RUI: Search for Verifiable Complex Diffusion Mechanisms

Northern Kentucky University Research Foundation (Do Not Use), Highland Height KY

Investigators

Abstract

NON-TECHNICAL SUMMARY Diffusion, 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 SUMMARY Computer 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.