L8: Diffusion, Solidification, Phase Diagrams Intro

Question 1

What are the two factors diffusion depends on

Answer 1

Time and temperature

Question 2

What are the three main mechanisms for the fusion in the solid-state

Answer 2

Bulk- vacancy and interstitial
grain Boundary
dislocation core

Question 3

What is the activation energy for diffusion?

What does it depend on?

Answer 3

The energy to break bonds and move through the bridge point between the intervening atoms

Also depends on bond strength and crystal structure

Question 4

What are six examples of applications of diffusion

Answer 4

Carburising Involves the diffusion of carbon into the surface of steals to harden the surface only

• difFusion bonding process
• sintering in powder metallurgy
• gas permeation of plastics used in food packaging
• Semiconductor device manufacture
• Intermetallic growth in soldered joints

Question 5

Why does solidification Not occur spontaneously at the same temperature as melting

Answer 5

Liquids must be cold below the melting temperature undercooled for solidification to commence

Question 6

Describe the solidification process from the moment it’s commences

Answer 6

Commences when several atoms in the liquid meet and have a sufficiently small amount of energy so that they are able to bind together and form a small solid region called an embryo

• If the Embryo gets big enough it will grow into a nucleus
• If the nucleus survives helped by undercooling more atoms will attach themselves and the nucleus will grow

Question 7

What is the difference between Homogenous nucleation and heterogeneous nucleation

Answer 7

Homo- Takes place in the liquid without any impurities or surfaces present this is very rare and requires a lot of undercooling to enable solidification to start

Hetero- Takes place when there are solid impurities in the liquid e.g. container walls or solid particles this requires much less undercooling