异质原子偏析对铜晶界力学特性影响的理论研究

The first-principles methods are employed to investigate the influence of the heteroatoms segregation on the cohesion of symmetric and asymmetric grain boundaries (GBs) of Cu. The heteroatoms include rear earth elements (Sc, Y, etc.), metal elements (Ag, Ni, etc.) and nonmetal elements (Si, etc.). In this project, several works will be performed on account of the current research situation in both the methods and the field of Cu GB segregation. Firstly, improve the traditional tensile scheme to avoid the influence on the fracture process of GB introduced by the symmetry of the supercell. Secondly, based on the improved tensile scheme, the atomic structure and fracture process of the clean symmetric and asymmetric GBs will be studied to get the information of stress and electronic property with the changing of strain. At the same time, this information can be taken as the benchmark and will be compared with that of segregated GBs. Thirdly, different types GB with different doses of segregated heteroatoms will be researched using both improved tensile scheme and Rice-Wang model. The mechanism of the influence could be obtained by analyzing the electronic property. The results proposed by this project can reveal the relationship between the materials strength and fracture behavior, and further, provide theoretical direction for improving strength and ductility simultaneously.

以对称倾侧和非对称倾侧的Cu晶界为研究对象，采用第一性原理方法研究以Sc、Y等稀土元素，Ag、Ni等金属元素、Si等非金属元素为异质原子的偏析对Cu晶界结合强度的影响。基于目前研究方法及Cu晶界偏析领域的研究现状，本课题拟展开一下三面的研究工作：优化传统的第一性原理拉伸方案，以消除模型对称性给研究结果带来的不确定性；应用优化的拉伸方案，研究清洁对称倾侧与非对称倾侧晶界的几何结构、力学性能及拉伸断裂过程；应用优化的拉伸方案和R-W热力学模型，研究不同浓度的异质原子在不同晶界偏析引起晶界结合强度的变化，通过分析电子结构，以获取产生影响的机制。该项目的研究对揭示晶界偏析与材料强度、断裂行为之间的关系有重要意义，为设计和发展具有高强度、高韧性于抗断裂能力强的金属材料提供理论指导。

利用第一性原理方法，针对异质元素在铜晶界偏析的各项性质开展了系统研究，并第一性原理拉伸方案进行了改进。研究结果表明晶界的结合强度因异质原子的不同而不同。Ti，W，Al ，Ag等元素对晶界结合起到显著强化作用，而Bi，Mg，Sn, Zn则是脆化作用。Ti，Mg 元素的晶界偏析位置表现处选择性。B在铜晶界间隙位偏析的浓度相对较低，不能构成二聚体，与实验现象相吻合。针对Ag元素偏析Cu晶界的断裂行为研究表明，Ag可稳定占据所有替换偏析位，当5个Ag偏析时，晶界沿晶断裂且具有最小的断裂能，其数值接近相应Ag晶界的断裂能。最后，课题还用经验是函数对Cu的∑9晶界的几何构型进行了搜索。