ch5 - Diffusion in Solids Chapter 5 What you should...

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1 Diffusion in Solids Chapter 5 What you should understand by the end of this chapter: ± The importance of atomic diffusion for altering the structure of materials. ± The nature and mechanisms of diffusion. The key role of vacancies in diffusion. ± Fick’s 1 st law for diffusion. ± How to solve simple diffusion problems.
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2 Diffusion ¾ Are solids inert? That is, once a solid forms is its structure fixed forever? ABSOLUTELY NOT ¾ One mechanism for altering a solid’s internal structure is diffusion . ± movement of atoms within a solid (crystalline or amorphous)
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3 Demonstration of Diffusion ¾ Diffusion in solids is . . . ± slow. ± hard to see. ¾ Can demonstrate related process in liquid (water) and a gas (air): ± How long does it take for plastic bag to swell? (~ 5 cm)? ± How long does it take for a scent to diffuse through the classroom (~ 5 m)? ¾ Will give a more quantitative treatment of this later on.
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4 Diffusion Alters Microstructure ¾ CuAl 2 precipitates in Al matrix. ± Heated at 320 o C for 15 minutes.
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5 Vacancy Diffusion ¾ Remember, all crystals have vacancies. (X v 10 -4 at T = T M ) ¾ Atoms easily hop into vacant lattice positions ± Process is called substitution. ¾ This is a statistical process. ¾ Eventually, all atoms have a chance to move.
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6 Interstitial Diffusion ¾ Interstitial atoms are small.
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ch5 - Diffusion in Solids Chapter 5 What you should...

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