2010-02-03 Chapter 05 Diffusion

2010-02-03 Chapter 05 Diffusion - Chapter 5: Diffusion in...

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Chapter 5 - 1 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? Chapter 5: Diffusion in Solids Why study Diffusion? Materials are often heat treated to improve their properties, and heat treatments involve atomic diffusion. Gears are often case hardened, i.e. their hardness and resistance to failure by fatigue have been enhanced by diffusing excess carbon or nitrogen atoms into the outer surface.
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Chapter 5 - 2 Diffusion Diffusion - Mass transport by atomic motion Mechanisms Solids – vacancy diffusion or interstitial diffusion
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Chapter 5 - 3 Interdiffusion : In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. Initially Adapted from Figs. 5.1 and 5.2, Callister 7e . Diffusion After some time
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Chapter 5 - 4 Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time Diffusion A B C D A B C D
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Chapter 5 - 5 Diffusion Mechanisms Vacancy Diffusion: atoms exchange with vacancies applies to substitutional impurities atoms rate depends on: --number of vacancies --activation energy to exchange. increasing elapsed time
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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)
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2010-02-03 Chapter 05 Diffusion - Chapter 5: Diffusion in...

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