Metals- Strengthening Mechanisms

Question 1

What is the Hall-Petch equation?

Answer 1

σy=70+17.4d^-1/2
Where σy is yield strength (MPa)
d is grain size (mm)
Describes effect of ferrite grain size on yield strength for low carbon steel

Question 2

What does reducing the grain size do?

Answer 2

It increases the strength and also the ductility of a material. Due to there being more grain boundaries to hinder dislocation motion

Question 3

Effect of phase boundaries

Answer 3

E.g boundary between ferrite and austenite. Also hinders dislocation motion but not as effectively as a grain boundary

Question 4

What is hot working?

Answer 4

Working where the deformation temperature divided by the melting temperature of the material is greater than 0.6

Question 5

What is cold working?

Answer 5

Working where the deformation temperature divided by the melting temperature of the material is less than 0.3 (roughly)

Question 6

What is warm working?

Answer 6

Working where the deformation temperature is between that for hot and cold working

Question 7

How does cold working work?

Answer 7

When we deform a crystal, dislocations are generated in large numbers. These move (slip) in certain directions. They are quite mobile so interact with and instruct each other. Aka work hardening

Question 8

How does flow stress vary for work hardening?

Answer 8

Flow stress increases with strain but not strain rate. As strain increases dislocation density increases so flow stress increases. Initially a sharp increase, then sharp transition to shallow curve

Question 9

What is cold working better for than hot working?

Answer 9

Better surface quality because there is no oxidation. Better control over dimensional tolerances

Question 10

Potential problem with cold working

Answer 10

The metal is not softened and gets stronger so needs a greater capacity mill to deform it

Question 11

What affect does cold working have on yield strength, tensile strength and ductility?

Answer 11

Increases yield and tensile strength. Can cause large decrease in ductility so the material becomes more brittle

Question 12

How to overcome problem of work hardening

Answer 12

When yield strength reaches a certain point, apply a heat treatment. The stored strain energy in the metal may revert back to the pre-cold worked state after the heat treatment

Question 13

What is the heat treatment called for Al and steel?

Answer 13

For Al it is called annealing. For steel it is called tempering

Question 14

3 mechanisms that cause restoration of mechanical properties

Answer 14

Recovery, recrystallisation, grain growth

Question 15

How are the mechanisms of recovery activated?

Answer 15

Thermally activated and occur at high homologous temperatures (T/Tm>0.3) and lead to microstructural changes in the metal

Question 16

Usual sequence of restoration mechanisms

Answer 16

Recovery, recrystallisation, grain growth. Although some metals more prone to recovery or recrystallisation than others.

Question 17

How does tensile strength and ductility vary through the restoration mechanisms?

Answer 17

Tensile strength starts high but starts to decrease in the recovery zone. It curves down more in recrystallisation zone. Then levels off entering grain growth zone. Ductility shows opposite shape where it starts low and ends high

Question 18

Recovery

Answer 18

Some stored internal strain energy can be relieved due to dislocation motion (in absence of externally applied stress) if temperatures are high enough (to give dislocations enough energy). When dislocations are more mobile they can cancel each other out by annihilation

Question 19

What is the recrystallisation mechanism?

Answer 19

The mechanism that leads to a much greater reduction of dislocation density and the nucleation and growth of new strain free, dislocation free grains. Driving force is the difference in internal energy between strained and untrained material. Metal becomes less strong and more ductile

Question 20

When does grain growth occur?

Answer 20

When a metal is held at elevated temperature, the recrystallised strain-free grains continue to grow