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ch06 - increases with increasing temperature D(900> D(700...

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Chapter 6 Diffusion Concept Check 6.1 Question: Rank the magnitudes of the diffusion coefficients from greatest to least for the following systems: N in Fe at 700 ° C Cr in Fe at 700 ° C N in Fe at 900 ° C Cr in Fe at 900 ° C Now justify this ranking. (Note : Both Fe and Cr have the BCC crystal structure, and the atomic radii for Fe, Cr, and N are 0.124, 0.125, and 0.065 nm, respectively. You may also want to refer to Section 5.4 .) Answer: The diffusion coefficient magnitude ranking is as follows: N in Fe at 900 ° C; D N (900) N in Fe at 700 ° C; D N (700) Cr in Fe at 900 ° C; D Cr (900) Cr in Fe at 700 ° C; D Cr (700) Nitrogen is an interstitial impurity in Fe (on the basis of its atomic radius), whereas Cr is a substitutional impurity. Since interstitial diffusion occurs more rapidly than substitutional impurity diffusion, D N > D Cr . Also, inasmuch as the magnitude of the diffusion coefficient
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