Chapter 5 Slides

Chapter 5 Slides - Chapter 5 Diffusion (atomic jumping) Why...

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Chapter 5 Chapter 5 Diffusion (atomic jumping) Diffusion (atomic jumping) • Why study diffusion? – Equilibrium state of materials changes with alloying state and with temperature • Above about half the melting point (T m measured in K not C), there is enough vibrational energy in the lattice to allow atomic jumps (diffusion) • Phase changes at high temperatures (HT) occur by diffusion, making quantitative study of T and t valuable – Reactions often occur by diffusion reactions – Sometimes we want to modify only the surface properties of a part • Surface treatments can often beneficially just change surface properties – Case harden gears by diffusing C into the surface – And many, many more reasons…….
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Chapter 5 Chapter 5 - - Diffusion Diffusion Lectures begin on 02-19-08
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Self-diffusion : In an elemental solid, atoms migrate under the action of the thermal energy of the crystal lattice kT/atom. For long range motion to occur, one must be above ~0.5T M (K) Label some atoms After some time Self Diffusion in Crystals Self Diffusion in Crystals A B C D A B C D •Notice that with time, atoms can move substantial distances •Self diffusion experiments have to be done by simulations or by adding radioactive isotopes whose position can be tracked by radiography….
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Interdiffusion : In an alloy, atoms tend to migrate from regions of high concentration to regions of low conc. Initially Diffusion of two mutually soluble species Diffusion of two mutually soluble species into each other into each other After some time What determines the shape and extent of the diffusion profile?
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Diffusion down a concentration gradient forming Diffusion down a concentration gradient forming solid solution (click 2x for animation) solid solution (click 2x for animation) A = black; B = white Begin with pure A and B, which have the same crystal structure, in contact with one another Allow diffusion to occur – A diffuses into B, and B diffuses into A Final product is a random solid solution of A and B
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Diffusion down a concentration gradient forming Diffusion down a concentration gradient forming AB (click 2x for animation) AB (click 2x for animation) A = black; B = white Begin with 1 mole of pure A and B in contact with one another. A and B have same crystal structure Diffusion occurs Form compound AB between A and B (AB has same structure as A and B) A and B diffuse through compound AB Final product is compound AB
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Diffusion up a concentration gradient forming A Diffusion up a concentration gradient forming A and B (click 2x for animation) and B (click 2x for animation) A = black; B = white Begin with random mixture of A and B Diffusion occurs separating the mixture into regions of pure A and pure B THIS COULD OCCUR – BUT ITS PROBABILITY OF HAPPENING IS ESSENTIALLY = 0
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Diffusion Mechanisms Diffusion Mechanisms Vacancy diffusion is the principal diffusion mechanism: • atoms exchange substitutionally with vacancies • exchange rate depends on:
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This note was uploaded on 04/20/2008 for the course EML 3234 taught by Professor Hellstrom during the Spring '08 term at FSU.

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Chapter 5 Slides - Chapter 5 Diffusion (atomic jumping) Why...

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