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Elements > Chapter 4 > Flashcards

Chapter 4 Flashcards

1
Q

What is solidification?

A

Result of casting of molten material

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2
Q

What are the two steps to solidification?

A
  1. nuclei of the solid phase form
  2. crystals grow until their boundaries meet each other– the crystals become grains
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3
Q

What do you have to start with in the process of solidification?

A

start with a molten material– all liquid

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4
Q

What are the two types of grains after solidification?

A

Equiaxed: roughly all the same dimension in all directions

Columnar: grains elongated in one direction

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5
Q

What is the function of a grain refiner?

A

Added to make smaller, more uniform, equiaxed grains

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6
Q

Where is it most common to see equiaxed grains/ columnar grains in a container when cooling?

A

Columnar will be in the middle since the region has slower cooling

Equiaxed grains will be near the outside because of rapid cooling near the wall

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7
Q

What are grain boundaries?

A

Regions between grains (crystals)

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8
Q

What are four properties of grain boundaries?

A
  1. Crystallographic misalignment across a grain boundary
  2. Slight atomic disorder
    • high atomic mobility
    • high chemical reactivity
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9
Q

What makes a grain boundary small or high?

A

the radius in between

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10
Q

Is there such a thing as a perfect crystal?

A

no

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11
Q

What is a crystalline defect?

A

Lattice irregularity with dimensions on the order of an atomic diameter

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12
Q

Are crystal imperfections bad?

A

No, it depends on the application

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13
Q

What are the three types of imperfections?

A
  • point defects (0 dimensional)
  • linear defects (1 dimensional)
  • interfacial defects (2 dimensional)
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14
Q

What are the different types of point defects?

A

3 types

Vacancies
Interstitial atoms
Substitutional impurity atoms

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15
Q

What is the type of linear defects?

A

dislocations

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16
Q

What is the type of interfacial defects?

A

Grain Boundaries

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17
Q

What are the properties of point defects in metals?

A

Vacancies
1. Vacant atomic site
2. distortion of planes
3. atoms close in around the vacancy

Self-Interstitials
1. atom placed in between other atoms
2. distortion of planes

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18
Q

What is the relationship between vacancies and temperature?

A

Vacancies increase with temperature

19
Q

What could happen to a material when temperature increases?

A

Surface area increases because there are more vacancies

20
Q

What are the two outcomes if impurity B atoms are added into a solid? What can often be a deciding factor?

A
  1. substitutional solid solution
  2. interstitial solid solution

atom size can be a deciding factor
- if B is small relative to A it will want to be interstitial

21
Q

What can happen to a solid solution B in A?

A

there is a different composition

22
Q

What are the four conditions for formation of substitutional solid solutions? What are these rules called?

A

W. Hume– Rothery rules
1. Δr (atomic radius) < 15%
2. Proximity in the periodic table
- similar electronegativities
3. Same crystal structure for pure metals
- FCC, BCC, Simple, etc…
4. High # valence electrons
- higher tendency to dissolve something with higher valence electrons

23
Q

What are dislocations? What are the two types of linear defects?

A

One-dimensional defects around which atoms are misaligned

  1. Edge dislocation
    • extra half-plane of atoms
      inserted in a crystal structure
    • b perpendicular to dislocation
      line
  2. Screw dislocation
    • spiral planar ramp resulting from shear deformation
    • b parallel to dislocation line
24
Q

What is the Burger’s vector?

A

measure of lattice distortion

25
Q

Where is the line of dislocation?

A

Where the atoms aren’t even

26
Q

What happens to dislocations when stresses are applied?

A
  1. Dislocations move
  2. Permanent (plastic) deformation results from dislocation motion
27
Q

Why do some dislocations look like they were done in steps?

A

It’s easier to dislocate something in steps

28
Q

What’s another name for interfacial defects?

A

Planar defects

29
Q

What are the three types of planar defects?

A
  1. Twin Boundaries
    - mirror reflections of atom positions on one side of twin plane to the other
  2. Stacking faults
    - occur when there’s an error in the stacking sequence
    ex: FCC: ABC ab ABC
  3. Grain Boundaries
30
Q

How many planar defects are there?

A

3

stacking, grain, twin

31
Q

What is a catalyst?

A

Something that increases the rate of a chemical reaction without being consumed

32
Q

Where do catalytic reactions normally occur?

A

Surface Defect sites

33
Q

What are the four parts of the single crystals used in automotive catalytic converters?

A

Ledge, step, terrace, adatom

34
Q

What are some examples of materials with large grain sizes?

A
  1. large single crystal of quartz
  2. diamond
  3. Si
35
Q

What is Optical Microscopy?

A

It uses light up to 2000X magnification
- polishing removes surface features
- etching changes reflectance

36
Q

What’s the relationship between grain boundaries and etching?

A
  1. Grain boundaries are more susceptible to etching
  2. after etching, grain boundaries appear as dark lines
37
Q

What is polarized light used for?

A
  1. metallographic scopes often use polarized light to increase contrast
  2. also used for transparent samples such as polymers
38
Q

In order to get higher resolution for optical microscopes, what do you do?

A

Use shorter wavelength radiation with electron beams
1. they have short wavelengths and atomic resolution is possible
2. focused by magnetic lenses

39
Q

What are the three kinds of microscopy?

A
  1. Optical microscopy– uses light
  2. Electron Microscopy– electron beams
  3. Scanning Tunneling Microscopy (STM)– microprobe tapers for a single atom
40
Q

What is Scanning Tunneling Microscopy?

A
  1. Surface atoms imaged used a microprobe that tapers to a single atom at its tip
  2. Surface atoms can be rearranged by pushing them into the desired position using the probe
41
Q

What are the dislocation types?

A
  1. Edge, screw, and mixed
42
Q

Up to what resolution can optical microscopy provide?

A

0.1μm

43
Q

Is optical or electron microscopy better?

A

Electron has higher magnifications and better resolution

Elements (15 decks)

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