy carbides. This will thus contribute to a better understanding of thermodynamics mechanism on precipitation and evolution of alloy carbides in Fe-based materials under the coupling of strong magnetic field and high temperature.

合金碳化物是核聚变、核裂变和火力发电用耐热钢中的重要组织相，其析出、粗化与演变对于耐热钢的性能，尤其是蠕变性能和服役寿命有重要影响。核聚变反应环形磁约束离子体用低活化钢，其服役环境即为高温和强磁场。因此，研究高温和强磁场协同作用下耐热钢中合金碳化物的析出与演变具有重要的科学与工程价值。本项目将充分考虑强磁场引起的磁自由能变化以及温度和成分引起的化学自由能变化对耐热钢中多元合金碳化物析出与演变的影响。主要研究内容为：利用扩散偶和电子探针方法研究溶质原子在高温和强磁场作用下不同相区的扩散行为，通过透射电镜等手段观察和鉴定析出相的磁结构，利用魏氏分子场理论和第一性原理计算析出相的磁学参数随温度与磁场强度的变化规律，通过修正磁热容公式计算强磁场和温度场耦合作用下的磁自由能，从而实现合金碳化物析出和演变的实验观察与物理模型的建立，揭示高温和强磁场协同作用下铁基材料中合金碳化物析出与演变热力学机制。

耐热钢中合金碳化物的析出行为决定着核聚变反应托卡马克装置在高温强磁场的蠕变服役性能。本项目借助于扫描电镜、电子探针等多种分析测试手段，发现强磁场可显著影响置换型溶质原子C和间隙型溶质原子（Cr, W, Mo）在纯铁和耐热钢中的扩散行为。通过分析磁性来源的Fe及Fe周围的结构信息（包括最近邻关系），构建出三种最小晶格类型。每一种晶格都是遵循框架+填充的方式。兼顾磁化强度及磁自由能的精度问题，最终量子数J取值为1；利用魏氏分子场理论和第一性原理计算析出相的磁学参数随温度与磁场强度的变化规律，发现Fe元素的含量与居里温度的改变量息息相关。借助小角中子散射、透射电镜等分析手段，发现磁场对某些特定合金碳化物的析出具有明显促进作用。磁场会改变α-Fe、Fe2C、Fe3C和Fe5C2，M7C3，M6C及M23C6的磁自由能，但是磁场对磁矩较大的Fe5C2及合金碳化物自由能影响最大，因此，磁场诱导磁性对碳化物的稳定性起决定性作用。上述研究成果将为核聚变极端条件下使用的结构材料的研究、开发及其服役行为提供理论支撑与参考。在本项目研究基础上，本团队积极响应号召，参与了国家磁约束聚变能发展研究专项关于先进结构材料的研发与制备，服务于国家核聚变试验堆的建设与运行。