MSE306Chap8Annealing-2

MSE306Chap8Annealing-2 - Chapter 8: Annealing The whole...

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Chapter 8: Annealing The whole idea is… We can first bring materials to states of metastability Free energy, G, is higher than the equilibrium There is a driving force, ΔG, to proceed towards equilibrium There must be an energy barrier for doing so Thermal vibrations can provide the energy to overcome the barrier (activation energy) We call this a thermally activated process Recall vacancy diffusion from last lecture…
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There will often be some driving force, ΔG, which directs diffusion. Obviously atoms/vacancies will do some random jumping around (back-and-forth). But if there is a concentration gradient or if the material is under stress, there will be a favored direction for the diffusion. This is a good archetype for annealing processes. Diffusion – Thermally Activated Process (II) Q Position Energy ΔG
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Possible Metastable Conditions of Interest The diffusion couple shown earlier Plastically deformed (cold-worked) material Fine grained material Supersaturated solid solutions Graphite (vs. diamond) or martensite (in steel) Metallic glasses
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When a metal is plastically deformed work is performed on the material dU = dW + dQ most of the work (~90%) becomes heat Stored energy (increase in free energy, dU) is relatively small because most of dW is lost to heat, dQ crystal defects are introduced What are they (2 or 3 types) How do they get there? How do defects represent excess energy? During annealing defects can be
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MSE306Chap8Annealing-2 - Chapter 8: Annealing The whole...

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