Nag Shubhadeep, Sharma Manju, Yashonath Subramanian
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India.
School of Chemistry, University of Hyderabad, Hyderabad, Telangana 500046, India.
J Chem Phys. 2020 Dec 28;153(24):244503. doi: 10.1063/5.0035420.
In some binary alloys, the solute exhibits high or fast diffusion with low activation energy. In order to understand this, diffusion of solute atoms through a lattice of body centered cubic solvent atoms has been investigated with molecular dynamics technique. Surprisingly, solutes exhibit two distinct diffusivity maxima. Solutes migrate through the lattice mainly by diffusion from one tetrahedral void to another (tt) and, less frequently, by diffusion from a tetrahedral to an octahedral void (to) or reverse jumps (ot). Solutes with maximum diffusivity show smooth decay of the velocity autocorrelation function without backscattering. The average force on the solutes of various diameters correlates well with the position and intensity of the diffusivity maxima exhibited by the solutes. This suggests that the explanation for the diffusivity maxima lies in the levitation effect, which suggests a lowered force on the solute at the diffusivity maxima. The activation energy computed for the solutes of different sizes confirms this interpretation as it is lower for the solutes at the diffusivity maxima. Calculations with blocking of octahedral voids show that the second diffusivity maximum has significant contributions from the to diffusion path. These findings obtained here explain the fast solute/impurity atom diffusivity and low activation energies seen in the literature in many of the alloys, such as Co in γ-U and β-Zr, Cu in Pr, or Au in Th.
在一些二元合金中，溶质表现出具有低活化能的高扩散率或快速扩散。为了理解这一点，利用分子动力学技术研究了溶质原子在体心立方溶剂原子晶格中的扩散。令人惊讶的是，溶质表现出两个不同的扩散率最大值。溶质主要通过从一个四面体间隙扩散到另一个四面体间隙（tt）在晶格中迁移，较少情况下，通过从四面体间隙扩散到八面体间隙（to）或反向跳跃（ot）。具有最大扩散率的溶质显示出自速度关联函数的平滑衰减且无反向散射。作用在各种直径溶质上的平均力与溶质所表现出的扩散率最大值的位置和强度具有良好的相关性。这表明扩散率最大值的解释在于悬浮效应，这意味着在扩散率最大值处溶质所受的力降低。为不同尺寸溶质计算的活化能证实了这一解释，因为在扩散率最大值处的溶质的活化能更低。对八面体间隙进行阻塞的计算表明，第二个扩散率最大值有来自to扩散路径的显著贡献。此处获得的这些发现解释了文献中在许多合金中看到的快速溶质/杂质原子扩散率和低活化能，例如γ-U和β-Zr中的Co、Pr中的Cu或Th中的Au。
