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Materials > Crystal Defects > Flashcards

Crystal Defects Flashcards

1
Q

Is there such a thing as a perfect crystal?

A

No! All crystals have defects.

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2
Q

What is a defect?

A

An irregularity in the structure.

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3
Q

What are the 4 broad categories of defects.

A

Point, line, planar, volume

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4
Q

Define a point defect and it’s types

A

A localized disruption the size of one or a few atoms, including vacancies, self - interstitial, and foreign impurity.

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5
Q

Define a vacancy. What materials have vacancies? Why?

A

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.

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6
Q

What is the relationship between temperature and optimum vacancies?

A

Optimum number of vacancies increases exponentially with temperature.

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7
Q

What is the interpretation of Qv in the following formula?

Nv = Ne^[-Qv/kT]

A

Energy required to produce one vacancy

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8
Q

What is the interpretation of Qv in the following formula?

Nv = Ne^[-Qv/RT]

A

Energy required to produce one mole of vacancies.

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9
Q

What is a self-interstitial defect? When do they occur?

A

An extra atom in a usually empty space that distorts the ones around it. They are rare and occur in low concentrations.

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10
Q

What is an impurity or foreign atom?

A

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.

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11
Q

What is alloying?

A

The addition of foreign atoms to crystals to create a desired structural property.

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12
Q

What is a solid solution alloy?

A

A single phase alloy, where all of the solute has ‘dissolved’ into the solvent. This has only one type of crystal structure present.

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13
Q

What factors affect alloy ‘solubility’?

A

Atomic size
Crystal structure
Electronegativity
Valence

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14
Q

What is the maximum atomic size difference that is able to produce a solid solution alloy?

A

15% size difference. Size difference will cause a new phase to be formed

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15
Q

What happens when two separate crystal structures are mixed?

A

Two different structures will be formed.

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16
Q

What happens when different electronegativies are mixed?

A

The alloy forms compounds rather than a solid solution.

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17
Q

What happens when different valences are mixed?

A

The alloy forms compounds rather than a solid solution.

18
Q

What is the maximum allowable concentrations of interstitial atoms in a solid solution alloy?

A

No more than 10%, and often lower.

19
Q

What is a line defect? What types are there?

A

A one-dimensional defect that causes a line of lattice distortion.
Screw and edge dislocations.

20
Q

When are dislocations introduced into materials?

A

During casting solidification and mechanical deformation.

21
Q

What is an edge dislocation?

A

The edge of an extra plane terminates within a crystal.

22
Q

Where is the edge dislocation line?

A

Down the length of the edge of the ending plane.

23
Q

Where are the lattice distortions of an edge dislocation?

A

Around the dislocation line. Atoms around the extra plane are compressed, while atoms in the other region are stretched.

24
Q

What is the conventional symbol for an edge dislocation?

A

T (the perpendicular symbol) with the horizontal edge facing opposite the terminating plane.

A A A A
A A T A A
A A A A A
A A A A A

25
Q

What is a screw dislocation?

A

Where a portion of the crystal is all shifted one atomic space over.

26
Q

Where is the screw dislocation line?

A

Down the length of the shift.

27
Q

Where are the lattice distortions of a screw dislocation?

A

Around the dislocation line.

28
Q

What is mixed dislocation? When does it occur?

A

It is a combination of both screw and edge dislocations, it is found in most materials

29
Q

What is the burgers vector?

A

b. It is the magnitude and direction of a dislocations, and is defined as the interatomic spacing in the direction of closest atomic packing.

30
Q

When is b perpendicular to the dislocation line? When is it parallel? Neither?

A

Perpendicular: Edge
Parallel: Screw
Neither: Mixed

31
Q

What are planar / interfacial defects?

A

2D boundaries that seperate regions of different structure or orientation.

32
Q

What are the types of planar defects?

A

External Surface, Grain Boundary, Twin Boundary, Stacking Fault, Phase Boundary

33
Q

Why is the external surface a defect? What is it’s energy signature? What kind of external surface area make a material ‘happiest’?

A

It’s a defect because it is a disruption in the crystal structure. It has a higher energy because it’s coordination number is lower. The happiest surface area is the smallest one, a sphere.

34
Q

Define a grain boundary.

A

A separation of regions of different orientation.

35
Q

What is atomic mismatch?

A

The transition from one orientation to another across a grain boundary.

36
Q

What is the difference between a low angle grain boundary and a high angle grain boundary.

A

How much one grain would have to rotate to match the other. The higher the angle, the smaller the coordination number, the higher the energy.

37
Q

What is a twin boundary?

A

Where the structures are mirror images of each other. Two identical orientations can be separated by a set of twin boundaries in the middle.

38
Q

What is a stacking fault?

A

An irregularity in the stacking of the planes. (A missing plane, an extra plane, or a misaligned plane)

39
Q

What is a phase boundary?

A

The boundary between two phases, or different crystal structures. The two structures will always have different chemical compositions. Only found in multi-phase alloys (obviously)

40
Q

What are volume defects? Examples? When do they occur?

A

3D defects, such as cracks, voids, or pores. They are introduced during material processing.

Materials (12 decks)

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