Subject 6 Diffusion 4 to 8

Subject 6 Diffusion 4 to 8 - Kinetics and Phase...

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Kinetics and Phase Transformations E. Ma 2010 6.1 6.4 Atomic mechanisms for diffusion in the solid state It will be useful to have a picture of the different kinds of microscopic diffusion mechanisms and the different diffusing species that may be encountered. The question is, how do we switch the positions for the impurity (dilute diffusing species A) and the host atoms? The list below is not exhaustive. Let’s first look at 1. Substitutional diffusion (swap positions). a. Direct Exchange In exchange, two atoms just change positions. This is uncommon, although has been implicated in silicon self-diffusion in the bulk. This mechanism involves significant crowding in the activated state. However, on the surface, a variation of exchange, push-out, is very common, because the crowding is avoided due to the open surface, b. Ring mechanism due to Zener See figure in class. This requires a many-body process and thus multi-atom coordination. It is unlikely, even when in less compact structures like bcc. c. Vacancies 1
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Kinetics and Phase Transformations E. Ma 2010 6.2 This is the accepted mechanism, exchange with vacancies. It is very common, and dominates diffusion in fcc metals. It can explain the Kirkendall effects (see later). We will single this one out and discuss more in 6.3. For example, we will ask: then, are there really many vacancies? 2. Interstitial diffusion In interstitial diffusion, the (small) diffusing atom lives in the interstitial sites of the lattice formed by the larger atom species. This is a very common mechanism; examples include H, C in Fe. But the species diffusing should be small enough for this to be possible. The generation of self-interstitial would require a high activation energy, say 4 eV relative to ~1 eV for vacancy formation. 3. Adatoms and surface diffusion 2
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Kinetics and Phase Transformations E. Ma 2010 6.3 “Adsorbed atoms”, or, adatoms, participate in surface diffusion. Because they have fewer bonds than vacancies, adatoms diffuse relatively quicker in general, at least in the absence of any chemical or mechanical effects. 4. Grain boundary diffusion This case is somewhere in between surface and bulk diffusion; atoms diffuse through the generally less dense regions associated with grain boundaries. 5. Dislocations Diffusion through the dislocation core is a short-circuit path due to excess space and tensile region. Also, dislocation climbs, in the presence of vacancy diffusion. In this process, a vacancy annihilates itself with an atom in the dislocation core, causing the dislocation to do negative climb. 6.5 The vacancy diffusion mechanism Vacancy diffusion is extremely common in fcc metals, and nicely pulls together the concepts we’ve developed in thermodynamics and diffusion of dilute, single component materials.
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Subject 6 Diffusion 4 to 8 - Kinetics and Phase...

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