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Lecture 5 - 8th Ed. Ch. 5

Lecture 5 - 8th Ed. Ch. 5 - Chapter 5 Diffusion in Solids 1...

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1 Chapter 5 Diffusion in Solids
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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 in Solids
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What is Diffusion? Diffusion - Mass transport by atomic motion Mechanisms Gases & Liquids random (Brownian) motion Solids vacancy diffusion or interstitial diffusion Definition to remember: Diffusion is a thermally activated and time dependent process!
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Interdiffusion: In an alloy, atoms tend to migrate from regions of high concentration to regions of low concentration in general. Initially Diffusion After some time
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Self-diffusion: In an elemental solid, atoms also migrate. It’s the mechanism by which vacancies are created thermally . Label some atoms After some time Diffusion in Pure Materials A B C D A B C D 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 driving force to achieve chemical equilibrium (for all mechanisms).
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Diffusion Mechanisms Vacancy Diffusion: atoms exchange with vacancies applies to substitutional impurities atoms/alloying elements rate depends on: --number of vacancies --activation energy to exchange. increasing elapsed time
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Diffusion Mechanisms Interstitial diffusion smaller atoms can diffuse between atoms. Faster than vacancy diffusion/self-diffusion!
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