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Understanding Diffusion in Materials: Activation Energy, Vacancies, and Movement of Atoms

Explore the fundamentals of diffusion in materials, covering topics such as activation energy, vacancy movement, and atom mobility through concentration gradients. Learn how vacancies occur naturally in materials and influence diffusion rates. Discover the diffusion coefficient and its relationship to temperature, along with practical examples like carburizing steel shafts. Gain insights into the mechanisms behind atom movement and the role of thermal vibrations in diffusion processes.

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Understanding Diffusion in Materials: Activation Energy, Vacancies, and Movement of Atoms

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

  2. Example - 718 Ni-base superalloy

  3. Example - 718 Ni-base superalloy

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

  5. Example - 718 Ni-base superalloy

  6. 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

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

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

  9. 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

  10. Diffusion Vacancy movement

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

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

  13. 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

  14. Diffusion Diffusion coefficient: D(T) = Do exp (-Q/RT) flux J = D(T) dC/dx 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

  15. 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

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