Lattice Location of Solutes and Diffusion in Intermetallics

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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.