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Engineering Materials > Defects in Crystals > Flashcards

Defects in Crystals Flashcards

1
Q

What are the types of defects?

A

0D, 1D, 2D, 3D

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2
Q

What is a 0D defect?

A

A point defect: atoms missing or in irregular places within a lattice i.e. lattice vacancies

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3
Q

What is a 1D defect?

A

Linear defect: groups of atoms in irregular positions i.e. screw and edge dislocations

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4
Q

What is a 2D defect?

A

Planar defects: the interfaces between homogenous regions of the material i.e. grain boundaries and external surfaces

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5
Q

What is a 3D defect?

A

Volume defects: extended defects i.e. pores and cracks

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6
Q

What is always present (absent) in crystals?

A

Vacancies

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7
Q

Why are there more vacancies at higher temperatures?

A

Atoms are frequently and randomly changing position leaving behind empty lattice sites.

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8
Q

What is the Boltzmann distribution?

A

The equation used to estimate the number of vacancies in a lattice in relation to temperature.

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9
Q

What is the Boltzmann equation?

A

Vacancies = nominal vacancies * e^(-Q/KbT)

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10
Q

What is Q?

A

The energy needed to form a vacant lattice site in a perfect crystal (J/mol)

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11
Q

What is Kb?

A

The Boltzmann constant, 1.38*10^-23 J/atomK

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12
Q

What is T?

A

Temperature (Kelvin)

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13
Q

What are the types of point defects?

A

vacancy, self-interstitial (uncommon), interstitial impurity, substitutional impurities (smaller and larger atoms)

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14
Q

What defines a very pure metal?

A

1 impurity per million atoms

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15
Q

What is added to iron to produce steel?

A

Carbon

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16
Q

What is added to silicon to change its electrical properties?

A

Boron

17
Q

What are alloys?

A

Deliberate mixtures of metals

18
Q

What is a factor for high solubility of substitutional impurities?

A
  1. Crystal structures of solute and solvent should be the same
  2. Electro-negativities of solute and solvent should be similar
  3. Atomic size factor - atoms need to “fit” ⇒ solute and solvent atomic radii should be within ~ 15%.
19
Q

What is a factor for interstitial impurities?

A
  1. Atomic radius of solute should be significantly smaller than solvent
    (For fcc, bcc, hcp structures the voids (or interstices) between the host atoms are relatively small ⇒ atomic radius of solute should be significantly less than solvent)
  2. Maximum solute concentration =< 10%
20
Q

What happens if too much of a solute is added to a lattice?

A

The second phase: new compounds/structures are formed or form local precipitates

21
Q

What is dislocation core?

A

The area of distortion created in the lattice structure

22
Q

What is dislocation?

A

The defect that produce distortion in the lattice situated around a line

23
Q

What are the types of dislocation?

A

Edge (Taylor), screw (Burgers), mixed

24
Q

What causes the formation of a dislocation?

A

concentration of vacancies, plastic deformation of crystal

25
Q

What is the burgers vector?

A

The magnitude and direction of the distortion within a lattice dislocation. Edge Dislocations (or TAYLOR)

26
Q

Which dislocation is parallel to the direction of crystal dislocation?

A

Screw.

27
Q

What is grain boundaries?

A

The atomic mismatch within the region where grains meet.

28
Q

What properties does a 3d defect affect?

A

External changes i.e Mechanical changes and optical properties

29
Q

Why are defects important?

A

Defects have a profound impact on the various properties of materials

30
Q

What is processing?

A

Processing allows the required properties to be achieved without changes in the composition of the material, but just by manipulating the crystal defects.
Control (and intentional introduction) of defects is in the core of many types of material processing.

31
Q

What is the definition of impurities?

A

Atoms which are different from the host. All real solids are impure.

32
Q

What are solid solutions made from?

A

Solid solutions are made of a host (the solvent or matrix) which dissolves the minor component (solute). The ability to dissolve is called solubility.

33
Q

Give an example of 3D defect

A

Pores and cracks

Engineering Materials (20 decks)

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