Week 2

Question 1

Define Diffusion

Answer 1

Mass transport by atomic motion.
Typically from an area of high concentration to an
area of low concentration.

Question 2

Identify which diffusion mechanisms occur in the following states:
Gases -
Liquids -
Solids -

Answer 2

Gases - Random (Brownain) motion
Liquids - Random (Brownain) motion
Solids - a) Vacancy diffusion and b) Interstitial diffusion

Question 3

Define Inter-diffusion and Self-“diffusion”

Answer 3

Inter-diffusion: In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. (aka impurity diffusion).

Self-“diffusion”: In a pure solids, atoms also migrate.

Question 4

;Define the diffusion mechanisms:

• Vacancy diffusion
• Interstitial diffusion

Answer 4

Vacancy diffusion (or substitutional diffusion) :
• atoms exchange with vacancies (point defects in metal structures)
• the exchanging atom could be an original atom or an impurity (termed doping if the latter)
Rate depends on:
- number of vacancies in the material
- activation energy to exchange (and therefore temperature)
- frequency of hopping (influenced by temperature)

Interstitial diffusion:
•Smaller* atoms (C, H, O, N) can diffuse between the atoms that form the parent lattice structure (* based on atomic radius).
• More rapid than vacancy diffusion (more space to move in).

Question 5

Discuss the factors of processing iron using diffusion

Answer 5

Processing Iron Using Diffusion:
High carbon iron fractures and crumbles when forged.

Wrought iron, with nearly no carbon in it, is very malleable and ductile, but not very hard.

Low-carbon iron is heated in an atmosphere of C, N, or both, to encourage carbon or nitrogen diffusion into the surface of the iron.

A thin surface layer of higher carbon steel is formed, with the carbon content gradually decreasing the deeper we go from the surface.

The resulting product combines much of the toughness of a low-carbon steel core, with the hardness and wear resistance of the outer high-carbon steel.

(For information: The iron structure on the surface transforms from austenite to martensite, while the core remains with ferrite and/or pearlite micro-structure)

Question 6

Discuss the use of Case hardening and Doping silicon processing as a result of diffusion

Answer 6

• Case Hardening:
-- Diffuse C or N atoms
into the host iron atoms
at the surface.
-- Interstitial diffusion can be used
in case hardening of self-drilling
screws & nitride drill bits.
• Result: The presence of C and N atoms makes iron (screws) and titanium (drill bits) harder and temp resistant.

Doping silicon (Si) with phosphorus for n-type semiconductors: ******