Imperfections in the Atomic Arrangement of Solids

Solvent Element

Element within an alloy present at the greatest amount

Solute Element

Element within an alloy present at the lowest amount

Degree of Solid Solution depends on…

Atomic size factor

• Solid solution takes place when the difference in atomic radii between two atom types is less than ±15%. Otherwise solute atoms create Substitutional lattice distortions and a new phase will be formed.

Crystal Structure

• For solid solution to take place, the crystal structures of both atoms (metals) must be identical.

Electronegativity

• The greater the difference in electronegativity of the two elements,
  the greater is the chance of solid solution

Valences

• The higher the valency, the higher is the chance for solid solution

Line Defects (Dislocations)

Screw Dislocation

• A dislocation is produced by skewing a crystal so that one atomic plane produces a spiral ramp about the dislocation.

Edge Dislocation

• A dislocation introduced into the lattice by adding an extra imaginary half plane of atoms. Burgers vector is the line from the end point to the start point

Significance of Dislocations

1. Force required for slipping is less than a fraction of the force required for breaking metallic bonds

2. Slipping provides ductility in metals

3. Mechanical properties of metals are controlled by interfering with the dislocation movements.

Point Defects

Vacancy

• produced when an atom is missing from a normal site

Interstitial Defect

• When an atom is inserted into the
  lattice structure at a site which is
  not a normal lattice point

Substitutional Defect

• An atom is replaced by a different type of atom

Frankel Defect

• When an ion jumps from normal lattice point to an interstitial site leaving behind a vacancy.

Schottky Defect

• Pair of vacancies in an ioncally bonded material.

Surface Defects

The grain boundary is the surface that separates the individual grains and is narrow zone in which the atoms are not properly spaced.

Atoms near the boundaries of three grains do not have an equilibrium spacing or arrangement.

Effect of Grain Size on Yield Strength

As size of grains reduces, grains increase in numbers and therefore the amount (length) of grain boundaries increases. Dislocations move only a short distance before encountering grain boundaries. This improves the strength of metallic materials

Microscopic Examination

Used to investigate the micro structural features of materials such as grain size, grain boundaries, composition (wt%) measurements,
material phase analysis

Optical Microscopy

Mag. 1000-2000, for grains and phases

Electron Microscopy

• Transmission Electron Microscopy : Mag. 106 ,

  Dislocation study
  

• Scanning Electron Microscopy:

  Mag. 50000,

  composition, Dislocation study

Scanning Probe Microscopy

Mag. 10^9 ,

3D images under various environmental conditions (vacuum, air and liquid)

Con-focal Scanning laser Microscopy:

resolution 0.25 microns and 3D profile of surface roughness