Intermetallic 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.
Intermetallic 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.
