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Materials Physics > Week 4 > Flashcards

Week 4 Flashcards

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1
Q

What is diffusion?

A

The migration of atoms from regions of high concentration to low concentration.

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2
Q

What is interdiffusion?

A

The diffusion of impurities between two regions in response to a concentration gradient.

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3
Q

What is self-diffusion?

A

Diffusion of atoms in a one element material where atoms of the same type exchange.

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4
Q

What is vacancy diffusion?

A

The atomic bonds around the vacancy break, allowing a new atom to fill the vacancy.

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5
Q

What are interstitial atoms in terms of diffusion?

A

Atoms that can diffuse fairly quickly as their bonding is weaker and there are more interstatial sites to jump to but the interstatial atoms need to be small.

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6
Q

What is diffusion flux?

A

The speed of diffusing atoms. This can be in terms of mass or number of atoms.

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7
Q

What is steady-state diffusion?

A

Diffusion in which the flux does not change with time.

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8
Q

What is a concentration profile?

A

The concentration of atoms of interest as a function of position.

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9
Q

What is Fick’s first law?

A

The diffusion flux along direction, x is proportional to the concentration gradient.

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10
Q

What is Fick’s second law?

A

In most scenarios, diffusion is not steady state so both the concentration profile and gradient change with time. Although the concentration drives diffusion, profiles change with time.

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11
Q

What does the Arrhenius equation describe?

A

The probability for an atom to jump to a vacancy i.e. when it has enough thermal energy to break its bonds and move (activation energy)

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12
Q

How do interstatial and vacancy diffusion compare?

A

Diffusion of interstatials is typically faster than vacancy diffusion. Smaller atoms cause less distortion of the lattice and diffuse faster. Diffusion is faster in open lattices or open directions.

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13
Q

What is the role of microstructure in diffusion?

A

Diffusion coefficient depends on the path that diffusing atoms take. It is easier to diffuse through regions with less packed structure. This depends on:

  • Grain boundaries
  • Dislocation cones
  • External surfaces
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14
Q

What factors increase diffusion rates?

A
  • Open crystal structures
  • Lower density materials
  • Cations
  • Polycrystalline materials
  • Materials with secondary bonding
  • Materials with low melting temperatures
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15
Q

What factors decrease diffusion rates?

A
  • Close-packed structures
  • Large diffusing atoms
  • High diffusing materials
  • Single crystal materials
  • Anions
  • Materials with covalent bonding
  • Materials with high melting temperatures
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16
Q

What is creep?

A

The tendency for a solid material to slowly deform under long-term mechanical stress

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17
Q

What is the rate of creep related to?

A

Time, temperature and the load

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18
Q

Can creep lead to failure?

A

Creep is a slow process but it can eventually lead to failure.

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19
Q

How can you test for creep?

A

Materials can be subjected to a constant load at elevated temperature for extended times. The sample will deform immediately due to elastic deformation by the stress followed by slower creep in three places (primary, secondary and tertiary creep).

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20
Q

What are primary, secondary and tertiary creep respectively?

A

Primary creep: The creep rate decreases with time
Secondary creep: Steady state creep with a constant slope
Tertiary creep: The creep increases with time, accelerating to rapture

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21
Q

What is diffusional creep?

A

When a material is transported across the specimen by several possible paths including Nabarro-Herring creep (diffusion through the bulk crystal lattice) and cable creep (diffusion along grain boundaries).

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22
Q

What is dislocation creep?

A

Creep due to dislocations. Also known as power law creep.

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23
Q

What does temperature diffusional transport allow?

A

Dislocations to climb from one slip plane to another making it easier to move past obstacles.

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24
Q

What are the mechanisms of creep?

A
  • Grain boundary sliding
  • Dislocation motion
  • Stress-assisted vacancy diffusion
  • Grain boundary diffusion
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25
What is a deformation mechanism map?
A map to explore the different deformation mechanisms in a material.
26
What is the homologous temperature in a deformation map?
The actual temperature over the melting temperature.
27
What is the dominant mechanism in deformation maps?
The mechanism that gives the fastest strain rate.
28
How can creep be minimised?
- High melting temperature - High elastic modulus - Large grain sizes
29
What are the 3 possibilities for cavity formation?
- Cavities nucleate at the carbide-austenite interface - Cavities nucleate at the ferrite-austenite interface - Cavities nucleate at points where only both carbide and ferrite are present
30
What is microstructure?
The structure of a material visible in a microscope at a magnification above 25 x.
31
What is texture?
The distribution of crystallographic orientations of a polycrystalline sample.
32
What is annealing?
Heating a material up to a certain temperature allows atoms within to move around through diffusion. The diffusion process can remove the effect of cold working by annihilating dislocations and vacancies and then forms new grains.
33
How do materials recover after annealing?
Heating leads to increased diffusion and dislocation movement. Dislocations are able to move to reduce the internal strain energy. This can include two dislocations of opposite sign annihilating or transformation of dislocations into lower energy configurations.
34
What is the driving force in annealing?
Difference between the internal energy in the strained and unstrained grains.
35
How can recrystallisation occur after annealing?
The strain energy stored gives rise to a driving force to recrystallise new grains with less strain. At the right temperature this can lead to a complete recrystallisation of grains into the new ones. It only occurs in materials with a large enough driving force so if there is not much cold working strain present, it will not happen.
36
What is recrystallisation temperature?
The temperature required to completely recrystalise the material in an hour.
37
How does quenching work?
Often a material has an optimal microstructure at a temperature above room temperature. If you cool the material slowly, the atoms will continue to move around so the microstructure will change as it cools. Often the materials will be rapidly cooled or quenched to keep the phase chemistry obtained at the heat treatment.
38
What is the solubility limit?
The maximum amount of a component that can be dissolved into a phase. Above that level, the component will precipitate.
39
What does changing the amount of an element past the solubility limit result in?
New phases form. These depend on: - The crystal structure of the main phase - The atomic radii of the elements involved - The electronegativity of the different elements - The valency of each element
40
What are the two ways of describing materials made of more than one element?
Weight percent and atomic percent
41
Where do alloying elements go when added to materials?
This depends on the strength of the bonds of the different elements. If there is no preferential bonding, the atoms will be randomly distributed in a solid solution. If the bond between two elements is stronger, the solution will order to alternate the atoms. If there is more of the alloying elements that can dissolve or if certain elements prefer to bond to themselves, this can lead to clustering and precipitation of a new region or phase with different chemical composition and structure.
42
What is a phase?
The region of a system that has uniform physical and chemical characteristics.
43
What is a heterogenous system?
A system that has two or more distinct phases with different physical and/or chemical characteristics separated by definite phase boundaries.
44
What are phase diagrams?
Diagrams that describe the change in phases of a material at different conditions.
45
What is a triple point?
A region where three phases can exist simultaneously.
46
What is a component?
Any variable where the amount or composition of each component can be changed.
47
What is a binary alloy?
A binary alloy contains two components e.g. Fe and C
48
What is a tertiary alloy?
A tertiary alloy contains three componenets.
49
When is a system considered to be at equilibrium?
When it does not change with time at a constant temperature, pressure and composition.
50
What is a metastable state?
If the kinetics are slow, a material may reach a state where is appears stable despite not being the lowest energy state.
51
What is the isomorphous system?
The simplest binary system. This is where the components of A can completely accommodate the components of B so that they are completely soluble with each other.
52
What are the three potential phases of an isomorphous system?
- Solid (All alpha phase) - Liquid (Both components melted) - Partially melted region (Some liquid and some solid)
53
What is the melting like in multicomponent regions?
Regions where instead of melting occurring at a single well defined temperature, the melting happens over a range of the melting points of the individual components between the solidus and liquidus lines.
54
What can we determine from the phase diagram for a given temperature and composition?
- The phases present - Composition of the phases - The relative fractions of the phases
55
What is the Lever Rule?
A rule that lets us determine the amount of each phase at those conditions adding a fourth step.
56
What is the fraction of the phase?
The length of the tie line to the phase boundary for the other phase, divided by the total length of the line.
57
What are binary eutectic systems?
In a binary eutectic, three phases are present (liquid, alpha and beta). Elements A and B are partially soluble with each other. This leads to the solidus line which indicates the limit of solubility (there are three in this case alpha, beta and liquid). There are also three phase regions.
58
What is the eutectic point ?
The point where the liquid and two solid phases coexist in equilibrium.
59
What is the eutectic isotherm?
The line across the eutectic point.
60
What is the eutectic composition?
The composition where the material is liquid above the eutectic temperature.
61
What is the eutectic temperature?
The temperature at which the liquid transforms entirely to alpha and beta solid phases.
62
What are terminal solid solutions?
Phases at the composition extremes
63
What are intermediate solid solutions?
Phases that don't exist at the extremes of the phase diagram.
64
What are intermetallics?
A phase of a different crystal structure to the main phase and very specific composition that does not deviate from that composition like a normal phase would.
65
How do intermetalics appear in phase diagrams?
As vertical lines as they can only be 100% of the phases at one specific place on the composition graph.
66
What is a eutectoid reaction?
Similar to the eutectic reaction but occurs from one solid phase to two new solid phases.
67
What is the invariant point?
A point where three solid phases are in equilibrium.
68
What are peritectic reactions?
Reactions that occur when a solid and liquid together form a new solid phase after cooling.
69
What are congruent and incongruent reactions?
A congruent transformation involves no change in composition. If at least one phase changes composition it is known as incongruent.
70
What are ternary phase diagrams used for?
Stainless steels can be approximated using these.

Materials Physics (6 decks)

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