Lecture note 24 (11-29-2011)

Lecture note 24 (11-29-2011) - MAE 581 Advanced Materials...

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1 MAE 581 Advanced Materials Science Lecture note 24 Diffusion The question of how long it takes for certain atoms in a solid alloy to “diffuse” from one place to another has always been the research focus of metallic alloys. Since the time of Faraday it has been known that such atomic migration was possible, in contradiction of the rule ‘corpora non agunt nisi fluida’. A knowledge of diffusion in the solid state is necessary to describe the kninetic of solid state reactions. In addition, the atomistic mechanism of diffusion is itself, mainly vacancies, without which it would be impossible for atoms in a close-packed metal to change places. We shall consider these lattice defects (dislocations). In the following paragraphs the course of diffusion in an alloy with time will be described using essentially a continuum model. Different diffusion mechanisms in alloys Isothermal diffusion We imagine a dilute single phase alloy (B in A, or an isotope A * in AB). Let the distribution of B
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Lecture note 24 (11-29-2011) - MAE 581 Advanced Materials...

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