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Dobson+CH6+Diffusion+2011

Dobson+CH6+Diffusion+2011 - DIFFUSION ISSUES TO ADDRESS How...

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ISSUES TO ADDRESS... 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? DIFFUSION
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Diffusion -­‐ Mass transport by atomic mo9on Mechanisms Gases & Liquids – random (Brownian) mo9on Solids – vacancy diffusion or inters99al diffusion Diffusion http://www.youtube.com/watch?v=gXJMBgyT_hk&feature=related http://www.youtube.com/watch?v=STLAJH7_zkY&feature=related
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Interdiffusion : In an alloy, atoms tend to migrate from regions of high concentration to regions of low concentration Initially Adapted from Figs. 5.1 and 5.2, Callister & Rethwisch 8e. After some time Diffusion Ni Cu
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Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms A B C D After some time A B C D Diffusion
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Vacancy Diffusion : atoms exchange with vacancies applies to substitutional impurities atoms rate depends on: -- number of vacancies -- activation energy to exchange. increasing elapsed time Diffusion Mechnaisms
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• Simulation of interdiffusion across an interface: Rate of substitutional diffusion depends on: -- vacancy concentration -- frequency of jumping (Courtesy P.M. Anderson) Diffusion Simula9on
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Inters99al diffusion – smaller atoms can diffuse between atoms. *More rapid than vacancy diffusion Adapted from Fig. 5.3(b), Callister & Rethwisch 8e. Diffusion Mechanisms
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What about our Cu/Ni example? -> Vacancy or Interstitial? -> Why? Diffusion
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•Food marinating •Carburization for Surface Hardening of Steels •Food packaging, beverage bottles •Mylar Balloons Applica9ons of Diffusion
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Adapted from chapter-opening photograph, Chapter 5, Callister & Rethwisch 8e. (Courtesy of Surface Division, Midland-Ross.) Case Hardening : -- Diffuse carbon atoms into the host iron atoms at the surface.
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