Metals 7

Question 1

Describe grains and the effect their boundaries have on dislocation motion

Answer 1

Grains have different orientations within a polycrystal

Dislocation motion is hindered by the boundaries

Question 2

What is the effect on material properties by reducing grain size?

Answer 2

Generally improves strength, ductility and toughness

ONLY method which increases both strength and ductility

Question 3

What other boundaries hinder dislocation motion?

Answer 3

Phase

Question 4

What happens when a crack tip is sharp and what is the effect of ductility?

Answer 4

More likely to propagate through material

Ductile material can ‘blunt’ crack tip

Question 5

What is the solution to having metals easy to shape in production but strong and resistant to deformation in service?

Answer 5

Use temp sensitivity of strength
Metals soft at high temps
USE at room temp

Question 6

What are the classifications of processes according to temperature?

Answer 6

Cold, warm and hot working

Question 7

Define hot working

Answer 7

T/Tm > 0.6

Question 8

How do dislocations behave in cold working?

Answer 8

Generated in large numbers 
They move (slip) in certain directions 
They interact with and obstruct each other

Question 9

What causes work hardening?

Answer 9

Dislocations created and move in cold working

Deformation becomes more difficult as dislocations can’t move as easily

Question 10

Describe room temperature flow stress

Answer 10

At room temp material work hardens
Flow stress increases with strain but not strain rate (rate of deformation has no effect)
As strain increases, dislocation density and flow stress increases

Question 11

How does cold working effect dimensional control and surface finish?

Answer 11

Gives greater control and better finish/quality

Question 12

What considerations are made when deep drawing drinks cans?

Answer 12

Material must be ductile

It is work hardened so should not be too thin to avoid cracking

Question 13

How does cold working effect material properties?

Answer 13

Increase in strength

Increase in brittleness

Question 14

How does annealing prevent metal from breaking in cold working?

Answer 14

The stored strain energy may revert back to pre-cold worked state

Question 15

What are the three restoration mechanisms?

Answer 15

Recovery, recrystallisation, grain growth

Question 16

When do the restoration mechanisms occur?

Answer 16

They are thermally activated and occur at high homologous temps (T/Tm > 0.3)

Question 17

What is the general order of restoration methods and what do steel/aluminium show?

Answer 17

Recovery, recrystallisation, grain growth
Or simultaneously
Steels show recrystallisation
Al shows recovery

Question 18

What is recovery?

Answer 18

Some stored internal strain energy can be relieved due to dislocation motion
Dislocations effectively cancel each other out and result in sharper grain boundaries

Question 19

What is recrystallisation?

Answer 19

Mechanism which leads to much greater dislocation density

Nucleation and growth of new, strain free, dislocation free grains

Question 20

What is the driving force for recrystallisation?

Answer 20

The difference in internal energy between strained and unstrained material

Question 21

When does grain growth occur?

Answer 21

When the metal is held at an elevated temp, the recrystallised grains continue to grow