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Explain the role that diffusion plays in both diffusion creep and dislocation creep; also explain how...

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Civil Engineering

Explain the role that diffusion plays in both diffusion creepand dislocation creep; also explain how these types of creep can bedesigned against.

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Sol The role that diffusion playsinDiffusion Creep Diffusion creep refers to the deformation ofcrystalline solids by the diffusion of vacancies through theircrystal latticeDiffusion creep results in plastic deformationrather than brittle failure of the materialDiffusional creep is an important mechanism of plasticdeformation in a polycrystalline material at relatively low stressand small grain size There is evidence that diffusional creepplays an important role in some regions of Earth At hightemperatures atoms move from their stable positions with someprobability due to thermally activated processes This is referredto as diffusion The driving force for diffusion is the gradient inchemical potential including the concentration gradient caused bythe contact of materials with different chemical compositions or bythe stress gradient at grain boundaries created by the appliedstress Consequently the rate of deformation due to diffusive masstransport is sensitive to diffusion coefficient as well as grainsize the rate of deformation is faster for a smaller grain sizeSimilar to other processes diffusional mass transport involves anumber of parallel independent and sequential dependentprocesses As a result the interplay of various diffusing speciescan be complicated and this also results in a complicated variationin grainsize sensitivity with grain size Deformation of apolycrystalline material is associated with grain boundary slidingLargestrain plastic flow involving grainboundary sliding issometimes referred to as superplastic flowPrincipleCrystalline materials are never perfect on a microscale Somesites of atoms in the crystal lattice can be occupied by pointdefects such as alien particles or vacancies Vacancies canactually be thought of as chemical species themselves or part of acompound speciescomponent that may then be treated usingheterogeneous phase equilibria The number of vacancies may also beinfluenced by the number of chemical impurities in the crystallattice if such impurities require the formation of vacancies toexist in the latticeA vacancy can move through the crystal structure when theneighbouring particle jumps in the vacancy so that the vacancymoves in effect one site in the crystal lattice Chemical bondsneed to be broken and new bonds have to be formed during theprocess therefore a certain activation energy is needed Moving avacancy through a crystal becomes therefore easier when thetemperature is higherThe most stable state will be when all vacancies are evenlyspread through the crystal This principle follows from FickslawIn which Jx stands for the flux flow ofvacancies in direction x Dxis a constant for the material in that direction and is the difference in concentration of vacancies in that directionThe law is valid for all principal directions in xy zspace so the x in the formula canbe exchanged for y or z The result will be thatthey will become evenly distributed over the crystal which willresult in the highest mixing entropyWhen a mechanical stress is applied to the crystal newvacancies will be created at the sides perpendicular to thedirection of the lowest    See Answer
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