Atomic Transport in Nanocrystalline Materials
Atomic Transport in Nanocrystalline Materials
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Date
2013-02-10
Auteurs
Chellali Mohammed Reda
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Éditeur
Université Oran1 Ahmed Ben Bella
Résumé
Triple junctions, singular topological defects of the grain boundary structure, which can get a dominant role for grain growth and atomic transport in nanocrystalline matter. In my thesis the atomic transport along triple junctions and grain boundaries has been studied using the three dimensional atom probe tomography technique in nanocrystalline Ni-Cu and Ni-Bi systems. In the completely miscible Ni-Cu system, we could demonstrate that in the triple junctions of nanocrystalline Cu the diffusivity of Ni is 100-300 times higher diffusivity than in standard high angle grain boundaries. The measurements also revealed that the chemical (or segregation) width of grain boundaries unexpectedly varies with temperature. This segregation layer at the grain boundaries grows from the 0.7 nm at 563 K to 2.5 nm at 643 K. This variation is not controlled by simple volume diffusion. It was proven by kinetic analysis that the effective transport width has to possess a significant dependence on temperature. The transport product of triple junction diffusivity has been accurately measured as a function of temperature.
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Mots-clés
Triple junction, Grain boundary, Segregation, Nanocrystalline, Atom probe Tomography, Ni/Cu, Ni/Bi, Embrittlement, Ductility, Alloy