Chapter 5- Diffusion in Solids

Chapter 5- Diffusion in Solids - Chapter 5: Diffusion in...

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Chapter 5: Diffusion in Solids 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 - 1
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Diffusion Diffusion (of mass) - spontaneous mass transport in solids, liquids, and gases by atomic or molecular motion due to a difference in concentration. ( Note : there are other transport phenomena that represent diffusion - heat flow, leakage current, energy transfer) Diffusion proceeds from an area of high concentration to an area of low concentration Mechanisms – Examples: ink in water, smoke in air Solids – vacancy diffusion or interstitial diffusion Chapter 5 - 2
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Diffusion Chapter 5 - 3 100% Concentration Profiles 0 Cu Ni 3 Interdiffusion : In an alloy, atoms tend to migrate from regions of large concentration. Initially After some time 100% Concentration Profiles 0 Adapted from Figs. 5.1 and 5.2, Callister 6e .
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Diffusion Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time A B C D A B C D Chapter 5 - 4
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Diffusion Mechanisms Vacancy Diffusion: • atoms exchange with vacancies applies to substitutional impurities diffusion rate depends on: --concentration of vacancies --activation energy to exchange. Chapter 5 - 5 increasing elapsed time
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Diffusion Mechanisms Interstitial diffusion – smaller atoms can diffuse between atoms. • More rapid than vacancy diffusion • Typical for interdiffusion of impurities - H, C, O, N Chapter 5 - 6 Adapted from Fig. 5.3 (b), Callister 7e .
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Chapter 5 - 7 Case Hardening : --Diffuse carbon atoms into the host iron atoms at the surface. --Example of interstitial diffusion is a case hardened gear. • Result: The "Case" is --hard to deform: C atoms "lock" planes from shearing . --hard to crack:
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This note was uploaded on 04/02/2008 for the course MSE 200 taught by Professor Wholedepartment during the Spring '08 term at N.C. State.

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Chapter 5- Diffusion in Solids - Chapter 5: Diffusion in...

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