Chapter 5_Part_4

Chapter 5_Part_4 - Diffusion Temperature Dependence (II) b...

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16 MSE 2090: Introduction to Materials Science Chapter 5, Diffusion Diffusion – Temperature Dependence (II) Graph of log D vs. 1/T has slop of –Q d /2.3R, intercept of log D o = 2 1 2 1 d T 1 T 1 D log D log R 3 . 2 Q = T 1 R 3 . 2 Q D log D log d 0 b ax y + = 0 logD b = 2.3R Q a d = 1/T x =
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17 MSE 2090: Introduction to Materials Science Chapter 5, Diffusion Temperature dependence: Interstitial and vacancy diffusion mechanisms (I) Diffusion of interstitials is typically faster as compared to the vacancy diffusion mechanism (self-diffusion or diffusion of substitutional atoms). = T 1 2.3R logD logD 0 d Q
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= T k Q exp D D B d 0 77 0.47 N in BCC Fe 145 0.34 N in FCC Fe 43 0.63 H in FCC Fe 15 0.12 H in BCC Fe 87 1.1 C in BCC Fe 138 23 C in FCC Fe Q d , kJ/mol D 0 , mm 2 /s -1 Impurity Smaller atoms cause less distortion of the lattice during migration and diffuse more readily than big ones (the atomic diameters decrease from C to N to H).
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Chapter 5_Part_4 - Diffusion Temperature Dependence (II) b...

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