Ch5 - Chapter 5 Diffusion Diffusion 1 Chapter 5: Diffusion...

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1 Chapter 5 Diffusion Chapter 5 Diffusion Diffusion
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2 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
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3 Chapter Outline Diffusion - how do atoms move through solids? Diffusion mechanisms Vacancy diffusion Interstitial diffusion Impurities The mathematics of diffusion Steady-state diffusion (Fick’s first law) Nonsteady-State Diffusion (Fick’s second law) Factors that influence diffusion Diffusing species Host solid Temperature Microstructure Nonsteady-State Diffusion
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4 Diffusion Diffusion - Mass transport by atomic motion Mechanisms Solids – vacancy diffusion or interstitial diffusion
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5 Diffusion Diffusion is a process of mass transport that involves the movement of one atomic species into another. It occurs by random atomic jumps from one position to another and takes place in the gaseous, liquid, and solid state for all classes of materials.
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6 Material transport by atomic motion. Within the same solid: high to low concentration From one solid to another solid From liquid to solid From Gas to solid Temperature plays a significant role in diffusion Diffusion
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7 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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8 Diffusion Mechanisms Two Conditions Empty site next to the diffusing atom Diffusing atom has enough energy to move Diffusion energy can dramatically increase by increasing temperature Two types Vacancy Diffusion Interstitial Diffusion
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9 Vacancy Diffusion Atoms in a normal site moves to a vacant lattice site There has to be a vacancy Could be self or inter diffusion This atom could be a host or substitutional atom
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10 Diffusion Mechanisms Vacancy Diffusion: Ex: Zinc diffuses in brass Zn into Cu brass alloy since size of Zn > size of Cu, but the sizes are comparable, Zn atom cannot fit into the interstices Cu Cu Cu Cu Cu Cu Cu Cu Zn
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11 Interstitial Diffusion Interstitial impurity atom moves to another interstitial lattice site Very fast compared to Vacancy diffusion. WHY? Only interdiffusion.
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12 Interstitial Diffusion Example: Carbons diffuse into Fe Steel size of C << size of Fe Carbons will fit into the interstitial sites
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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
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This note was uploaded on 03/15/2011 for the course CHE 215 taught by Professor Aboyousef during the Spring '11 term at American University of Sharjah.

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Ch5 - Chapter 5 Diffusion Diffusion 1 Chapter 5: Diffusion...

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