RUI: Search for Verifiable Complex Diffusion Mechanisms

扩散是原子的远距离运动，是影响材料加工和性能的一个主要因素。这种运动对于形成金属间化合物所需的原子的彻底混合是必不可少的，而不必要的扩散可能导致化合物降解。对扩散有一个坚定的理论认识；然而，对于复杂扩散机制的情况，验证这种理解的正确性的实验证据有限。这是因为很难用实验观察扩散过程的单个原子步骤。本研究的主要目的是利用计算机模拟来预测可以直接观察复杂扩散机制过程和评估理论的有利实验条件。模拟和相应的后续实验将为扩散过程提供更好的基础理解，使研究人员能够改进金属间化合物的制备方法，并找到控制材料中潜在破坏性扩散的方法。由于金属间化合物在整个工业中使用，其应用范围从医学到国家安全，因此通过加强对扩散的了解而可能对社会产生的影响可能是广泛的。研究结果将通过会议报告和同行评议的期刊文章向科学界传播，并通过信息网页和公开讲座向公众传播。预计将有5至10名本科生在参与本研究期间接受先进计算机模拟和实验技术的培训。技术总结：基于嵌入原子法（EAM）的计算机模拟被用于计算点缺陷形成、溶质位置占据、缺陷关联和迁移的自由能，所有这些都包括振动贡献，以预测二元金属间化合物中选定示踪剂的扩散途径。基于密度泛函理论的额外计算有助于参数化EAM模型并预测这些化合物中示踪剂所经历的电场梯度。本研究的主要目标是确定放射性示踪剂和金属间化合物的相容组合，使用摄动角相关光谱（PAC）测量可能允许确定有效扩散机制。特别令人感兴趣的是，通过直接观察在合作跳跃序列中形成的瞬态缺陷复合物引起的信号，有可能验证复杂扩散机制存在的预测系统，即那些机制涉及的不仅仅是简单的示踪-空位交换。考虑的候选体系包括B2和l12结构化合物中的Cd、Fe、Hf、In、Ni、Pd、Rh、Ru、Ta和Ti示踪剂。这个项目的其他活动包括检查在一系列稀土三铟的扩散机制的变化，正如实验发现的，并通过PAC测量Cd-111m在Pd3Ga7中的跳变率为未来的实验奠定基础。

NON-TECHNICAL SUMMARYDiffusion, the long-range movement of atoms, is a major factor in the processing and performance of materials. Such movement is essential for the thorough intermixing of atoms needed to form intermetallic compounds whereas unwanted diffusion can lead to compound degradation. There is a firm theoretical understanding of diffusion; however, there is limited experimental evidence verifying the correctness of that understanding for the case of complex diffusion mechanisms. This is because it is difficult to observe the individual atomic steps of diffusion processes experimentally. The main goal of this research is to use computer simulations to predict favorable experimental conditions for which the processes of complex diffusion mechanisms can be observed directly and theories assessed. The simulations and corresponding follow-up experiments will provide a better fundamental understanding of diffusion processes and allow researchers to refine preparation methods of intermetallic compounds and to find ways to control potentially destructive diffusion in materials. Since intermetallic compounds are used throughout industry, with applications ranging from medicine to national security, the impacts on society made possible by an enhanced understanding of diffusion are likely to be widespread. Results are to be disseminated to the scientific community through conference presentations and peer-reviewed journal articles and to the general public through informative webpages and public lectures. It is anticipated that between five and ten undergraduate students will receive training in advanced computer simulation and experimental techniques during their participation in this research. TECHNICAL SUMMARYComputer simulations based on the embedded atom method (EAM) are being used to calculate free energies of point defect formation, solute site occupation, defect association, and migration, all including vibrational contributions, to predict diffusion pathways of selected tracers in binary intermetallic compounds. Additional calculations based on density functional theory serve to parameterize the EAM model and predict the electric field gradients experienced by tracers in those compounds. The main goal of this research is to determine compatible combinations of radiotracers and intermetallic compounds for which measurements using perturbed angular correlation spectroscopy (PAC) are likely to allow determination of operative diffusion mechanisms. Of particular interest is predicting systems for which it is possible to verify the existence of complex diffusion mechanisms, that is, those mechanisms involving more than simple tracer-vacancy exchanges, through direct observation of the signals induced by the transient defect complexes formed during cooperative jump sequences. Candidate systems under consideration include Cd, Fe, Hf, In, Ni, Pd, Rh, Ru, Ta, and Ti tracers in B2- and L12-structured compounds. Other activities of this project include examination of the change in diffusion mechanism across the series of rare earth tri-indides, as was discovered experimentally, and laying the groundwork for future experiments through PAC measurements of Cd-111m jump rates in Pd3Ga7.