Chapter 5 Diffusion

Question 1

Mechanism of Diffusion

Answer 1

Vacancy Diffusion
• exchange of atoms between different sites with vacan­cies [mechanism of self diffusion and substitutional diffusion]

Interstitial Diffusion
• motion from an interstitial site to an adjacent site

Question 2

Describe the Rate of Diffusion

Answer 2

• Rate can be described via: steady or NON-steady state

* Faster in interstitial than vacancy diffusion

Question 3

What is Steady State Diffusion?

Answer 3

Also called as Fick’s First Law of Diffusion
• Rate of diffusion independent of time
• Flux proportional to gradient

Question 4

What is Non-Steady State Diffusion?

Answer 4

Also know as Fick’s Second Law of Diffusion
• Concentration of diffusing species is a function of both
TIME and POSITION

Question 5

Contrast Steady State Vs. Non-Steady State Diffusion

Answer 5

The major difference is time dependency. SS is time independent while NS is time dependent

Question 6

Which factors influence diffusion rate?

Answer 6

DIFFUSING SPECIES
• Interstitial Diffusion is Faster than vacancy diffusion
• Atomic size significantly influence diffusion

TEMPERATURE
• Diffusion is exponentially dependent to Temperatur me and the relationship can be visualized using Arrhenius Equation

Question 7

Describe the difference between interdiffusion versus self diffusion

Answer 7

• Interdiffusion: In an alloy, atoms tend to migrate
from regions of high conc. to regions of low conc.

• Selfdiffusion: In an elemental solid, atoms also migrate within itself.