Materials Science and Engineering Ch 4

What are the types of imperfections

Point defects, line defects, area defects, volume defects.

Pont defects - Vacancy

Missing one lattice point/atom. All materials have vacancies, it's not possible to create a vacancy free material.

Equilibrium Vacancies

Nv = N exp(-Qv/k*T)

used to predict the number of vacancies present in a material. N atomic sites, Qv activation energy, k boltzmann's constant, T abs temp.

Point defects - self interstitial

an atom of the crystal occupies an otherwise non-occupied small void between lattice points. Requires a large amount of energy to form and is not highly probable.

Other point defects - Impurities

Impurity atoms are intentionally added to achieve a desired material property.

Solid Solution

Many alloys are created through solid solution. A solute (smaller quantity) is added to a solvent (largest quantity) which adds a random distribution of point defects while maintaining the crystal structure.

Impurity point defects - Substitutional

An atom is replaced by another different atom and maintains the same crystal structure.

Hume-Rothery Rules

1. Atomic size

2. Crystal structure

3. Electronegativity

4. Valency

Atomic size Hume-Rothery Rule

1st rule. the difference in atomic size between atoms must be less than 15% the radius of the original atom

Crystal Structure Hume-Rothery Rule

2nd rule. Both atoms must have the same crystal structure.

Electronegativity Hume-Rothery Rule

3rd Rule. Both atoms must have similar electronegativities. (be near each other on the periodic table)

Valency Hume-Rothery Rule

4th Rule. If all else is similar than a metal is more likely to dissolve in a metal of higher valency than one with lower valency. (ex: valency of +2 is more likely to dissolve than +3)

Impurity point defect - interstitial

An atom will fill in the voids or interstices between host atoms. Due to high packing factor the impurities must be much smaller than the host atom.

What are the two types of dislocations?

Edge defect and screw dislocation

What are edge defects?

An extra portion of a plane of atoms who's edge terminates inside the crystal. Above the dislocation line lattice is squeezed, and below the lattice is pulled apart. Perpendicular to dislocation line.

What are screw dislocations?

Formed by shear stress. Upper part of the crystal is shifted to the right relative to the bottom. Distortion is linear along a dislocation line. Parallel to dislocation line.

What is slip?

Magnitude and direction of lattice distortion are expressed as the Burger's vector. Motion along these vectors is what causes permanent deformation of a material.

What are the five types of interfacial defects?

External surface, Grain boundaries, Phase boundaries, Twin boundaries, Stacking faults

How can we see defects?

Electron microscopy, scanning electron microscopy, transmission electron microscopy, scanning probe microscopy

What are the characteristics of dislocations in metals?

Dislocation motion is easy, Non-directional bonds, close packed direction for slip.

What are the characteristics of dislocations in covalent ceramics?

Dislocation motion is difficult. Directional bonds with fixed angles.

What are the characteristics of dislocations in Ionic ceramics?

Dislocation motion is difficult. Opposite charges from nearest neighbors create repulsive forces. Breaks applied for motion.