Defects and Diffusion in Intermetallic Compounds

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9612306 Collins Point defects in intermetallic compounds are investigated using perturbed angular correlations of gamma rays (PAC), a spectroscopy technique providing excellent resolution of different local environments of probe atoms in solids. Lattice vacancies and antisite defects belonging to higher- order equilibrium defects are detected in atomic shells close to indium probe atoms. The equilibrium defect of greatest interest is the high-temperature one that makes diffusion possible, such as the Schottky vacancy-pair in a binary alloy. There are three objectives to the grant: (1) Properties of equilibrium defects are determined through measurements on quenched samples and samples at high temperature. Binary alloys selected have the B2 (CsC1) structure, including high-temperature structural aluminides, such as NiAl and model systems such as PdIn and AuCd in which disturbances from PAC probes are minimal. For the first time vacancy migration enthalpies are determined from nuclear relaxation of PAC signals caused by vacancy motion at high temperature. (2) A study is made of ordering among Au and Ag atoms in the ternary compound AuAgCd2. Because the order parameter is determined locally, there is a much greater sensitivity to order-parameter fluctuations than found when using conventional diffraction methods. If the transition is found to be second-order, the measurements give insight into the range of interactions leading to the ordering. (3) The fraction of stored energy associated with lattice vacancies in mechanically-milled PdIn is examined using PAC to determine vacancy concentrations and calorimetry to determine the stored energy. To date there is inadequate knowledge of how excess energy is stored in materials during milling, and the goal of this study is to determine the amount of energy stored in vacancies. %%% The material defects under study here affect phase stability and diffusion in advanced materials used in a variety of applic ations. ***