强磁场下低活化钢中合金碳化物(Fe,Cr)xCy析出的热力学机制研究

The reduced activation steels used in the ring which magnetically confines plasma in the nuclear fusion reaction are operated in the condition of high temperature and high magnetic field. Alloy carbide precipitation plays an extremely important role in high temperature creep-resistant property. The magnetic free energy change, as one of the vital thermodynamic factors under the magnetic field and temperature coupling, contribute to characterize and predict the structure variation in steels. The main contents of this project are as follows: the variation of magnetic parameters of M7C3, M23C6, M6C and Fe atom with the external magnetic field and temperature coupling by the first principle calculation which is associated with the Weiss molecular field theory; the calculation of the internal magnetization of alloy carbides under strong magnetic field; the exploration of the Curie temperature for alloy carbides with the fluctuation of the Fe concentration and the strength of the magnetic field; the investigation of the evolution of magnetic heat capacity and magnetic free energy change with Curie temperature, magnetic moment of Fe atom in strong magnetic field and temperature field of the alloy carbides based on the heat capacity of the ferrite to austenite, and finally the establishment of thermodynamic mechanism of reduced activation steel in the strong magnetic field and temperature field coupling. The research can provide the theoretical basis for the performance evaluation for the reduced activation steels and enrich the metallographic principles in the heat-resistant steel under the extreme conditions.

低活化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。强磁场和温度耦合作用下磁自由能的变化规律是表征和预测此类钢在极端环境下结构变化的重要热力学因素。本项目借助第一性原理计算0K时合金碳化物M7C3，M23C6，M6C及其Fe原子的磁矩，结合Weiss分子场理论研究合金碳化物及其Fe原子磁化强度在强磁场和温度耦合场作用下的演变规律；计算强磁场下合金碳化物的内部磁化强度；探究合金碳化物的居里温度随着Fe原子浓度和磁场强度的波动情况；同时以铁素体-奥氏体的磁热容公式为基础，通过分析合金碳化物的居里温度、单位Fe原子磁矩在强磁场和温度场耦合下的变化情况来探索合金碳化物的磁热容和磁自由能的演变规律，最终建立低活化钢中合金碳化物在强磁场和温度场相互耦合作用下析出的热力学机制，为评价此类钢的服役性能提供理论依据，丰富极端条件下使用耐热钢的金属学原理。

低活化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。强磁场和温度耦合作用下磁自由能的变化规律是表征和预测此类钢在极端环境下结构变化的重要热力学因素。本项目突破了第一性原理零磁场和绝对温度的限制，将磁场和温度的影响综合考虑，研究合金碳化物及其Fe原子磁化强度在强磁场和温度耦合场作用下的演变规律。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；晶粒退磁能与磁自由能相差近两个数量级，因此忽略退磁能；Fe元素的含量与居里温度的改变量息息相关。Fe含量越高，居里温度的改变量就越大；引入比例因子，借此来说明磁自由能占总自由能的比例，发现磁自由能是具有大磁矩合金碳化物（M23C6 和M6C）析出的决定性因素，然而热效应决定碳化物M7C3, M3C 及M2C的析出稳定性。此项目为研究合金碳化物的析出与演化提供定量的评价方法，是揭示钢失效和开发新钢种的前沿科学课题，同时丰富极端条件下使用耐热钢的金属学原理。