L10: Phase Diagrams- Tin/ Lead System (and Mg/Pb System)

Question 1

What are the alpha and beta phase regions in the tin lead alloy eutectic system

Answer 1

Alpha is a solid solution of tin in lead

Beta is a solid solution of lead in tin

Question 2

What is a eutectic reaction

Answer 2

A three phase reaction which occurs when a liquid phase forms two solid phases at the same time

(Eutectoid reaction is another type of three-phase reaction but involves one solid-phase reacting to form to other solid phases)

L=a + b

Question 3

What structure do many eutectic alloys are produced on solidification

Answer 3

Lamellae structure

• lamellae are 1-3 micro m in width
• For tin and lead the layers consist of alternating alpha and beta phases

Question 4

What is the difference between the cooling of a hypoeutectic/hypereutectic alloy and an alloy with wire tic composition/pure metal

Answer 4

Hypo/hyper alloy solidified over a temp range depending on composition, then at eutectic temp the remaining liquid solidifies

Eutectic composition solidifies and stays at one temp- stays at that temp for a certain time while latent heat is removed

Question 5

What do the vertical lines mean on phase diagrams?

Answer 5

These are compounds with specific compositions, eg Mg2Pb

Question 6

What are the four main strengthening methods and what are they based on

Answer 6

They are all based on restricting dislocation movement
Increased number of dislocations
Presence of grain boundaries
Presence of solute atoms
Presence of second phase particles in alloys