Diffusion

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  Diffusion  Introduction Diffusion is the phenomenon of material transport by atomic motion . This chapter discusses  The atomic mechanisms by which diffusion occurs  The mathematics of diffusion  The influence of temperature and diffusing species on the rate of diffusion Phenomenon of diffusion It may be demonstrated with the use of a diffusion couple, which is formed by  joining bars of two different metals together so that there is intimate contact between the two faces For Example copper and nickel Chemical analysis will reveal   Pure copper and nickel at the two extremities of the couple, separated by an alloyed region  Concentrations of both metals vary with position  This result indicates that copper atoms have migrated or diffused into the nickel, and that nickel has diffused into copper Many reactions and processes that are important in the treatment of materials rely on the transfer of mass either within a specific solid ordinarily on a microscopic level or from a liquid, a gas, or another solid phase  (b) Schematic representations of Cu (colored circles) and Ni (gray circles) atom locations within the diffusion couple.(c) Concentrations of Cu and Ni as a function of  position across the couple.  A Cu-Ni diffusion couple before a high-temperature heat treatment  A Cu-Ni diffusion couple after a high-temperature heat treatment Showing the alloyed diffusion zone. For Example  interdiffusion or impurity diffusion  Is the process, whereby atoms of one metal diffuse into another changes in concentration which occur over time  For Example for the Cu –  Ni diffusion couple  There is a net drift or transport of atoms from high to low concentration regions Self-diffusion  Is the diffusion that occurs for pure metals, but all atoms exchanging positions are of the same type  It is not normally subject to observation by noting compositional changes. Diffusion Mechanisms  From an atomic perspective: Diffusion is  just the stepwise migration of atoms  from lattice site to lattice site.  In fact: The atoms in solid materials are in constant motion, rapidly changing  positions For an atom to make such a move, two conditions must be met:  There must be an empty adjacent site  The atom must have sufficient energy to break bonds with its neighbor atoms and   Then cause some lattice distortion during the displacement   This energy is vibrationalin nature   At a specific temperature some small fraction of the total number of atoms is capable of diffusive motion, by virtue of the magnitudes of their vibrationalenergies.  This fraction increases with rising temperature.  Vacancy Diffusion The mechanism of vacancy diffusion involves the interchange of an atom from a normal lattice  position to an adjacent vacant lattice site or vacancy, figure  Diffusing atoms and vacancies exchange  positionsThe diffusion of atoms in one direction corresponds to the motion of vacancies in the opposite direction  Both self-diffusion and interdiffusionoccur by this mechanism  Interdiffusion, the impurity atoms must substitute for host atoms.The mechanism of vacancy diffusion necessitates  The presence of vacancies  The extent of vacancy diffusion is a function of the number of vacancies that are present in the metals  Concentrations of vacancies in metals increases significantly at elevated temperatures For metallic diffusion, they are two models for this atomic motion :  Vacancy Diffusion  Interstitial Diffusion
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