Composition:

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Composition 52.5Ni 19Cr 3Mo 19Fe 0.4Al
5Nb-1Ti 0.04C
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Possibilities for strengthening 718
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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
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Movement of atoms (or molecules) down a
concentration gradient
How??
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One way - vacancy movement
No. fract. of vacancies exp (-Q/RT)
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One way - vacancy movement
No. fract. of vacancies exp (-Q/RT) For Cu _at_ rm
T 1/1014 _at_ 1080C 1/1600
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Vacancy movement
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Movement of atoms (or molecules) down a
concentration gradient
I lied
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Movement of atoms (or molecules) over a hump
Where do we get the activation energy?
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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 _at_ rm T 2.2 x 1010 _at_ 1080 C

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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 _at_ 500
C 4 x 10-7 cm2/sec _at_ 1000 C
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