Diffusion and Solute-Solute Interactions in Intermetallic Compounds

金属间化合物是关键的技术材料。 例如，NiAl或镍铝化物用作波音飞机喷气涡轮机的结构材料。 它必须在巨大的离心力和白热的温度下保持强度。 物理和材料特性最终取决于原子水平上的结构和动力学。 与常见的岩盐晶体结构一样，金属间化合物如NiAl和本项目所研究的金属间化合物具有高度有序的结构，但不可避免地含有点缺陷。 这些包括原子在错误的位置和丢失的原子，称为“空位”。 空位使固体中的原子有可能通过原子跳跃进入相邻的空位，这一过程称为扩散。 然而，这种运动可能导致缺陷的聚集，弱化材料并增加断裂的可能性。 因此，研究晶体中缺陷的行为是很重要的。 该项目将使用一种可以在原子尺度上监测探测原子位置的核光谱学。 光谱学PAC测量杂质探针原子的核与探针的局部环境产生的场的相互作用。 不同的局部环境导致不同的场，用于标记和识别探针的位置。 此外，还可以测量探针原子在高温下跳跃的速率。 该项目有两个主要目标。 (1)通过对一系列相关化合物的研究，加深对扩散系统学的理解。 (2)测量化合物中杂质原子对相互吸引或排斥的相互作用能。 实现这些目标将有助于更好地了解金属间化合物在高温下的行为，在真正的原子尺度上测量。 PI将通过培训研究生、本科生和高中生，将研究与教育结合起来。将努力让女学生和代表性不足的少数民族成员参与进来。将从大学和当地高中寻找有前途的学生，并将维护一个指导小组研究网站。技术概述金属间化合物是关键的技术材料。 物理和材料特性最终取决于原子水平上的结构和动力学。 在这个项目中，加里S.柯林斯和学生们正在应用一种专门的核超精细光谱学，γ射线的扰动角相关（PAC），研究金属间化合物中的扩散现象和溶质相互作用。 学生接受固态物理，核实验室方法和数据分析的跨学科培训。 主要的可测量的是由局部电场梯度（EFG）引起的PAC探针核的核四极相互作用。 该项目建立在柯林斯实验室在过去十年中开创的两种方法的基础上。 第一个是通过分析由涨落EFG引起的PAC谱中的核弛豫来测量探针原子的扩散跳跃频率。 第二个是在不同的晶体学网站的探针原子溶质的能量差异的测定。 这两种测量都是在高温下的热力学平衡中进行的。 PAC的结果将得到支持的经典扩散率测量，其中杂质扩散的相关因素将被确定为第一次在一个直接的方式。 将进行以下三个主要工作：1）将在选定的系统中测量镉示踪原子的跳跃频率，以完善已经对具有Cu 3Au结构的五个系列的稀土相进行的广泛研究。 2)关联因子是杂质扩散理论中的一个重要参数，但还没有直接测量过。 扩散率测量将在几种合金上进行，这些合金已经有精确的PAC跳频数据。 将扩散率除以跳频得到相关因子;还将确定其温度依赖性。 3)溶质原子对之间的相互作用能将被测量的第一次，与一个原子对是PAC探针原子。 晶体EFG和由PAC探针附近的溶质原子引起的EFG将用于识别探针和溶质原子的位置。 将从各种溶质和相的溶质位点分数的温度依赖性来确定缔合物，所述溶质和相包括具有共同的立方晶体结构的相。 这项工作是基础和跨学科的范围，桥接固态物理和材料科学。 PAC具有在高温下确定详细原子排列和原子运动的出色能力。 对其他系列稀土合金的跳频测量将有助于巩固规则，显示探针原子的扩散行为（和位置偏好）如何取决于相，成分和温度。 详细的扩散机制可以阐明复杂的晶体结构。 测量的相关系数也可能取决于扩散机制，并使人们深入了解扩散机制。 溶质-溶质相互作用的研究将被用来制定预测网站的偏好和估计溶质原子对之间的关联能的启发式规则。

Non-technical SummaryIntermetallic compounds are key technological materials. As an example, NiAl, or nickel aluminide, is used as a structural material in jet turbines of Boeing aircraft. It must retain strength under enormous centrifugal forces and white-hot temperatures. Physical and material properties ultimately depend on structure and dynamics at the atomic level. Like the familiar rock-salt crystal structure, intermetallics such as NiAl and the ones to be studied in this project have highly ordered structures, but inevitably contain point defects. These include atoms on wrong sites and missing atoms, called "vacancies". Vacancies make it possible for atoms in solids to move through jumps of atoms into neighboring vacancies, a process known as diffusion. However, such motion may lead to agglomeration of defects, weakening the material and increasing the potential for fracture. For this reason, it is important to study the behavior of defects in crystals. This project will use a nuclear spectroscopy that can monitor the locations of probe atoms on an atomic scale. The spectroscopy, PAC, measures interactions of nuclei of impurity probe atoms with fields produced by the local surroundings of the probes. Different local surroundings lead to distinct fields that serve to label and identify the locations of the probes. In addition, the rate at which the probe atoms jump at high temperature can be measured. This project has two main goals. (1) To obtain a better understanding of the systematics of diffusion through studies on series of related compounds. (2) To measure interaction energies of pairs of impurity atoms in compounds, which may attract or repel each other. Achieving these goals will contribute to a better understanding of the behavior of intermetallic compounds at high temperatures, measured on a true atomic scale. The PI will integrate the research with education by training graduate, under-graduate as well as high-schools students. Efforts will be made to involve female students and members of under-represented minorities. Promising students from within the university and local high schools will be sought out and a tutorial group research website will be maintained.Technical SummaryIntermetallic compounds are key technological materials. Physical and material properties ultimately depend on structure and dynamics at the atomic level. In this project, Professor Gary S. Collins and students are applying a specialized nuclear hyperfine spectroscopy, perturbed angular correlation of gamma rays (PAC), to study diffusion phenomena and solute interactions in intermetallics. Students receive interdisciplinary training in solid-state physics, nuclear laboratory methods, and data analysis. The principal measurable is the nuclear quadrupole interaction at PAC probe nuclei caused by local electric field gradients (EFGs). The project builds on twin methodologies pioneered in Collins's laboratory over the previous ten years. The first is measurement of diffusional jump-frequencies of probe atoms made through analysis of nuclear relaxation in PAC spectra caused by fluctuating EFGs. The second is determination of differences in energies of probe-atom solutes at different crystallographic sites. Both kinds of measurements are made in thermodynamic equilibrium at high temperature. PAC results will be buttressed by classical diffusivity measurements out of which correlation factors for impurity diffusion will be determined for the first time in a direct way. Three major efforts will be undertaken, as follows: 1) Jump-frequencies of cadmium tracer atoms will be measured in select systems to round out extensive studies already made for five series of rare-earth phases having the Cu3Au structure. 2) The correlation factor is an important parameter in the theory of impurity diffusion but has never been measured in a direct way. Diffusivity measurements will be carried out on several alloys for which precise PAC jump-frequency data already exist. Dividing the diffusivity by the jump-frequency yields the correlation factor; its temperature dependence will also be determined. 3) Interaction energies between pairs of solute atoms will be measured for the first time, with one atom of the pair being a PAC probe atom. Crystal EFGs and EFGs caused by solute atoms near the PAC probes will be used to identify locations of probe and solute atoms. Association enthalpies will be determined from temperature dependences of solute site fractions for a variety of solutes and phases, including phases having common, cubic, crystal structures. This work is fundamental and interdisciplinary in scope, bridging solid-state physics and materials science. PAC has an excellent ability to determine detailed atomic arrangements and atom movements at high temperature. Jump-frequency measurements on additional series of rare-earth alloys will help to firm up rules showing how diffusion behavior (and site-preferences) of probe atoms depend on phase, composition and temperature. Detailed diffusion mechanisms may be elucidated for complex crystal structures. Measured correlation factors may also depend upon, and give insight into, diffusion mechanisms. Studies of solute-solute interactions will be used to formulate heuristic rules for predicting site preferences and estimating association energies between solute-atom pairs.