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Materials Science > Topic 3- Structure of Crystalline Solids > Flashcards

Topic 3- Structure of Crystalline Solids Flashcards

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1
Q

Define crystalline.

A

Crystalline materials have atoms situated in an ordered, periodic array over large atomic distances.

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2
Q

What materials are crystalline?

A

Metals and most ceramics.

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3
Q

Give two examples of properties that depend on the crystal structure of a material.

A

Density and ductility.

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4
Q

Define amorphous/non-crystalline.

A

Amorphous materials have no long range order. They are a random arrangement of atoms.

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5
Q

What materials are amorphous?

A

Glasses (silica), plastics and rapidly cooled metals.

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6
Q

How fast is rapid cooling for a metal?

A

1x10^5 degrees C/s.

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7
Q

When do amorphous structures usually occur?

A

When cooling happens too quickly for nucleation to occur.

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8
Q

What is a region in a crystalline structure called?

A

Grain.

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9
Q

What are the lines in crystalline regions called?

A

Twins.

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10
Q

Define polycrystalline.

A

Polycrystalline materials are made up of lots of crystalline regions and form like a lake freezing over.

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11
Q

What is the name of the line where two crystalline regions meet?

A

Grain boundary.

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12
Q

Describe the formation of a polycrystalline structure.

A

‘Nuclei’ form during solidification, each of which grows into a crystal. The crystals grow and meet. These crystals are separated by an amorphous grain boundary which influences properties.

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13
Q

How does packing affect energy?

A

Dense, regular-packed structures tend to have lower energy.

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14
Q

Are dense, regularly-packed atoms stable or unstable?

A

Stable - this is the preferred atomic arrangement.

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15
Q

For a dense, regularly-packed atoms will separation distance be slightly greater or slightly less than equilibrium position?

A

Slightly greater.

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16
Q

Why are non-dense, randomly packed atoms less stable?

A

A range of bond lengths, means average bond length is greater so there is a higher energy state.

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17
Q

What is one negative impact of non-dense, random packing?

A

Resistance to corrosion would be worse.

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18
Q

What is a lattice?

A

A 3D array of regularly spaced points.

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19
Q

What is hard sphere representation?

A

Atoms denoted by hard, touching spheres.

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20
Q

What is reduced sphere representation?

A

Atoms denoted by small circles allowing position in 3D to be seen.

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21
Q

What is a unit cell?

A

Basic building block that repeats in space to create the crystal structure, usually a parallelpiped or prizm.

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22
Q

Roughly how many different types of unit cell are there?

A

14.

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23
Q

What does FCC stand for?

A

Face Centered Cubic.

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24
Q

What does BCC stand for?

A

Body Centered Cubic.

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25
Q

What does HCP stand for?

A

Hexagonal Close Packed.

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26
Q

What is the atomic packing factor?

A

Volume of atoms in unit cell / volume of unit cell.

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27
Q

What to things is ductility (ease of plastic deformation) linked to?

A

Crystal structure and close packed planes.

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28
Q

Where does slip occur?

A

On specific atomic planes and in specific crystallographic slip directions. I.e. slip systems.

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29
Q

What are slip planes/directions?

A

The most densely packed planes, and in that plane the closely packed direction.

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30
Q

What is the coordination number (CN)?

A

Number of nearest neighbours or touching atoms.

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31
Q

What is CN for simple cubic structure (SC)?

A

6

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32
Q

What is APF for SC?

A

0.52

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33
Q

Which element has simple cubic structure?

A

Polonium (Po)

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34
Q

Where are atoms located in an FCC structure?

A

At each corner and the centre of the cube faces.

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35
Q

What is CN for FCC?

A

12

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36
Q

What is APF for FCC?

A

0.74

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37
Q

How many slip planes in FCC?

A

4 (face diagonals)

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38
Q

How many slip directions in FCC?

A

3

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39
Q

How many slip systems in FCC?

A

4x3 = 12

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40
Q

Which is the most efficient way to pack? E.g. supermarket fruit

A

FCC

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41
Q

What materials have a FCC structure? Give four examples.

A

Ductile metals as there are many opportunities for planes to slide over each other.
Copper, aluminium, silver, gold.

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42
Q

Where are atoms located in BCC structure?

A

At each corner and the cube centre.

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43
Q

What is CN for BCC?

A

8

44
Q

What is APF for BCC?

A

0.68

45
Q

How many slip planes does BCC have?

A

6 (body diagonal)

46
Q

How many slip directions does BCC have?

A

2

47
Q

How many slip systems does BCC have?

A

6x2 = 12

48
Q

What do BCC materials experience?

A

A ductile-brittle transition.

49
Q

What do BCC materials feature? Give 3 examples.

A

A fatigue limit.

Chromium, Ferrite Iron (alpha), Tungsten.

50
Q

What is the arrangement of a HCP unit cell?

A

Hexagonal unit cell where top and bottom consists of 6 atoms with another atom in the centre, and the middle plane consists of 3 atoms.

51
Q

What is CN for HCP?

A

12

52
Q

What is APF for HCP?

A

0.74 (like FCC)

53
Q

How many slip planes does HCP have?

A

1 (hex. side)

54
Q

How many slip directions does HCP have?

A

3

55
Q

How many slip systems does HCP have?

A

1x3 = 3

56
Q

Which metals have HCP structure? Give four examples.

A

Least ductile.

Cadmium, magnesium, titanium, zinc.

57
Q

What is the difference between HCP and FCC structure?

A

HCP is ABA, FCC is ABC. FCC has more symmetry.

58
Q

What are single crystal materials?

A

When the periodic and repeating arrangement of atoms is perfect and extends through the entirety of the specimen.

59
Q

Give 3 examples of extreme technology single crystal materials are used for.

A

Electronic and optical material (Si wafers).
High performance turbine blades.
Abrasive materials (synthetic diamond).

60
Q

What does anisotropic mean?

A

Properties vary with direction.

61
Q

What impacts the production of silicon wafers?

A

Silicon in elemental abundance but high embodied energy and cost of production.

62
Q

What are single crystal turbine blades made form?

A

Nickel-based superalloys incorporating chromium, cobalt and rhenium.

63
Q

What crystal structure do most engineering materials have?

A

Polycrystalline.

64
Q

Polycrystalline materials often have isotropic properties, what does this mean?

A

Same properties in all directions.

65
Q

When do polycrystalline materials have anisotropic properties?

A

When the grains are textured.

66
Q

When do amorphous materials occur?

A

For complex structures and rapid cooling.

67
Q

What are many important material properties due to?

A

Presence of imperfections.

68
Q

What are the three types of point defects (0D)?

A

Vacancy atoms.
Interstitial atoms.
Substitutional atoms.

69
Q

What are line defects (1D)?

A

Dislocations.

70
Q

What are area defects (2D)?

A

Grain boundaries.

71
Q

What are the three types of volume defects (3D)?

A

Precipitates.
Cracks.
Porosity.

72
Q

What are vacancies?

A

Vacant atomic sites in a structure.

73
Q

How does diffusion in a solid occur?

A

Interchange of a vacancy and a neighbouring atom.

74
Q

What are self-interstitials?

A

“Extra” atoms positioned between atomic sites.

75
Q

Why does the equilibrium concentration of point defects vary with temperature?

A

Greater temperature means greater energy, therefore atoms move around more so there are more defects.

76
Q

What are intrinsic point defects?

A

Defects due imperfections without an impurity.

77
Q

What are extrinsic point defects?

A

Defects due to an impurity being added to a host (alloy).

78
Q

What are substitutional point defects?

A

When an impurity atom replaces a host atom.

79
Q

What are interstitials?

A

When a smaller impurity atom positions between host atomic sites.

80
Q

What occurs when your have larger amounts of an impurity?

A

Second phase particle which has a different composition and often a different structure.

81
Q

What are the four conditions for substitutional solid solution point defects?

A
  1. Difference in atomic radii <15%.
  2. Proximity in periodic table. I.e. similar electronegatives.
  3. Same crystal structure for pure metals.
  4. Valency - all else being equal, a metal will have greater tendency to dissolve a metal of higher valency than lower valency.
82
Q

What is an edge dislocation?

A

Extra plane of atoms inserted in a crystal structure.

83
Q

What is a screw dislocation?

A

Spiral planar ramp resulting from shear deformation.

84
Q

What is the Burger’s Vector?

A

A measure of lattice distortion.

85
Q

What causes dislocations to move?

A

Applied stress.

86
Q

How do dislocations move?

A

Incrementally breaking/re-making bonds.

87
Q

How does plastic deformation occur in metals and alloys?

A

Large numbers of dislocation move (or will just fracture like a ceramic).

88
Q

Why is dislocation motion easier in metals?

A

Non-directional bonding.

Close-packed directions for slip.

89
Q

Why is dislocation motion hard in covalent ceramics? E.g. Si, diamond.

A

Directional (angular) bonding.

90
Q

Why is dislocation motion hard in ionic ceramics? E.g. NaCl

A

Need to avoid ++ and – neighbours.

91
Q

What typically determines the strength of a material?

A

Dislocation density.

92
Q

What are the two definitions of dislocation density?

A
  1. Total dislocation length per unit volume of material.

2. Number of dislocations that intersect a unit are of a random section.

93
Q

Typical dislocation density for carefully solidified metals?

A

10^3 / mm^2

94
Q

Dislocation density for heavily deformed metals?

A

10^9 - 10^10 / mm^2

95
Q

Dislocation density for heat treated metals?

A

10^5 - 10^6 / mm^2

96
Q

Dislocation density for ceramics?

A

10^2 - 10^4 / mm^2

97
Q

Dislocation density for single crystal silicon for ICS?

A

0.1 - 1 / mm^2

98
Q

What does the low density in grain boundaries allow for?

A

High mobility, high diffusivity, high chemical reactivity.

99
Q

How does the angle of a grain boundary influence mechanical properties?

A

The angle influences the way dislocations move past which influences properties.

100
Q

What happens everytime a dislocation meets a grain boundary?

A

There is an increase in force.

101
Q

Where do impurities collect?

A

Grain boundaries. = weakness.

102
Q

How does grain size affect yield stress?

A

A reduction in grain size leads to an increase in yield stress.

103
Q

How can we view grain boundaries?

A

Using electron backscatter diffraction (EBSD).

104
Q

How do crystallites (grains) vary in size?

A

Large - single crystal of quartz, grains on a Al lamppost.

Small - mm or less, have to use a microscope to observe.

105
Q

What is an example of a deliberate volume defect? Why?

A

Precipitates in Al alloys. Precipitates act as barriers to dislocations and increase strength.

106
Q

What volume defects affect brittle materials?

A

Cracks and damage.

107
Q

What volume defect occurs in ceramics and metals?

A

Porosity

Materials Science (8 decks)

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