Structure of Crystalline Solids Flashcards
What are crystalline materials?
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.
What are amorphous materials?
Amorphous materials have no periodic packing / long-range order.
- Examples include glasses, plastics and other rapidly cooled materials.
- Sometimes known as non-crystalline.
How are polycrystalline materials formed?
- ‘Nuclei’ form during solidification, each of which grow into crystals.
- These grains are separated by amorphous grain boundaries.
How does the density/packing of a material affect its bonding energy.
- Dense, regularly packed materials have a high bonding energy.
- Non-dense, randomly packed materials have a low bonding energy.
What is a unit cell?
A unit cell is the smallest repeating arrangement of a material.
What is the coordination number (CN)?
The number of nearest neighbours or touching atoms.
What is the atomic packing factor (APF)?
The volume of atoms within a unit cell.
What is a simple cubic (SC) structure?
- Close packing direction is on the cube edges
- CN = 6
- APF = 0.52
- Rare, only Po has this structure
What is a body centred cubic (BCC) structure?
- 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
What is a face centred cubic (FCC) structure?
- 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)
What is a hexagonal close packed (HCP) structure?
- CN = 12
- APF = 0.74
- Slip systems = 3
- E.g. cadmium, magnesium, titanium, (non-ductile materials)
What are slip systems?
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.
What are single-crystalline materials?
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.
What are poly-crystalline materials?
- Isotropic if grains are randomly orientated.
- Anisotropic if grains are textured.
What is the difference between isotropic and anisotropic materials?
- Isotropic = properties do not vary with direction.
- Anisotropic = properties vary with direction.
What are the different types of defect?
Point defects (0D):
- Vacancy atoms
- Interstitial atoms
- Substitutionary atoms
Line defects (1D):
- Dislocations
Area defects (2D):
- Grain boundaries
Volume defects (3D):
- Precipitates
- Cracks
- Porosity
What is the effect of dislocations / imperfections on a material?
It affects many of the important properties of a material.
What are the different types of line defects?
- 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.
How are dislocations affected in different types of material?
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.
What is the dislocation density?
- 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.
What are the different types of area defects?
- 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.
What are the different types of volume defects?
- Cracks affect brittle metals.
- Porosity can be introduced into ceramics and metals.
