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Materials Science > Structure of Crystalline Solids > Flashcards

Structure of Crystalline Solids Flashcards

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1
Q

What are crystalline materials?

A

Crystalline materials have atoms situated in a periodic 3D array over large atomic distances.

  • Examples include metals and most ceramics.
  • Properties such as ductility and density depend on the crystal structure of the material.
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2
Q

What are amorphous materials?

A

Amorphous materials have no periodic packing / long-range order.

  • Examples include glasses, plastics and other rapidly cooled materials.
  • Sometimes known as non-crystalline.
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3
Q

How are polycrystalline materials formed?

A
  1. ‘Nuclei’ form during solidification, each of which grow into crystals.
  2. These grains are separated by amorphous grain boundaries.
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4
Q

How does the density/packing of a material affect its bonding energy.

A
  • Dense, regularly packed materials have a high bonding energy.
  • Non-dense, randomly packed materials have a low bonding energy.
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5
Q

What is a unit cell?

A

A unit cell is the smallest repeating arrangement of a material.

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6
Q

What is the coordination number (CN)?

A

The number of nearest neighbours or touching atoms.

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7
Q

What is the atomic packing factor (APF)?

A

The volume of atoms within a unit cell.

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8
Q

What is a simple cubic (SC) structure?

A
  • Close packing direction is on the cube edges
  • CN = 6
  • APF = 0.52
  • Rare, only Po has this structure
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9
Q

What is a body centred cubic (BCC) structure?

A
  • Close packing direction is in the body diagonal
  • CN = 8
  • APF = 0.68
  • Slip systems = 6 x 2 = 12
  • E.g. chromium, a-iron, tungsten
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10
Q

What is a face centred cubic (FCC) structure?

A
  • Close packing direction is on the face diagonal
  • CN = 12
  • APF = 0.74
  • Slip systems = 4 x 3 = 12
  • E.g. copper, aluminium, gold, (ductile materials)
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11
Q

What is a hexagonal close packed (HCP) structure?

A
  • CN = 12
  • APF = 0.74
  • Slip systems = 3
  • E.g. cadmium, magnesium, titanium, (non-ductile materials)
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12
Q

What are slip systems?

A

Slip systems = slip planes x slip directions

  • Slip planes are the most densely packed plane of the unit cell.
  • Slip occurs on specific atomic planes and in specific slip directions.
  • Ductility is linked to the crystal structure and close packed planes.
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13
Q

What are single-crystalline materials?

A

Single crystal materials ones where the repeating arrangements of atoms are perfect and extend throughout the entire specimen.

  • Benefit electronic and optical materials
  • Improve strength.
  • Anisotropic meaning properties vary with direction.
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14
Q

What are poly-crystalline materials?

A
  • Isotropic if grains are randomly orientated.
  • Anisotropic if grains are textured.
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15
Q

What is the difference between isotropic and anisotropic materials?

A
  • Isotropic = properties do not vary with direction.
  • Anisotropic = properties vary with direction.
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16
Q

What are the different types of defect?

A

Point defects (0D):

  • Vacancy atoms
  • Interstitial atoms
  • Substitutionary atoms

Line defects (1D):

  • Dislocations

Area defects (2D):

  • Grain boundaries

Volume defects (3D):

  • Precipitates
  • Cracks
  • Porosity
17
Q

What is the effect of dislocations / imperfections on a material?

A

It affects many of the important properties of a material.

18
Q

What are the different types of line defects?

A
  • Edge dislocation
  • Screw dislocation
  • Dislocations are mobile under applied stress and move by incrementally breaking and remaking bonds.
  • If a large number of dislocations are present, plastic deformation will occur in metals and alloys.
19
Q

How are dislocations affected in different types of material?

A

Metals:

  • Dislocation is easy as bonding is non-directional.

Covalent ceramics:

  • Dislocation is hard due to directional bonding.

Ionic ceramics:

  • Dislocation is hard as it would produce +/+ and -/- neighbouring atoms which repel each other.
20
Q

What is the dislocation density?

A
  • The total dislocation length per unit volume of a material, or the number of dislocations that intersect a unit area of random section.
  • The dislocation density typically determines the strength of a material.
21
Q

What are the different types of area defects?

A
  • Grain boundaries are the regions found between two crystals where there is a transition from the lattice of one crystal to that of another.
  • The angle of misalignment acts as a measure of dislocation.
22
Q

What are the different types of volume defects?

A
  • Cracks affect brittle metals.
  • Porosity can be introduced into ceramics and metals.

Materials Science (8 decks)

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