Atomic Level Study of Grain Boundary Diffusion in Intermetallic Compounds using Polysynthetically Twinned TiAl as a Model System

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9615228 Luzzi An opportunity exists to carry out a basic investigation of the relationship between interfacial structure and diffusion along interfaces in intermetallic alloys. The crystallographically sharp twin-like and fault interfaces found in the lamellar structure of the technologically important intermetallic compound TiAl provide a model system which is ideal for such a study. The structure of the interfaces in this alloy system allow high-resolution electron microscopy (HREM) analysis to be used to study the location of atoms. Together with high resolution compositional analysis, electron energy loss spectroscopy, and theoretical atomistic modeling, the location of solute atoms on well-characterized interfaces in TiAl are determined. Large crystals of the lamellar TiAl are grown by optical float zone processing. High temperature Rutherford backscattering spectroscopy experiments measure the time for penetration of the solute atoms along the interfaces at various temperatures through a thin film. Incorporation of the results of the HREM analysis allows the interface diffusion coefficients and activation energies to be calculated via a Hwang-Balluffi analysis as a function of interface structure and diffusion direction. Theoretical studies are tied to the experimental program through the determination of minimum energy interfacial structures by molecular statics calculations employing potentials that include the directional character of bonding in TiAl. An important aspect of the theoretical studies is the development of these potentials. The diffusion properties at the interface are investigated through a detailed analysis of vacancy configurations within and near the interfaces and vacancy migration energies. The results of this theoretical analysis are correlated with the experimental measurements to develop a clearer understanding of the atomic level diffusion processes on interfaces. This research program which combines synergistic expe rimental and theoretical studies provides an excellent vehicle for graduate education in which the students will be immersed in in-depth experimental or theoretical investigations and will be continuously exposed to the complementary approach. %%% Intermetallic alloys are candidate materials for high temperature applications where stability of structure is important. This research should provide increased understanding of the influence of structure on diffusion of alloying elements. ***