diffusion cards new

What are the five diffusion mechanisms examiners expect you to know?

Interstitial diffusion vacancy diffusion direct exchange diffusion grain boundary diffusion and dislocation pipe diffusion

What type of atoms diffuse by interstitial diffusion?

Small atoms such as H C N and O

How do atoms move in interstitial diffusion?

They move between interstitial sites

Are vacancies required for interstitial diffusion?

No vacancies are required

Is interstitial diffusion fast or slow?

Fast with low activation energy

How does atom size affect interstitial diffusion?

Strong effect only very small atoms can diffuse interstitially

How does temperature affect interstitial diffusion?

Strong effect diffusion increases rapidly with temperature

Give examples of atoms in interstitial diffusion

C or N in steel and H in metals

What type of atoms diffuse by vacancy diffusion?

Atoms similar in size to host atoms

How does vacancy diffusion occur?

An atom jumps into a neighbouring vacancy

What energy contributions are required for vacancy diffusion?

Vacancy formation energy and migration energy

Is vacancy diffusion faster or slower than interstitial diffusion?

Slower than interstitial diffusion

How does atom size affect vacancy diffusion?

Moderate effect atoms must be similar in size to host atoms

How does temperature affect vacancy diffusion?

Strong effect higher temperature increases vacancy concentration

what needs to be similar to allow for vacancy diffusion

atomic radius

What is direct exchange or ring diffusion?

Atoms exchange positions directly without vacancies

When is direct exchange diffusion possible?

When atoms are very similar in size

Are vacancies required for direct exchange diffusion?

No vacancies are required

Why is direct exchange diffusion rare at low temperatures?

It has a very high activation energy

How does atom size affect direct exchange diffusion?

Strong effect atoms must be nearly identical in size

How does temperature affect direct exchange diffusion?

Strong effect only occurs at very high temperatures

Give an exam example of direct exchange diffusion

Ag–Au diffusion couple

What is grain boundary diffusion?

Diffusion along grain boundaries where atomic packing is looser

How does grain boundary diffusion compare to lattice diffusion?

It is faster than lattice diffusion

Why is grain boundary diffusion faster than lattice diffusion?

Grain boundaries have a more open atomic structure

How does atom size affect grain boundary diffusion?

Minimal effect compared to lattice diffusion

How does temperature affect grain boundary diffusion?

Important at intermediate temperatures

Give processes where grain boundary diffusion is important

Creep sintering and precipitation kinetics

What is dislocation pipe diffusion?

Diffusion along the cores of dislocations

How does dislocation pipe diffusion compare to lattice diffusion?

It is much faster locally

Why does dislocation pipe diffusion have limited contribution?

It occurs over a very small volume fraction

How does atom size affect dislocation pipe diffusion?

Minimal effect

How does temperature affect dislocation pipe diffusion?

Occurs even at relatively low temperatures

Give processes where dislocation pipe diffusion is important

Creep and precipitation kinetics

How should you choose the correct diffusion mechanism in an exam?

By considering atom size temperature vacancy availability and diffusion path

If the atom is small which diffusion mechanism dominates?

Interstitial diffusion

If diffusion requires vacancies which mechanism dominates?

Vacancy diffusion

If atoms are similar in size and no vacancies are involved which mechanism applies?

Direct exchange diffusion

If diffusion occurs along grain boundaries which mechanism applies?

Grain boundary diffusion

If diffusion occurs along dislocation cores which mechanism applies?

Dislocation pipe diffusion