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Materials Physics > Week 5 > Flashcards

Week 5 Flashcards

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1
Q

What is steel?

A

An alloy conaining iron and other elements

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2
Q

What is stainless steel?

A

Steel that contains more than 10 percent chromatin.

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3
Q

What are the five phases of the iron-carbon phase diagram?

A
  • Alpha ferrite (Solid solution of C in BCC Fe)
  • Gamma ferrite (Solid solution of C in FCC Fe)
  • Delta ferrite (Solid solution of C in BCC Fee)
  • Fe3C (Metastable intermetallic compound)
  • FeC liquid solution
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4
Q

What is Fe-C phase behaviour?

A
  • Since the carbon atom is smaller than iron atom so C is an interstitial impurity and forms a solid solution with alpha, gamma and delta phases at different solubility levels
  • The BCC lattice in alpha ferrite has relatively small interstitial positions so C only has a low solubility
  • Alpha ferrite is magnetic below 768 degrees Celsius but austenite is non-magnetic
  • Cementite is hard and brittle which can be used to strengthen steel via precipitation hardening
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5
Q

What do eutectic and eutectoid reactions in steel allow?

A

The control of steel microstructure by careful use of heat treatments

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6
Q

What are hypoeutectoid alloys?

A

Alloys that contain proeutectoid ferrite formed above Te and a eutectoid pearlite structure containing eutectoid ferrite and cementite.

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7
Q

What are hypereutectoid steels?

A

Steels that contain cementite formed above the eutectoid temperature plus eutectoid ferrite and cementite.

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8
Q

What are the phases of hypereutectoid microstructure evolution?

A

1) Liquid Phase
2) Alloy begins to solidify to gamma austenite
3) Gamma austenite completely solidified forming grain boundaries
4) Proeutectoid cementite begins to form along gamma austenite grain boundaries
5) Thick proeutectoid cementite has formed along most grain boundaries
6) Alloy falls below eutectoid temperature so gamma austenite converts to pearlite structure of alpha ferrite and new Fe3 cementite

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9
Q

What are the three types of phase transformations that can occur?

A

1) Diffusion dependent with no changes in composition or number of phases (melting, solidification, recrystallisation)
2) Diffusion dependent with changes in phase composition and/or number of phases (eutectic and eutectoid transformations)
3) Diffusionless phase transformations (Small displacements of all atoms in the structure)

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10
Q

Where do most phases form?

A

Non-equilibrium conditions

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11
Q

What are the two ways that nucleation can occur?

A

Homogenous and heterogenous nucleation.

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12
Q

What is homogenous nucleation?

A

Nucleation that occurs randomly within the bulk material

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13
Q

What is heterogenous nucleation?

A

Nucleation that occurs on sites favourable to nucleation. These typically have lower energies such as defects, dislocations and grain boundaries.

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14
Q

How does the rate of heterogenous and homogenous nucleation compare?

A

As the energy required for heterogenous nucleation at defects is lower. Heterogenous nucleation tends to be faster than homogenous.

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15
Q

What must happen for homogenous nucleation to occur?

A

The temperature typically needs to be substantially beyond the transition point (when the material crosses the phase line).

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16
Q

What is supercooling?

A

When the temperature falls significantly below the transition point.

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17
Q

What does little super cooling mean?

A

Slow nucleation rate, few nuclei form, resulting in large crystals

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18
Q

What does large super cooling mean?

A

Rapid nucleation rate, lots of nuclei form, leading to small crystals

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19
Q

What does the Gibbs phase rule tell you?

A

How many phases are present in a material

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20
Q

What is Gibbs free energy?

A

A thermodynamic potential that can be used to calculate the maximum amount of non-volume expansion work that may be performed by a thermodynamically closed system at constant temperature and pressure.

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21
Q

What happens when the Gibbs Free energy decreases during a phase transformation?

A

The phase transformation occurs spontaneously.

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22
Q

What are the two contributions to the free energy difference between the solid and liquid phase?

A
  • The volume (bulk) free energy (the energy required to change the volume)
  • The surface tension
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23
Q

What are the energy effects from nucleation?

A
  • The surface term is positive, requiring energy to form a new surface interface, destabilising the nuclei from forming
  • The volume energy is negative so once the nuclei grows this term releases energy and stabilises the nuclei
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24
Q

What happens to the Gibbs energy above/below the critical radius?

A

Above, the nuclei are stable and want to grow to release energy via the volume term. Below, the surface term dominates and the nuclei shrink.

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25
What are heat treatments used for?
Heat treatments can create features of a desired size, shape and chemistry to alter the physical properties of the material
26
What does the graph of the time dependence of a solid state phase transformation at a fixed temperature look like?
It is an S-shaped curve.: 1) The rate increases as interfacial surface area increases and nuclei grow 2) The maximum rate is reach and amount unconverted decreases so rate slows 3) Transformation complete
27
What is the temperature dependence of transformation rate?
The rate increases with increasing temperature. At low temperatures the rate can be so slow that we never get to the equilibrium state.
28
What are time temperature transformation (TTT) diagrams/curves?
They show how quickly a transformation will occur at a particular temperature. They are for isothermal transformations where the material is cooled quickly and then kept at the temperature. It basically combines multiple S-shaped curves for different temperatures.
29
What is pearlite?
A finely laminated mixture of ferrite and cementite present in cast iron and steel, formed by the cooling of austenite.
30
What does the thickness of pearlite depend on?
The rate of change of the temperature.
31
When does coarse pearlite form?
At temperatures just below eutectoid.
32
When does fine pearlite form?
If the material cools rapidly to 540 degrees, thin layers of fine pearlite are formed.
33
What does reaction rate depend on?
Nucleation and growth rates.
34
What are the nucleation and growth rates for temperatures just below, moderately below and far below the eutectoid temperature?
Just below - Low nucleation, high growth rate Moderately below - Medium nucleation, medium growth rate Far below - High nucleation, low growth rate.
35
What is bainite?
A nonequilibrium transformation that occurs once the temperature falls too low for pearlite to form. It is also a transformation of austenite, but occurs at medium temperatures below the eutectoid point, where diffusion can take place.
36
What does bainite consist of?
Bainite consists of α-ferrite and Fe3C cementite like pearlite but with acicular (needle-like) particle shapes.
37
What is spheroidite?
A diffusion dependent structure. It consists of spherical Fe3C cementite within an α-ferrite matrix
38
What does spheroidite need to form?
It requires diffusion to form – the spherical shape is due to the driving force to reduce the interfacial area between the phases.
39
How does spheroidite form?
By heating bainite or pearlite at temperatures just below the eutectoid for long times.
40
What is martensite?
If an Fe-C alloy is rapidly cooled (or quenched) to a low temperature (i.e. room temperature), there is not time to form pearlite or even bainite. Instead, an instantaneous transformation occurs from FCC austenite to a BCT (body centred tetragonal) phase.
41
What is the rate of martensite growth and nucleation?
The martensite grains nucleate and grow at an incredibly rapid rate – comparable to the speed of sound in austenite.
42
When does the martensitic transformation occur?
The martensitic transformation is instantaneous, so the start of the transformation appears as a straight line on the TTT curve at 215°C
43
What is the structure of martensite?
It has a distinctive jagged structure. The new martensite phases forms its laths within the previous austenite structure.
44
What does tempering do to martensite?
Improves the ductility and toughness and anneals out internal stress. Typically the martensite is taken to a fixed temperature below the eutectoid temperature for a specific period of time. The martensite partially decomposes into separate ferrite and cementite phases.
45
What is a continuous cooling transformation (CCT) curve?
A diagram that shows the transformations with a constant rate of cooling. This delays the time for the reaction of start and end. These can be more helpful to predict how a metal will change as it cools.
46
What is the Bessemer process?
A way of making steel by bubbling air through molten steel to remove carbon. This makes the steel less brittle and more resistant to fatigue.
47
What process is used in the Bessemer process?
The Bessemer process uses a pear-shaped container, with air blown through bottom. Air reacts with manganese, silicon and carbon which rise to the top as slag. This slag reaction is exothermic, which also increases temperature
48
How did Mushet improve the Bessemer process?
By adding a certain kind of scrap that contained manganese after all the carbon was burned away.
49
How did open hearth steel improve steel making?
The regenerative heating in the honeycomb hearth allowed carbon to be burned out of pig iron in much larger volumes than before.
50
How is modern steel made?
Modern steel works tend to use electric arc furnaces to melt the raw materials. The melt is lanced with oxygen to burn off impurities.
51
What are welds used for?
They are used to join two components together. Typically a filler metal is melted in between the two parts to fuse them together.
52
Why can welds be the weakest parts of a system?
The microstructure is disrupted.
53
What is additive manufacturing?
3D printing of components from plastic, resin and even metal.
54
What are the two methods of corrosion?
Cry and wet.
55
What is the mechanism of dry corrosion?
In dry air corrosion the corrosion product (usually oxide) forms at the site of the corrosion, and forms uniformly. Dry corrosion is controlled by the rate of movement of ions/electrons through the oxide at the surface. This means that at low temperature, even a small layer of oxide at the surface can provide a protective barrier to further oxidation.
56
How do we measure the oxidation rate of dry corrosion?
Weight gained by the oxide forming.
57
What are the two primary types of oxidation at high temperature?
Linear and parabolic oxidation.
58
Why is wet corrosion faster than dry corrosion?
Due to electrochemical attack by the moisture. The moisture allows electron transfer, removing electrons from the metal (oxidation) and using them to generate OH- ions (reduction), resulting in an anodic region where M+ ions are formed and a cathodic region where OH- ions are formed, with a voltage difference between them due to the transfer of electrons.
59
What are REDOX potentials?
The potential required to either reduce or oxidise is important to the rate and mechanism of the corrosion that occurs.
60
What signs are the potentials for reduction and oxidation respectively?
Reduction = positive Oxidation = negative
61
What are Pourbaix diagrams?
Diagrams used to determine reaction produces of wet corrosion. This is similar to a phase diagram, but maps possible stable phases of an aqueous electrochemical system
62
What is pitting?
Downward propagation of small pits and holes as a result of localized attack at defects on the surface
63
What is erosion-corrosion?
Combined chemical attack and mechanical wear (e.g. pipe elbows).
64
What is a crevice corrosion?
In narrow and confined spaces, the concentration of corrosive species can be higher.
65
When can wet corrosion be a problem?
For materials with a difference in oxidation potential.
66
What are galvanic couples?
Where two metals are in the same corrosive environment but with differing potentials, the less noble metal will corrode.
67
What is intergranular corrosion?
In intergranular corrosion, the microstructure of the grain boundary is more susceptible to attack, and so the corrosion spreads along the edges of the grain.
68
What is stress corrosion?
In stress corrosion, a crack started from a corroded region can propagate through a material under stress. The crack continues to corrode into the material, growing and pushing the crack apart.
69
How can corrosion be prevented?
- Materials Selection - Use metals that are relatively unreactive in the corrosion environment -- e.g., Ni in basic solutions - Use metals that passivate - These metals form a thin, adhering oxide layer that slows corrosion. - Lower the temperature (reduces rates of oxidation and reduction) - Apply physical barriers -- e.g., films and coatings - Use a sacrificial material that is more cathodic that will corrode first. - Add inhibitors
70
Why is stainless steel corrosion resistance?
The Cr in stainless steel reacts quickly with oxygen to form a CrO surface layer. Unlike FeO which keeps growing, this CrO layer is stable and doesn’t grow further than a few 100 nm. This alloy is used for so many different applications today.
71
What is cathodic protection?
Attaching a more anodic material to the one to be protected - the corrosion will attack the sacrificial layer first.
72
What is wear?
When two surfaces are in contact with each other and in motion, the effects of friction lead to the removal of material.
73
What do lubricants do?
Reduce friction and so create less wear.
74
What is the study of wear called?
tribology
75
What is the process of wear?
- Energy initiation - Energy transformation - Energy dissipation Friction between two surfaces transfers stress to both surfaces. Depending on their material, this can lead to plastic deformation, cracking or grooves.
76
What is adhesive wear?
Where two surfaces bond together, and the plastic deformation causes the weaker surface to pull away
77
What is abrasive wear?
If one surface is harder, it can dig into the other surface and plough away material.
78
What is fatigue wear?
Repeated contact can lead to build up of cracks and damage due to fatigue.
79
What is corrosive wear?
In the presence of corrosive liquid and gases, wear can be enhanced by chemical reactions weakening the surface.
80
How does wear lead to failure?
Wear can lead to the formation of defects on the surface of components. As we know, these can lead to initiation of fatigue or brittle cracking
81
What is thermal cycling?
Thermal cycling causes expansion and contraction – can lead to thermal fatigue stresses.
82
Why do things expand as they get hotter?
As temperature increases, the amplitude of atomic oscillations increases. If the potential well is symmetric, then the average separation doesn’t change with increased T. For more asymmetric potentials, higher oscillation results in an increase of the average interatomic separation.
83
How does interatomic bonding affect thermal expansion?
The stronger the interatomic bonding, the deeper the potential energy curve, so the less this effect matters.
84
What is irradiation?
When the impact of a single high energy particle sets off a cascade of damage that spreads through the material.
85
What is a primary knock on atom?
An atom directly knocked out of position by the neutron, creating a vacancy.
86
What can radiation damage cause?
- Dislocations - Gas bubbles at grain boundaries - Intragranular gas bubbles - High burnup region

Materials Physics (6 decks)

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