Lattice Location of Solutes and Diffusion in Intermetallics

技术支持：溶质的晶格位置和金属间化合物中原子的扩散将使用超精细相互作用方法，扰动角相关的γ射线（PAC）进行研究。该研究建立在现有NSF资助下开发的工具和理解的基础上。溶质的晶格位置影响材料的许多性质。作为溶质原子的PAC探针的位置可以通过测量四极相互作用来确定。PAC测量的一个特点是溶质以化学稀释极限存在。晶格位置已被观察到显着变化，在非常狭窄的范围内的组成，即使是“线”的化合物在二元相图。已经观察到溶质在合金成分的置换位点、具有间隙特征的空亚晶格位点和诸如晶界中的位点之间进行各种切换。一个热力学模型和一套启发式规则的开发，解释这些意见。在这个赠款PAC测量具有其他晶体结构的阶段，以发展理解的系统性的网站偏好行为。一些核磁共振测量也计划用于测量其他溶质。原子运动是所有扩散现象的基础。发现了一种研究化合物中原子运动的新方法。当一个超精细探针原子跳跃的非立方网站的电场梯度重定向在每个跳跃的子晶格，跳跃导致退相干或松弛的四极相互作用，可以拟合得到的跳跃频率。激活跃迁频率的hapropies确定具有Cu3Au结构的相位与示踪剂扩散研究中的精度一样好。以同样的方式为晶格位置，跳跃频率已被发现是高度敏感的小组成变化。PAC的研究计划为其他Cu3Au相和更复杂的晶体结构，其中探针原子之间的不等价的亚晶格跳跃。对于一些复杂的结构，它将被证明，可以区分内部和内部的示踪原子，不可能做到使用示踪剂扩散方法的子晶格跳跃频率。非技术性的：这些研究将增加对固体的两个基本方面的理解。稀溶质的晶格位置更清楚地表现出强大的组成和温度依赖性预测的解决方案的热力学和增加我们的理解的网站偏好行为。跳跃频率测量，区分不同种类的跳跃提供了其他方法无法获得的信息，也可以深入了解详细的扩散机制。该项目涉及两名研究生和几名本科生的参与和培训，为期四年，使用不同的学生群体。结果将在出版物和专题介绍中广泛传播。这项研究与更广泛的受众之间的联系将通过本科生和研究生级别的研讨会进行。

TECHNICAL: Lattice locations of solutes and diffusion of atoms in intermetallics will be studied using a hyperfine interaction method, perturbed angular correlation of gamma rays (PAC). The research builds on tools and understandings developed under a current NSF grant. Lattice locations of solutes affect many properties of materials. Locations of PAC probes, used as solute atoms, can be determined by measurement of quadrupole interactions. A feature of PAC measurements is that solutes are present at the thermodynamically dilute limit. Lattice locations have been observed to change dramatically over very narrow ranges of composition, even for "line" compounds in binary phase diagrams. Solutes have been observed to switch variously among substitutional sites of alloy constituents, empty-sublattice sites having interstitial character, and sites such as in grain boundaries. A thermodynamic model and set of heuristic rules were developed that explain these observations. In this grant PAC measurements are made for phases having other crystal structures in order to develop understanding of the systematics of site preference behavior. Some nuclear magnetic resonance measurements also are planned for measurements on additional solutes. Atom movement underlies all diffusion phenomena. A new method has been discovered to study atom movement in compounds. When a hyperfine probe atom jumps on a sublattice of noncubic sites for which the electric field gradient reorients in each jump, the jumps lead to decoherence or relaxation of the quadrupole interaction that can be fitted to obtain the jump frequency. Activation enthalpies for jump frequencies were determined for phases having the Cu3Au structure with precision as good as in tracer diffusion studies. In the same way as for lattice locations, jump frequencies have been found to be highly sensitive to small compositional changes. PAC studies are planned for other Cu3Au phases and for more complex crystal structures in which probe atoms jump between inequivalent sublattices. For some complex structures, it will be shown that one can discriminate between intra- and inter-sublattice jump frequencies of tracer atoms, something impossible to do using the tracer diffusion method. NONTECHNICAL: These investigations will increase understanding of two fundamental aspects of solids. Lattice locations of dilute solutes more clearly exhibit strong composition and temperature dependences predicted by the thermodynamics of solutions and increase our understanding of site preference behavior. Jump-frequency measurements that discriminate among different kinds of jumps provide information unattainable by other methods and may also give insight into detailed diffusion mechanisms. The project involves participation and training of two graduate students and several undergraduates over a four-year period, using a diverse group of students. Results will be disseminated broadly in publications and presentations. Contact between this research and a broader audience will be made through seminars at the undergraduate and graduate levels.