MCE 230_F10_lec_16 - MCE 230 Materials Science Fall 2010...

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MCE 230 Materials Science Fall 2010 Lecture - 16
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Diffusion
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http://www.youtube.com/watch?v =pYQuqNFG2ro Example: Vacuum carburization process
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Diffusion Material transport by atomic motion. Within the same metal: high to low concentration (self-diffusion) Between two metals ( interdiffusion or impurity diffusion)
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Before diffusion After diffusion
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Diffusion Mechanisms For atoms to move from one position to another two conditions have to be met: 1. Empty site next to the diffusing atom 2. Diffusing atom has enough energy to move Temperature can dramatically increase the diffusion energy Two dominate mechanisms for metallic diffusion: 1. Vacancy Diffusion 2. Interstitial Diffusion
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Vacancy Diffusion Atoms in a normal site moves to a vacant lattice site There has to be a vacancy In metals more vacancies exists at higher temperature Could be self or inter diffusion
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Interstitial Diffusion Interstitial impurity atom moves to another interstitial lattice site
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