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

9615228 Luzzi有机会对金属间化合物合金的界面结构和界面扩散之间的关系进行基础研究。在具有重要技术意义的金属间化合物TiAl的片层结构中发现的结晶学上尖锐的孪晶和断层界面为此类研究提供了理想的模型系统。这种合金系统的界面结构使高分辨电子显微镜(HREM)分析可以用来研究原子的位置。结合高分辨成分分析、电子能量损失谱和理论原子模拟，确定了TiAl中溶质原子在界面上的位置。用光学浮区法生长了片层状TiAl的大晶体。高温卢瑟福背散射光谱实验测量了不同温度下溶质原子穿透薄膜界面的时间。结合HREM分析的结果，可以通过Hwang-Balluffi分析计算界面扩散系数和激活能，作为界面结构和扩散方向的函数。理论研究与实验程序相结合，通过分子静力学计算确定最小能量界面结构，采用包含TiAl中成键的方向性的势。这些潜能的开发是理论研究的一个重要方面。通过对界面内和界面附近的空位组态和空位迁移能的详细分析，研究了界面的扩散特性。理论分析的结果与实验测量结果相关联，以便更清楚地了解界面上的原子能级扩散过程。这一结合了协同实验和理论研究的研究计划为研究生教育提供了一个很好的工具，学生将沉浸在深入的实验或理论研究中，并将继续接触到互补的方法。%的金属间化合物合金是结构稳定性要求很高的高温应用的候选材料。这项研究应该有助于加深对结构对合金元素扩散的影响的理解。***

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. ***