Crystal Defects Flashcards
Is there such a thing as a perfect crystal?
No! All crystals have defects.
What is a defect?
An irregularity in the structure.
What are the 4 broad categories of defects.
Point, line, planar, volume
Define a point defect and it’s types
A localized disruption the size of one or a few atoms, including vacancies, self - interstitial, and foreign impurity.
Define a vacancy. What materials have vacancies? Why?
A vacancy is a hole where an atom should be. All materials have them, because they lower the energy of an atom. The optimal number of defects is the one where energy is lowest.
What is the relationship between temperature and optimum vacancies?
Optimum number of vacancies increases exponentially with temperature.
What is the interpretation of Qv in the following formula?
Nv = Ne^[-Qv/kT]
Energy required to produce one vacancy
What is the interpretation of Qv in the following formula?
Nv = Ne^[-Qv/RT]
Energy required to produce one mole of vacancies.
What is a self-interstitial defect? When do they occur?
An extra atom in a usually empty space that distorts the ones around it. They are rare and occur in low concentrations.
What is an impurity or foreign atom?
An atom of a different element that occupies an interstitial or substitutional location. They are often introduced to provide specific properties.
100% is impossible to manufacture, so all materials have some foreign atoms.
What is alloying?
The addition of foreign atoms to crystals to create a desired structural property.
What is a solid solution alloy?
A single phase alloy, where all of the solute has ‘dissolved’ into the solvent. This has only one type of crystal structure present.
What factors affect alloy ‘solubility’?
Atomic size
Crystal structure
Electronegativity
Valence
What is the maximum atomic size difference that is able to produce a solid solution alloy?
15% size difference. Size difference will cause a new phase to be formed
What happens when two separate crystal structures are mixed?
Two different structures will be formed.
What happens when different electronegativies are mixed?
The alloy forms compounds rather than a solid solution.
What happens when different valences are mixed?
The alloy forms compounds rather than a solid solution.
What is the maximum allowable concentrations of interstitial atoms in a solid solution alloy?
No more than 10%, and often lower.
What is a line defect? What types are there?
A one-dimensional defect that causes a line of lattice distortion.
Screw and edge dislocations.
When are dislocations introduced into materials?
During casting solidification and mechanical deformation.
What is an edge dislocation?
The edge of an extra plane terminates within a crystal.
Where is the edge dislocation line?
Down the length of the edge of the ending plane.
Where are the lattice distortions of an edge dislocation?
Around the dislocation line. Atoms around the extra plane are compressed, while atoms in the other region are stretched.
What is the conventional symbol for an edge dislocation?
T (the perpendicular symbol) with the horizontal edge facing opposite the terminating plane.
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