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Materials Science > Metals and Alloys > Flashcards

Metals and Alloys Flashcards

1
Q

What are the features of dislocations in metals?

A
  • 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.
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2
Q

What are the different strengthening mechanisms?

A

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
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3
Q

What is grain size reduction?

A

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.
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4
Q

What is the Hall-Petch equation?

A

σyield = σo + ky d-1/2

σyield = yield strength (N/mm2)

σo = starting stress for dislocation motion (constant)

ky = strengthening coefficient

d = average grain diameter

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5
Q

What is solid solution strengthening?

A
  • 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.
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6
Q

What is precipitation strengthening?

A
  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: σy = 1 / S
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7
Q

What is cold work strengthening?

A

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
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8
Q

How does annealing affect strengthening?

A
  • 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.
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9
Q

What is an alloy?

A

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
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10
Q

How can you control the properties of an alloy?

A

The properties can be controlled by:

  • The type of bonding
  • It’s crystal structure
  • Presence of defects
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11
Q

What are components?

A

The elements or compounds which make up an alloy.

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12
Q

What are phases?

A

The resulting physically and chemically distinct material regions.

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13
Q

What is the solubility limit?

A
  • The maximum concentration for which an allow is only a solution.
  • As the temperature increases, the solubility limit also increases.
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14
Q

What are ferrous alloys?

A

Ferrous alloys contain iron which is used because it is:

  • Abundant
  • Economical
  • Versatile
  • Stiff, strong and ductile
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15
Q

What are steels?

A

Steels are iron / carbon alloys.

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16
Q

How is cast iron produced?

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

It is frequently known as pig iron.

17
Q

What is the iron-carbon system?

A
18
Q

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

A

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.
19
Q

What are the different phase reactions?

A

Eutectic:

  • Occurs at a solubility of 4.3 wt%

Eutectoid:

  • Occurs at a solubility of 0.77 wt%
20
Q

How are pearlite and martensite formed?

A
  • 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.
21
Q

What are the features of pearlite?

A
  • 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.
22
Q

What are the features of martensite?

A
  • 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.
23
Q

Which factors affect the hardness of steels?

A
  • 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.
24
Q

What are the features of pure iron steels?

A

0 to 0.008% carbon

25
Q

What are the features of low carbon steels?

A
  • 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
26
Q

What are the features of medium carbon steels?

A
  • 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
27
Q

What are the features of high carbon steels?

A
  • 0.6 to 1.4% carbon
  • Hardest and strongest
  • Least ductile
  • Excellent wear resistance
  • Heat treatable in thick sections but avoid welding
28
Q

What are the features of cast iron steels?

A
  • 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
29
Q

What are the different types of non-ferrous alloys?

A
  • Copper alloys
  • Titanium alloys - Reactive
  • Magnesium alloys - Ignited easily
  • Aluminium alloys - Unreactive
  • Refractory alloys - High melting point
30
Q

What is metal fabrication?

A

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
31
Q

What are the different types of casting?

A

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.
32
Q

What are the different types of forming?

A

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.
33
Q

Why is plastic deformation carried out at high temperatures?

A

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

34
Q

What is the difference between cold and hot working?

A

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.
35
Q

What are the different types of joining?

A

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.

Materials Science (8 decks)

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