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Understanding Diffusion in Materials: Vacancy Movement and Activation Energy

Learn about the movement of atoms through materials and the factors that influence diffusion, including vacancy motion and activation energy. Explore examples of diffusion in solids and its effects on material properties. This educational text is designed to enhance your understanding of materials science.

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Understanding Diffusion in Materials: Vacancy Movement and Activation Energy

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  1. Try not to have a good time...this is supposed to be educational -Peanuts

  2. Example - 718 Ni-base superalloy Composition: 52.5Ni 19Cr 3Mo 19Fe 0.4Al 5Nb-1Ti 0.04C

  3. Example - 718 Ni-base superalloy

  4. Example - 718 Ni-base superalloy

  5. Example - 718 Ni-base superalloy Possibilities for strengthening 718:

  6. Example - 718 Ni-base superalloy

  7. Example - 718 Ni-base superalloy 52.5Ni Provides friendly matrix for alloying elements 19Cr Oxidation and corrosion resistance, solid solution strengthening 3Mo Solid solution strengthening, stabilizes chromium oxide film, carbide formation for Hi T strength 19Fe Solid solution strengthening 0.4Al Increases oxidation resistance, forms strengthening pptates 5Nb-1Ti Form strengthening pptates 0.04C Carbide formation for creep resistance

  8. Diffusion concn concn concn concn distance distance distance distance Movement of atoms (or molecules) down a concentration gradient How??

  9. Diffusion Vacancies occur naturally in materials vacancy One way - vacancy movement No. fract. of vacancies = exp (-Q/RT)

  10. Diffusion Vacancies occur naturally in materials vacancy One way - vacancy movement No. fract. of vacancies = exp (-Q/RT) For Cu @ rm T: 1/1014; @ 1080°C: 1/1600

  11. Diffusion Vacancy movement

  12. Diffusion concn concn concn concn distance distance distance distance Movement of atoms (or molecules) down a concentration gradient I lied

  13. Diffusion Movement of atoms (or molecules) over a hump Where do we get the activation energy?

  14. Diffusion Where do we get the activation energy? From energy stored as thermal vibrations # jumps/sec = A exp (-Q/RT) For Cu: # jumps/sec = 1015 exp (-29/RT) = 1.1 x 10-6 @ rm T = 2.2 x 1010 @ 1080° C

  15. Diffusion Diffusion coefficient: flux J = D(T) dC/dx D(T) = Do exp (-Q/RT) For C diffusing in Fe: D(T) = 0.21 exp (-33.8/RT) = 6.7 x 10-11 cm2/sec @ 500° C = 4 x 10-7 cm2/sec @ 1000° C

  16. Diffusion For C diffusing in Fe: D(T) = 0.21 exp (-33.8/RT) = 6.7 x 10-11 cm2/sec @ 500° C = 4 x 10-7 cm2/sec @ 1000° C Ex - carburizing a steel shaft: how long to carburize to depth of 1 mm x2 = D t D(1000° C) = 3 x 10-7 cm2/sec t = 10 hr D(1200° C) = 1.8 x 10-6 cm2/sec t = 1.5 hr

  17. Assignments for next class: 1. review today's classnotes a. vacancies & vacancy motion b. rates of diffusion in solids c. effect of temperature on diffusion rates d. carburization example 2. Read text: chapter 5

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