【作者】 李阳；

【导师】 汤富领； 薛红涛；

【作者基本信息】 兰州理工大学 ， 材料物理与化学， 2021， 硕士

【摘要】 目前合金材料中存在的过度合金化现象以及稀贵元素回收困难或成本高昂问题已成为制约合金材料可持续发展的突出“瓶颈”,因此在不(少)依赖合金元素的前提下如何提高合金性能急需新的解决方案。晶界弛豫是提高晶界稳定性或稳定纳米金属材料的有效策略,它可以使原有的晶界转变为一种更低能态的晶界从而达到稳定纳米晶金属材料的效果。以往对晶界弛豫的研究主要集中在纯纳米晶金属材料中,虽然取得了一定的成果,但还有不足之处,极少考虑到杂质或合金原子等外来原子在晶界弛豫中的作用。因此,本文采用分子动力学方法系统研究了CuΣ3[110](112)和Σ17[110](223)两种对称倾斜晶界在剪切载荷作用下的能量变化和结构弛豫,以及杂质元素(Ag、Fe和Ni)和温度对晶界弛豫的影响;并通过详细观察晶界在不同剪切应变和温度下的结构和位错变化等,揭示了杂质原子和温度对晶界弛豫的微观作用机制。我们的工作不仅可为不(少)依赖合金元素的Cu晶界调控和性能提升提供理论指导,还可为合金成分设计提供有益参考。取得的主要结果如下:(1)高能CuΣ17[110](223)和低能CuΣ3[110](112)对称倾斜晶界的结构单元在能量最小化的过程中均会自发地解离成由Σ11(113)和Σ3(111)晶界的结构单元组成的两个晶界。与优化的CuΣ3[110](112)晶界相比,在优化后的Σ17[110](223)晶界中,除肖克莱不全位错外,还存在Hirth、Lomer-Cottrell位错和其他位错。(2)剪切应变通过在优化后的Cu晶界处发射肖克莱不全位错从而引发纯CuΣ3[110](112)对称倾斜晶界以及含Fe、Ni或Ag晶界的结构转变。尽管Fe、Ni或Ag原子在整个晶界模型中的浓度非常低(0.00165 at.%),但它们对剪切应变诱导的结构弛豫仍具有阻碍作用。在本文所考虑的温度范围内(50 K～400 K),温度并不能独立地触发晶界弛豫,但在剪切应变诱导的纯Cu以及含Fe、Ni或含Ag晶界的结构弛豫过程中起到积极的促进作用,也即随着温度升高,触发和完成结构弛豫所需要的剪切应变均减小。(3)由于在优化后的CuΣ17[110](223)晶界处存在Hirth位错锁和Lomer-Cottrell位错锁,导致该晶界结构不会随着剪切应变的增加而发生改变。此外,在50 K时,Ni原子在晶界处的存在会促进紊乱晶界区域原子的能量弛豫,但随着温度升高,这种促进作用会减弱甚至消失;Ag原子在晶界处的存在会阻碍紊乱晶界区域原子的能量弛豫,且这种阻碍作用不受温度影响;Fe原子在晶界处的存在对紊乱晶界区域原子的能量弛豫无任何促进或抑制作用。更多还原

【Abstract】 At present,the over alloying phenomenon and the difficulty or high cost of rare precious elements recovery have become the prominent bottleneck to restrict the sustainable development of alloy materials.Therefore,how to improve the properties of alloys without(less)reliance on alloying elements is in urgent need of new solutions.Grain boundaries(GBs)relaxation is a promising and effective strategy to improving GB stability or stabilizing nanocrystalline metals,it can transform the original GB into a lower energy state GB to achieve the effect of stabilizing nanocrystalline metal materials.The previous studies mainly focused on nanocrystalline pure metals and GB behaviors therein,which rarely considering the role of foreign atoms such as impurity or alloying atoms in GB relaxation.In this work,the energy changes and structural relaxation under shear load of Cu Σ3[110](112)and Σ17 [110](223)two kinds symmetric tilt GBs(STGBs),and the effects of impurity elements(Fe,Ni and Ag)and temperature on the GB relaxation were investigated in systematic by molecular dynamics method.The microscopic mechanism of the effect of impurity atoms and temperature on GB relaxation were revealed by observing the GB structure and dislocation changes under different shear strains and temperatures.Our work can not only provide theoretical guidance for Cu GB control and performance improvement,but also provide useful reference for alloy composition design.The main results obtained are as follows:(1)The structural units(SUs)of the high-energy Cu Σ17 [110](223)GBs and low-energy Cu Σ3 [110](112)GBs dissociate spontaneously into two GBs that are made up of SUs of Σ11(113)and Σ3(111)GBs in the process of energy minimization.Compared with the optimized Cu Σ3 [110](112)GBs,there are also Hirth,Lomer-Cottrell and other unidentified dislocations in the optimized Σ17 [110](223)GBs,besides the Shockley partial dislocations.(2)The shear strain can trigger the structural transformation of pure Cu Σ3[110](112)GB and Fe-,Ni-and Ag-containing Cu GBs by the emission of Shockley partial dislocations from the optimized Cu GBs.However,Fe,Ni and Ag,all three have impediment effects on the shear-strain induced GB relaxation,though the content of Fe,Ni or Ag atom(0.00165 at.%)is quite low in the GB model.The temperature cannot trigger GB relaxation independently within the considered temperature range(50 K-100 K)in this paper,but play a positive role in the shear-strain induced structural relaxations of pure,Fe-,Ni-and Ag-containing Cu GBs,the shear strains required to start and finish the SU transformation become smaller with the increase of temperature.(3)Because of the Hirth dislocation lock and lomer-cottrel dislocation lock at the optimized Cu Σ17 [110](223)GBs,the SUs of the GB would not change with the increase of shear strain.In addition,at 50 K,the presence of Ni atoms at the GB can promote the energy relaxation of atoms in the disordered GB region,but this promotion would weaken or even disappear with the increase of temperature.The existence of Ag atoms at the GB can hinder the energy relaxation of atoms in the disordered GB region,and this inhibition is not affected by the temperature.The existence of Fe atoms at the GB does not have any promote or inhibit effect the energy relaxation of atoms in the disordered GB region.更多还原