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Chapter 5: Problem 3

(a) Compare interstitial and vacancy atomic mechanisms for diffusion. (b) Cite two reasons why interstitial diffusion is normally more rapid than vacancy diffusion.

Short Answer

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Answer: Two main reasons why interstitial diffusion is generally faster than vacancy diffusion are: 1) atoms involved in interstitial diffusion are generally smaller, allowing them to easily move through interstitial sites, and 2) interstitial diffusion requires a lower energy barrier for movement, as atoms do not need to physically swap positions with other atoms like in vacancy diffusion, leading to a faster diffusion process.

Step by step solution

01

Define Interstitial Diffusion

Interstitial diffusion is a diffusion process in which atoms or molecules migrate through a lattice by moving from one interstitial site to another. The interstitial sites are spaces between the main lattice points where the migrating atom or molecule can fit.
02

Define Vacancy Diffusion

Vacancy diffusion is a diffusion process in which atoms or molecules exchange positions with vacancies, or empty lattice points, allowing them to move within the lattice structure. This movement generally requires atoms to physically swap positions, and requires more energy to be applied in order to overcome the potential energy barrier of doing so.
03

Compare Interstitial and Vacancy Diffusion

In interstitial diffusion, the atoms or molecules travel in the open spaces between the atoms of the host lattice structure, whereas in vacancy diffusion, the diffusing elements move by exchanging positions with an adjacent vacancy in the lattice structure. Because the two mechanisms involve different paths for atomic movement, their rate and ease of diffusion can differ greatly.
04

Describe Reason 1 for Interstitial Diffusion being more rapid

One reason for interstitial diffusion being more rapid than vacancy diffusion is that the atoms or molecules involved are generally smaller than the host lattice atoms. Due to their smaller size, they can easily move through the interstitial sites, while in vacancy diffusion, the migrating atoms need to be nearly the same size as the host lattice atoms to successfully exchange positions with vacancies. The larger the atom to move, the more energy is required and the slower the diffusion process.
05

Describe Reason 2 for Interstitial Diffusion being more rapid

Another reason why interstitial diffusion is generally faster is that it requires a lower energy barrier for movement, as the migrating atoms do not need to physically swap positions with other atoms, unlike vacancy diffusion. The energy barrier in vacancy diffusion is the energy required for an atom to move out of its initial position and into an adjacent vacancy, whereas, in interstitial diffusion, atoms simply move between the spaces in the lattice, requiring less energy input. Less energy to overcome the energy barrier leads to a faster diffusion process.

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Most popular questions from this chapter

Consider the diffusion of some hypothetical metal Y into another hypothetical metal \(Z\) at \(950^{\circ} \mathrm{C}\); after \(10 \mathrm{~h}\) the concentration at the \(0.5 \mathrm{~mm}\) position (in metal \(Z\) ) is \(2.0 \mathrm{wt} \% \mathrm{Y}\). At what position will the concentration also be \(2.0 \mathrm{wt} \% \mathrm{Y}\) after a \(17.5-\mathrm{h}\) heat treatment again at \(950^{\circ} \mathrm{C}\) ? Assume preexponential and activation energy values of \(4.3 \times 10^{-4} \mathrm{~m}^{2} / \mathrm{s}\) and \(180,000 \mathrm{~J} / \mathrm{mol}\), respectively, for this diffusion system.

An FCC iron-carbon alloy initially containing \(0.55 \mathrm{wt} \% \mathrm{C}\) is exposed to an oxygen-rich and virtually carbon-free atmosphere at \(1325 \mathrm{~K}\) \(\left(1052^{\circ} \mathrm{C}\right)\). Under these circumstances, the carbon diffuses from the alloy and reacts at the surface with the oxygen in the atmosphere-that is, the carbon concentration at the surface position is maintained essentially at \(0 \mathrm{wt} \%\) C. (This process of carbon depletion is termed decarburization.)At what position will the carbon concentration be \(0.25 \mathrm{wt} \%\) after a \(10-\mathrm{h}\) treatment? The value of \(D\) at \(1325 \mathrm{~K}\) is \(3.3 \times 10^{-11} \mathrm{~m}^{2} / \mathrm{s}\).

(a) Briefly explain the concept of a driving force. (b) What is the driving force for steady-state diffusion?

The preexponential and activation energy for the diffusion of chromium in nickel are \(1.1 \times 10^{-4}\) \(\mathrm{m}^{2} / \mathrm{s}\) and \(272,000 \mathrm{~J} / \mathrm{mol}\), respectively. At what temperature will the diffusion coefficient have a value of \(12 \times 10^{-14} \mathrm{~m}^{2} / \mathrm{s} 2\)

Briefly explain the difference between selfdiffusion and interdiffusion.
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