Metals and Alloys

Question 1

What are the features of dislocations in metals?

Answer 1

• Dislocations produce field strains which create areas of compression and tension, and these forces decrease the further away you move from the dislocation.
• The strain field from one dislocation will interact with that of another as they move close together, two like dislocations repel each other and two dislike dislocations will attract and annihilate each other.
• The strength of a metal is increased by reducing dislocation motion.

Question 2

What are the different strengthening mechanisms?

Answer 2

Plastic deformation is caused by the motion of large numbers of dislocations, and so the ability of a material to plastically deform depends on the ability of dislocations to move.

Strengthening techniques involve restricting the dislocation motion in order to reduce plastic deformation.

• Grain size reduction
• Solid-solution strengthening
• Precipitation strengthening
• Cold work strengthening

Question 3

What is grain size reduction?

Answer 3

Grain boundaries prevent dislocation motion, so by decreasing grain size you increase the grain boundary area, therefore increasing resistance to dislocations.

Finely grained materials and therefore stronger and harder but less ductile.

Grain size can be decreased by:

• Increasing the rate of cooling
• Cold work and heat treatments
• Rolling - you turn an isotropic metal with large grains into and anisotropic material with smaller grains of a different shape.

Question 4

What is the Hall-Petch equation?

Answer 4

σ_yield = σ_o + k_y d^-1/2

σ_yield = yield strength (N/mm²)

σ_o = starting stress for dislocation motion (constant)

k_y = strengthening coefficient

d = average grain diameter

Question 5

What is solid solution strengthening?

Answer 5

• Alloys containing substitutional/interstitial impurity atoms are deliberately produced, to increase lattice strains which interact with dislocations, restricting their movement by decreasing overall strain energy.
• If a dislocation wants to move it has to tear itself from the impurity atoms which costs energy.
• Increases tensile and yield strength, with the degree of strengthening dependent on relative atomic sizes.

Question 6

What is precipitation strengthening?

Answer 6

1. Precipitates require large shear stresses to move dislocations towards them and shear them.
2. Therefore, as dislocations advance the precipitates act as pinning sites, preventing it from moving the whole way through a material.

• The yield strength is inversely proportional to the spacing of the pinning sites: σ_y = 1 / S

Question 7

What is cold work strengthening?

Answer 7

Cold work causes the metal to plastically deform, causing the slip planes and dislocations in interact and dislocations to entangle with each other. This impedes further dislocation motion.

This is achieved through:

• Forging - a mould squeezes a metal into shape.
• Rolling
• Drawing - metal is pulled between two die.
• Extrusion - metal is forced between two die.

This causes:

• Yield strength to increase
• Tensile strength to increase
• Ductility to decrease

Question 8

How does annealing affect strengthening?

Answer 8

• Heating (annealing) reduces strengthening, as diffusion allows dislocations to rearrange and annihilate.
• It also reduces dislocation density and increases grain size.
• As tensile strength increases, ductility decreases.
• The material changes greatest during recrystallisation.

Question 9

What is an alloy?

Answer 9

An alloy is a metallic substance made up of more than one element and are produced because they are:

• Stronger
• Easier to cast
• Have good electrical/magnetic properties

Question 10

How can you control the properties of an alloy?

Answer 10

The properties can be controlled by:

• The type of bonding
• It’s crystal structure
• Presence of defects

Question 11

What are components?

Answer 11

The elements or compounds which make up an alloy.

Question 12

What are phases?

Answer 12

The resulting physically and chemically distinct material regions.

Question 13

What is the solubility limit?

Answer 13

• The maximum concentration for which an allow is only a solution.
• As the temperature increases, the solubility limit also increases.

Question 14

What are ferrous alloys?

Answer 14

Ferrous alloys contain iron which is used because it is:

• Abundant
• Economical
• Versatile
• Stiff, strong and ductile

Question 15

What are steels?

Answer 15

Steels are iron / carbon alloys.

Question 16

How is cast iron produced?

Answer 16

• Cast iron is produced by re-melting iron without refinement.

It is frequently known as pig iron.

Question 17

What is the iron-carbon system?

Answer 17

Question 18

What are the different phases in the iron - carbon system?

Answer 18

Ferrite (α):

• The maximum solubility of carbon in ferrite is 0.022 wt% at 727 deg.
• Ferrite is soft, ductile and magnetic.
• BCC.

Austenite (γ):

• The maximum solubility of carbon in ferrite is 2.11 wt% at 1148 deg.
• Austenite is non-magnetic.
• FCC.

Cementite (Fe₃C):

• Above 6.7 wt% cementite is formed.
• Cementite is hard and brittle.
• Cementite is not in equilibrium.

Question 19

What are the different phase reactions?

Answer 19

Eutectic:

• Occurs at a solubility of 4.3 wt%

Eutectoid:

• Occurs at a solubility of 0.77 wt%

Question 20

How are pearlite and martensite formed?

Answer 20

• If at the eutectoid phase, the temperature falls slowly, alternating layers of ferrite and cementite are formed, known as pearlite.
• If at the eutectoid phase, the temperature falls quickly, a formation of ferrite and cementite known as martensite is formed.

Question 21

What are the features of pearlite?

Answer 21

• Pearlite is made up of hard, brittle cementite as well as softer, ductile ferrite making it very tough.
• It has a BCC structure.
• Slow diffusion of carbon is required for pearlite growth.
• Carbon atoms slowly diffuse out of the area where ferrite is being produced and to the area where cementite is being produced, as ferrite has less carbon than cementite. This forms pearlite.

Question 22

What are the features of martensite?

Answer 22

• Martensite is formed when the quenching rate is high, meaning carbon diffusion cannot occur making it very hard and brittle.
• It has a BCT structure.

Question 23

Which factors affect the hardness of steels?

Answer 23

• Quenching (heat treatment) can be used to improve strength and hardness of steels by preventing martensite formation.
• Martensite formation during welding causes the heat affected zone to become weak.

Question 24

What are the features of pure iron steels?

Answer 24

0 to 0.008% carbon

Question 25

What are the features of low carbon steels?

Answer 25

• 0.008 to 0.25% carbon
• Soft and weak
• Very ductile
• Weldable and cheap
• Unresponsive to heat treatments as it is difficult to form martensite

Question 26

What are the features of medium carbon steels?

Answer 26

• 0.25 to 0.6% carbon
• Stronger
• Less ductile
• Wear resistant and moderate toughness
• Heat treatable to improve mechanical properties but only in thin sections

Question 27

What are the features of high carbon steels?

Answer 27

• 0.6 to 1.4% carbon
• Hardest and strongest
• Least ductile
• Excellent wear resistance
• Heat treatable in thick sections but avoid welding

Question 28

What are the features of cast iron steels?

Answer 28

• 2.0 to 4.0% carbon
• Low melting point due to high carbon content
• Made up of pearlite and flakes of graphite
• Large wear resistance but low toughness and ductility

Question 29

What are the different types of non-ferrous alloys?

Answer 29

• Copper alloys
• Titanium alloys - Reactive
• Magnesium alloys - Ignited easily
• Aluminium alloys - Unreactive
• Refractory alloys - High melting point

Question 30

What is metal fabrication?

Answer 30

The process in which metals are turned into finished objects:

• Casting - pouring liquid metals into moulds
• Forging - deforming the metals
• Machining - cutting and grinding into shape
• Joining

Question 31

What are the different types of casting?

Answer 31

Sand casting:

1. Make required shape out of wood and place in moulding sand.
2. Remove wood and pour liquid alloy into mould.

Investment casting:

1. Produce a master mould and from this create wax patterns.
2. Coat these patterns in ceramics and then heat the wax away leaving a ceramic mould.
3. Pour in the liquid alloy.

Die casting:

1. Inject the liquid alloy into a reusable mould under pressure.
   * This produces complex shapes and reduces metal waste.

Question 32

What are the different types of forming?

Answer 32

Forging:

1. Shaping by a hammer into a mould.
   * Able to form strong complex parts.

Rolling:

1. Material is passed through cylindrical rollers.
   * Used for I beams and rails.

Extrusion:

1. Metal is forced through a shaped die.
   * Forms parts with constant cross sections and able to produce complex shapes.

Drawing:

1. Metal is pulled through a shaped die.
   * Compressed the metal making it harder.

Question 33

Why is plastic deformation carried out at high temperatures?

Answer 33

To reduce the stress required to provide ductility in the finished part.

Question 34

What is the difference between cold and hot working?

Answer 34

Cold working:

• More energy required to deform.
• Oxidation provides a good finish.
• Higher strength.
• Generally anisotropic.

Hot working:

• Recrystallisation occurs.
• Less energy to deform.
• Oxidation provides a poor finish.
• Lower strength.

Question 35

What are the different types of joining?

Answer 35

Powder processing:

1. Powders are places under high pressure and heat.
2. This causes densification to occur between particles forming solid compounds.

• This is achieved with materials of low ductility.

Welding:

• Heat affected zone is the location where microstructure has changed and is often slightly weaker.