Experimental measurements of the correlation factor for solute diffusion

替代原子通过跳入邻近的空格位置进行扩散。但是，一个刚刚跳跃的原子可能会向后跳跃到空位中，因此，与假设的随机行走过程相比，原子的净“向前运动”减少了所谓的关联系数。对于宿主原子，关联因子只是一个反映扩散机制和扩散亚晶格几何形状的数值常数。另一方面，溶质原子与空位相互作用，改变空位的局部浓度和微观跳跃速率。这种溶质-空位相互作用使得溶质扩散的关联因子对原子相互作用的细节和温度高度敏感。它的测量应该有助于识别扩散机制，解释相互作用，并通过原子模拟促进扩散速度的可靠预测。到目前为止，还没有直接确定溶质扩散的相关系数。本提案的目的是将同一基质中溶质的扩散系数和跳跃速率的互补测量结果合并在一起，首次做到这一点。这些将分别使用放射性示踪技术和微扰角相关光谱进行测量。在一个常数范围内，相关系数只是扩散系数和跳跃率的比率。选择具有共同L12结构的A3B相作为基准系统进行研究。这些有序相中的扩散引起了人们的极大兴趣，因为存在许多可能的扩散机制，并且具有不同活化能的溶质可能发生亚晶格间和亚晶格内的跳跃。确定了组成和温度对相关因子的影响。具体地说，将测量不同温度下In3Gd相的In富组分和In贫组分的镉溶质扩散的相关系数。不同的扩散机制可以在这两种组合物上操作，这将使用具有相反相界组合物的样品来获得。明斯特大学(放射性示踪剂实验室)和美国华盛顿州立大学(扰动角度相关实验室)的最先进设施将使测量具有所需的精度。在没有任何先前测量的情况下，结果可能会令人惊讶。

Substitutional atoms diffuse by jumping into neighboring, vacant lattice sites. But an atom that has just jumped may jump backwards into the vacancy so that, compared with a hypothetical random walk process, the net 'forward movement' of the atom is decreased by a so-called correlation factor. For host atoms, the correlation factor is simply a numerical constant reflecting the diffusion mechanism and geometry of the diffusion sublattice. Solute atoms, on the other hand, interact with vacancies and modify the local concentration of vacancies and microscopic jump rates. Such solute-vacancy interactions make the correlation factor for solute diffusion highly sensitive to details of the atomic interactions and temperature. Its measurement should help identify diffusion mechanisms, interpret the interactions, and facilitate reliable predictions of diffusion rates via atomistic simulations. As yet there has been no direct determination of a correlation factor for solute diffusion. The present proposal aims to do that for the first time by combining results of complementary measurements of the diffusion coefficient and jump-rate of a solute in the same matrix. These will be measured using the radiotracer technique and perturbed angular correlation spectroscopy, respectively. To within a constant, the correlation factor is simply the ratio of the diffusion coefficient and the jump-rate. A3B phases having the common L12 structure have been chosen for study as a benchmark system. Diffusion in these ordered phases has attracted enormous interest because there exists a number of possible diffusion mechanisms and likely occurrence of inter- and intra-sublattice jumps for solutes with different activation energies. The influence of composition and temperature on the correlation factor will be determined. Specifically, correlation factors for diffusion of cadmium solutes will be measured for In-rich and In-poor compositions of the In3Gd phase at a variety of temperatures. Different diffusion mechanisms may operate at the two compositions, which will be obtained using samples having the opposing phase boundary compositions. State-of-the-art facilities at Münster University (radiotracer laboratory) and at Washington State University, USA (perturbed angular correlation laboratory) will enable measurements to be made having the required accuracy. In the absence of any previous measurements, results are likely to surprise.