铌微合金化钢中铌的晶界偏聚行为及其微观机理研究

Niobium atom has a strong grain boundary segregation tendency in steel which plays an important role in controlling phase transformation, microstructure and mechanical properties. Its segregation behavior and mechanism research is scarce and seriously limits the development of its grain boundary engineering. This proposal intends to reveal the grain boundary segregation behavior of niobium in steel and its mechanism systematically. Fe-Nb binary alloy and Fe-C-Nb ternary alloy are chosen to explore intrinsic segregation behavior of Nb in iron matrix and the interaction of Nb and C atoms influence on niobium segregation, respectively. The grain boundary segregation behavior will be measured after a series thermal history of isothermal quenching or continuous cooling under nearly niobium dissolved condition. Kinetics and mechanism of grain boundary segregation of Nb will be investigated by using the experimental measurement results and segregation theory. Furthermore, it will also use the first principles calculation to study the thermodynamic micromechanism of grain boundary segregation of Nb. The research results can not only enrich and improve the grain boundary segregation theory, but also provide guidance in grain boundary engineering for utilizing Nb in controlling phase transformation, grain boundary strengthening and improving the corrosion resistance, etc. This proposal exhibits important scientific significance and application value.

铌微合金化钢是先进钢铁材料研发的重要方向，铌原子的强晶界偏聚倾向对相变及组织性能控制有重要作用。但关于钢中铌的偏聚行为、偏聚机制及微观机理的系统研究相对匮乏，制约其晶界工程的发展。针对此问题本课题拟采用Fe-Nb二元合金、Fe-C-Nb三元合金作为试验材料，通过对等温冷淬和连续冷却等一系列铌基本溶解的热历史条件下晶界偏聚行为的测定，探索Nb、C原子交互作用对铌晶界偏聚的影响，建立铌晶界偏聚的动力学模型，预测并验证其偏聚机制及条件；并利用第一性原理研究铌晶界偏聚的热力学微观机理。最终综合实验测量和理论研究从热力学、动力学以及微观机理的角度揭示其偏聚机制。进一步丰富和完善晶界偏聚理论，为利用铌在相变与组织控制、晶界强化、提高耐蚀性等方面提供科学依据，具有重要的科学意义和应用价值。

铌微合金化钢是先进钢铁材料研发的重要方向，铌原子的强晶界偏聚倾向对相变及组织性能控制有重要作用。但关于钢中铌的偏聚行为、偏聚机制及微观机理的系统研究相对匮乏，制约其晶界工程的发展。针对此问题本课题采用电子探针微区分析-波谱仪（EPMA-WDS）测定了等温冷淬热历史条件下超低碳铌钢Nb、C的晶界偏聚行为，采用APT测定了连续冷却条件下超低碳铌钢Nb、C的晶界偏聚行为, 根据晶界偏聚动力学理论建立了铌晶界偏聚的动力学模型；利用第一性原理研究了铌晶界偏聚的热力学微观机理。研究发现：连续冷却样品中，铁素体/马-奥组织相界、原奥氏体晶界、铁素体晶界，这三种不同晶界Nb的富集浓度依次递减，根据测量数据获得了950℃原奥氏体晶界铌的晶界偏聚能为22.91 kJ/mol。等温实验中，从室温直接升至高温（如1200℃），在原奥氏体晶界处未观察到明显偏聚，晶界浓度与晶内差别不大；在奥氏体相区内快速降温（如1200℃-1000℃）后等温，由于过饱和空位的存在，Nb元素在晶界快速偏聚，测定的其偏聚动力学曲线基本符合非平衡晶界偏聚理论；同时，通过第一性原理计算的奥氏体相中最近邻Nb-空位复合体结合能比经典方法计算结果略小，但仍处于0.15~0.60eV范围内，作为置换型原子易于形成非平衡晶界偏聚。该研究进一步丰富和完善了晶界偏聚理论，为利用铌在相变与组织控制、晶界强化、提高耐蚀性等方面提供科学依据。