ENGI1002 - Topic 3

What is diffusion?

Diffusion is mass transport by atomic motion.

How does diffusion happen in solids?

In solids, diffusion happens by vacancy jumping (substitutional) or interstitial hopping.

Is interstitial or substitutional diffusion faster?

Interstitial diffusion is faster than substitutional diffusion.

What are fast diffusion paths in crystalline materials?

Grain boundaries, surfaces, and dislocations are fast diffusion paths.

What is a phase?

A phase is a physically and chemically distinct region.

How many phases in a solution?

A solution is one phase.

How many phases in a mixture?

A mixture is more than one phase.

What to dislocations do to metals?

Dislocations give metals plasticity and reduce strength.

Where is slip strongest?

Slip occurs on close-packed planes in close-packed directions.

Which is stronger, polycrystals or single crystals?

Polycrystals are stronger than single crystals.

What are the four strengthening methods blocking dislocations?

1. Smaller grains

2. Solid solution

3. Precipitates

4. Work hardening

What does rate of substitutional diffusion depend on?

• Vacancy concentration

• Frequency of jumping

What is inter-diffusion?

Between two different elements.

What is self diffusion?

Of the same element.

What is phase equilibrium?

Phase equilibrium is when phases reach their lowest energy point and do not change with time.

What is the solubility limit?

Solubility limit is the maximum concentration for which only a single phase solution exists.

What can be determined from a phase diagram?

1. Phases present

2. Composition of each phase

3. Relative amounts of each phase

What is a eutectic reaction?

When a liquid goes to two solid phases. All eutectic reactions look similar.

What increases the rate of diffusion?

Temperature increases rate of diffusion.