Lecture 5 - 8th Ed. Ch. 5

Lecture 5 8th Ed. - Chapter 5 Diffusion in Solids 1 Diffusion in Solids ISSUES TO ADDRESS How does diffusion occur Why is it an important part of

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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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Diffusion Mechanisms Substitutional diffusion – atoms can only diffuse when a vacant lattice site is available! Slower than interstitial diffusion and self-diffusion!
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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 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). Substitutional diffusion involves the migration of the diffusing species into a vacant site. Therefore, this diffusion mechanism is more energetically costly (hence slower) than interstitial diffusion as one needs to create vacancies first! This mechanism is very common in oxide ceramics (Al 2 O 3 , TiO 2 , Fe 3 O 4 , BaTiO 3 etc.) and in nonmetallic materials in general. Self- diffusion is essentially this mechanism. As a rule of thumb the following holds: interstitial substitutional interstitial interstitial jump substitutional subtitutional vacancy jump QQ Q Q Q
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Case Hardening : --Diffuse carbon atoms into the host iron atoms at the surface.
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This note was uploaded on 02/20/2012 for the course MSE 14:635:407 taught by Professor Dr.e.korayakdoğan during the Spring '12 term at Rutgers.

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Lecture 5 8th Ed. - Chapter 5 Diffusion in Solids 1 Diffusion in Solids ISSUES TO ADDRESS How does diffusion occur Why is it an important part of

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