挥发条件下溶质表面偏聚动力学的实验和理论研究

Surface segregation of solute atoms is an important phenomenon which affects the surface properties of materials significantly. Many experiment have proved the occurrence of the evapoeration of segergated atoms and the consequent change in the kinetics of surface sergation as well as the resulting effect on the surface properties of materials. However, most researchers neglected the fact of the evaporation directly when studying surface segregation. Therefore, the influence of the evaporation on the kinetics of surface segregation has yet not been investigated thoroughly. In this case, the relationship between the surface segregation of solute atoms and the surface properties of materials still keeps ambiguous until now. Under these circumstances, the experimental study on the surface segregation of solute atoms will be carried out in the present project by emphasizing on the phenomenon of the evaporation of Bi in Ni(Bi) alloy in order to explore that how the evaporation affects the surface segregation. Then, the incomplete model proposed by the applicant concerning the kinetics of surface segregation in the case of the evaporation will be confirmed through the experimental results, theory of regular solution and Henry’s law. At the same time, the equations corresponding to the model will be derived for the sake of quantifying the influence of the evaporation on the kinetics of surface segreation. Based on the experimental results and the derived equations, computer simulation will be initiated subsequently to obtain the values of physical parameters used in the equations, including diffusion constant and diffusion activation energy both in bulk and at grain boundary. The present project will provide experimetal results, therectical basis and quantitative data to the development of segregation theory, modification of surface property and purifying of materials.

溶质的表面偏聚是影响材料表面性能的重要物理现象。实验已证实，偏聚在表面的溶质会产生挥发，从而直接改变其表面偏聚动力学和材料表面性能。目前，绝大多数的研究结果直接忽略了挥发这一事实，未能深入探讨挥发对表面偏聚动力学的影响规律，因而也未能真正掌握表面偏聚行为与材料表面性能之间的联系。由此，本项目针对前期实验确定的Bi挥发特性，开展Bi在Ni-25ppmBi二元合金表面的偏聚和挥发实验研究，掌握挥发对表面偏聚动力学的影响过程；然后基于实验结果、正规溶液理论以及亨利定律，修正和完善申请人初步建立的“挥发条件下的溶质表面偏聚动力学”模型并推导相应方程，以此定量阐明挥发的影响机制；同时依据实验结果和理论方程进行模拟计算，获得重要物理参量（体扩散常数、体扩散激活能、晶界扩散常数、晶界扩散激活能等）的定量结果。本项目可在偏聚理论发展、材料表面改性以及材料纯净化等方面提供新的实验结果、理论基础和定量数据。

溶质的表面偏聚是影响材料表面性能的重要物理现象。实验已证实，偏聚在表面的溶质会产生挥发，从而直接改变其表面偏聚动力学和材料表面性能。目前，绝大多数的研究结果直接忽略了挥发这一事实，未能深入探讨挥发对表面偏聚动力学的影响规律，因而也未能真正掌握表面偏聚行为与材料表面性能之间的联系。由此，本项目针对前期实验确定的Mg挥发特性，开展Mg在Al-0.67at%Mg二元合金表面的偏聚和挥发实验研究，掌握挥发对表面偏聚动力学的影响过程；然后基于实验结果、一阶反应动力学理论以及亨利定律，建立“挥发条件下的溶质表面偏聚动力学”模型并推导相应方程，以此定量阐明挥发的影响机制；同时依据实验结果和理论方程进行模拟计算，获得重要物理参量（富集比、挥发参数、挥发激活能等）的定量结果。本项目提出了一个完整且合理的动力学模型。通过将模型边界条件考虑为溶质原子在表面层的累积量等于在基体内的扩散通量与在表面层的挥发通量之差，建立了挥发条件下表面偏聚动力学方程，进而利用数学求解得到合理且可靠的挥发条件下表面偏聚动力学公式。通过挥发速率常数的一般表达形式和简化形式赋予了挥发参数明确的物理意义。量化了挥发激活能，进一步揭示了挥发机制。拟合了Mg在Al-0.8wt%Mg中195℃时的偏聚动力学，获得了拟合最好的富集比16.484和挥发参数0.575。此外计算得出Mg从合金表面挥发的挥发激活能为142.23kJ/mol，与Mg原子和Al原子间的键能（127.48kJ/mol）非常接近，揭示了Mg原子从吸附于表面的位置挥发。通过采取变换挥发速率常数简化形式的方法，揭示了动力学方程无法反映出表面浓度最大值与温度的变化规律这一问题的实质，即挥发参数不是与温度无关的常数，而是随着温度的升高而减小的。本项目可在偏聚理论发展、材料表面改性以及材料纯净化等方面提供新的实验结果、理论基础和定量数据。