Lecture 8 - Diffusion Diffusion in Solids ISSUES TO...

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1 Diffusion Diffusion 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 Diffusion in Solids
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2 Diffusion Diffusion Diffusion - Mass transport by atomic motion Mechanisms Solids – vacancy diffusion or interstitial diffusion 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 Diffusion After some time
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3 Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time Diffusion Diffusion A B C D A B C D Diffusion Mechanisms 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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4 • Simulation of interdiffusion across an interface: • Rate of substitutional diffusion depends on: --vacancy concentration --frequency of jumping. (Courtesy P.M. Anderson) Diffusion Simulation Diffusion Simulation Diffusion Mechanisms Diffusion Mechanisms Interstitial diffusion – smaller atoms can diffuse between atoms. More rapid than vacancy diffusion Adapted from Fig. 5.3 (b), Callister 7e .
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5 Adapted from chapter-opening photograph, Chapter 5, Callister 7e. (Courtesy of Surface Division, Midland-Ross.) Case Hardening : --Diffuse carbon atoms into the host iron atoms at the surface. --Example of interstitial diffusion is a case hardened gear. • Result: The presence of C atoms makes iron (steel) harder. Processing Using Diffusion
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This note was uploaded on 04/27/2009 for the course MEEN 222 taught by Professor Radovic during the Fall '08 term at Texas A&M.

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Lecture 8 - Diffusion Diffusion in Solids ISSUES TO...

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