lecture5 - Chapter Chapter 5 Diffusion in Solids Diffusion...

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Unformatted text preview: Chapter Chapter 5 Diffusion in Solids Diffusion in Solids 1 Diffusion in Solids Diffusion in Solids ISSUES TO ADDRESS... How does diffusion occur? • How does diffusion occur? • Why is it an important part of processing? • How can the rate of diffusion be predicted for some simple cases? • How does diffusion depend on structure and temperature? What is Diffusion? What is Diffusion? iffusion- ass transport by atomic motion Diffusion Mass transport by atomic motion Mechanisms • Gases & Liquids – random (Brownian) motion olids acancy diffusion or interstitial iffusion • Solids – vacancy diffusion or interstitial diffusion Definition to remember: Diffusion is a thermally activated and time dependent rocess! process! Diffusion Diffusion • Interdiffusion: In an alloy, atoms tend to migrate from regions of high concentration to regions of low concentration in general. Initially After some time Diffusion in Pure Materials Diffusion in Pure Materials • Self-diffusion: In an elemental solid, atoms also migrate. It’s the mechanism by which vacancies are eated ermally created thermally . Label some atoms After some time C A C D A D B B For any T>0 K, diffusion takes place. However, the rate might be very low at low temperature but still finite! Diffusion takes place under the (chemical) thermodynamic riving force to achieve chemical equilibrium (for all driving force to achieve chemical equilibrium (for all mechanisms). Diffusion Mechanisms Diffusion Mechanisms Vacancy Diffusion: atoms exchange with vacancies • atoms exchange with vacancies • applies to substitutional impurities atoms/alloying elements • rate depends on: umber of vacancies--number of vacancies--activation energy to exchange. increasing elapsed time Diffusion Mechanisms Diffusion Mechanisms • Interstitial diffusion – smaller atoms can diffuse between atoms. Faster than vacancy diffusion/self-diffusion! Diffusion Mechanisms Diffusion Mechanisms • Substitutional diffusion – atoms can only diffuse when a vacant lattice site is available! Slower than interstitial diffusion and self-diffusion! Interstitial vs. Substitution Diffusion Interstitial vs. Substitution Diffusion Interstitial diffusion involves the “jump” of the diffusion species from one interstitial site to another in a given host lattice. Therefore, the activation energy for this diffusion mechanism is due to hopping from interstitial site interstitial site and is governed by the kinetic energy supplied to the to interstitial site and is governed by the kinetic energy supplied to the diffusing species. Very common in FCC metals with small size alloying elements such as C in Fe (steel)....
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This note was uploaded on 02/08/2011 for the course GENERAL EN 407 taught by Professor Akdogan during the Fall '10 term at Rutgers.

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lecture5 - Chapter Chapter 5 Diffusion in Solids Diffusion...

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