RUI: Search for Verifiable Complex Diffusion Mechanisms

非技术和扩散，即原子的长距离运动，是材料加工和性能的一个主要因素。这种移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的降解。对扩散有坚定的理论理解；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。这是因为很难在实验上观察扩散过程的单个原子步骤。这项研究的主要目的是利用计算机模拟来预测有利的实验条件，从而可以直接观察复杂扩散机制的过程并对理论进行评估。这些模拟和相应的后续实验将提供对扩散过程的更好的基本了解，并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工业中广泛使用，应用范围从医学到国家安全，因此对扩散的进一步了解可能对社会产生广泛的影响。结果将通过会议发言和同行评议的期刊文章向科学界传播，并通过内容丰富的网页和公开讲座向公众传播。预计在参与这项研究期间，将有5到10名本科生接受高级计算机模拟和实验技术方面的培训。技术总结基于嵌入原子方法(EAM)的计算机模拟被用来计算点缺陷形成、溶质位置占据、缺陷缔合和迁移的自由能，所有这些都包括振动贡献，以预测选定示踪剂在二元金属间化合物中的扩散路径。基于密度泛函理论的附加计算用于将EAM模型参数化，并预测示踪剂在这些化合物中经历的电场梯度。这项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，对于这些组合，使用微扰角相关光谱(PAC)的测量可能允许确定有效的扩散机制。特别令人感兴趣的是预测系统，对于这些系统，可以通过直接观察在合作跳跃序列期间形成的瞬时缺陷复合体所诱导的信号来验证复杂扩散机制的存在，即那些涉及不仅仅是简单的示踪剂-空位交换的机制。正在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。该项目的其他活动包括检查实验中发现的一系列稀土三铟化合物的扩散机制的变化，并通过PAC测量Pd3Ga7中的Cd-111M跳跃速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.