辐照引发应力场下铁素体/马氏体钢中合金元素偏聚机理的模拟研究

One important task of the development and research of structural metallic materials serving in nuclear plants is to improve radiaion-resistant performance. The nonuniform stress field induced by radiation defects promotes the diffusion of alloying elements, and consequent segregation near grain boundaries, and extended defects (e.g., voids and dislocations), which causes service performance degradation of metallic materials. However, it has been not clear so far about the stress field promoted segregation mechanism of alloying solutes. To gain such knowledge, this proposal manages to investigate the segregation behaviors of a spectrum of alloying solutes in Fe-based alloys under stress fields of different radiation defects by molecular dynamical simulation, molecular statics as well as first-principles methods. The research plan includes: characterization of the stress fields induced by extended defects inside grains, and the stress fields near and on grain boundaries (GB) in different damage states; study on the influence of stress fields on the chemical potential and migration barrier of alloying solutes inside grains, and near as well as in GB; study on the concentration distribution of different alloying solutes in Fe-based alloys containing extended defects as well as damaged GB. From these studies, alloying elements whose segregation are less influenced by radiation defects can be picked out, which is instructive for chemical component design of long life Fe-based alloys serving in radiation as well as high-stress environments.

提高金属结构材料的抗辐照性能是核能系统材料研发的重要任务之一。辐照环境下金属材料中合金原子在不均匀应力场中会加快扩散并在晶界、空洞及位错等扩展缺陷附近偏聚，导致材料使用性能的急剧退化。然而，合金元素在应力场驱动下的偏聚机理至今仍不清楚。本项目针对辐照环境下材料中固溶合金元素稳定性及其对应力场的不同响应问题，拟采用计算模拟方法（结合分子动力学模拟、静态计算和第一性原理方法），以铁基合金为研究对象，研究辐照缺陷引发的应力场中不同合金原子的偏聚行为。本项目将考察晶粒内扩展缺陷周围、不同损伤状态晶界面及其附近应力场特征，进而研究应力场中合金原子化学势及扩散能垒的变化，并揭示不同合金原子在包含受损晶界及辐照缺陷的体系中浓度分布的差异。最终筛选出一些扩散行为受应力场影响较小的合金元素，为延长在辐照及应力环境中铁基合金材料成分的优化设计提供理论指导与建议。

金属结构材料中合金元素的辐照诱导偏析机理至今仍不清楚。本项目从应变对合金元素的稳定性影响来揭示辐照诱导偏聚机理。本项目采用第一性原理方法模拟研究体心立方铁基合金中替代位合金元素在两种典型外形畸变的应变模式下的稳定性。研究发现，在体心立方铁中的合金原子稳定性和它们的内禀性质有直接关联。在切应变作用下，每一族的过渡金属原子的替代能随它们的d价轨道半径线性增加，斜率随它们的Watson 电负性与铁的差值的绝对值指数式下降，这归因于过渡金属原子与最近邻铁原子的Pauli排斥作用受到它们的d价轨道之间杂化的调制作用。在正应变作用下，过渡金属原子的替代能变化很小，而主族及后过渡金属族合金原子的替代能大幅下降，这归因于正应变作用下它们的s、p价带电子与第二层近邻铁原子的相互作用。本项目采用第一性原理方法，系统计算了五种浓度下过渡金属元素在体心立方铁中的尺寸因子，用来表征在体积应变下它们的稳定性差别；还计算了过渡金属元素在体心立方铁中的体积应变模量因子和切应变模量因子，用来反映对弹性模量的影响。通过分子动力学模拟发现，在体心立方铁晶界及附近区域间隙原子和空位的迁移能垒都比块体内的低，可以快速扩散并复合，部分地揭示了晶界修复辐照缺陷的机理。本项目还发现，体心立方铁锰合金随着锰浓度增加，发生反铁磁相到铁磁相的同构相转变，根源于锰原子的d能带内电子的转移。