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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
Where is the line of dislocation?
Where the atoms aren't even
26
What happens to dislocations when stresses are applied?
1. Dislocations move 2. Permanent (plastic) deformation results from dislocation motion
27
Why do some dislocations look like they were done in steps?
It's easier to dislocate something in steps
28
What's another name for interfacial defects?
Planar defects
29
What are the three types of planar defects?
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
How many planar defects are there?
3 stacking, grain, twin
31
What is a catalyst?
Something that increases the rate of a chemical reaction without being consumed
32
Where do catalytic reactions normally occur?
Surface Defect sites
33
What are the four parts of the single crystals used in automotive catalytic converters?
Ledge, step, terrace, adatom
34
What are some examples of materials with large grain sizes?
1. large single crystal of quartz 2. diamond 3. Si
35
What is Optical Microscopy?
It uses light up to 2000X magnification - polishing removes surface features - etching changes reflectance
36
What's the relationship between grain boundaries and etching?
1. Grain boundaries are more susceptible to etching 2. after etching, grain boundaries appear as dark lines
37
What is polarized light used for?
1. metallographic scopes often use polarized light to increase contrast 2. also used for transparent samples such as polymers
38
In order to get higher resolution for optical microscopes, what do you do?
Use shorter wavelength radiation with electron beams 1. they have short wavelengths and atomic resolution is possible 2. focused by magnetic lenses
39
What are the three kinds of microscopy?
1. Optical microscopy-- uses light 2. Electron Microscopy-- electron beams 3. Scanning Tunneling Microscopy (STM)-- microprobe tapers for a single atom
40
What is Scanning Tunneling Microscopy?
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
What are the dislocation types?
1. Edge, screw, and mixed
42
Up to what resolution can optical microscopy provide?
0.1μm
43
Is optical or electron microscopy better?
Electron has higher magnifications and better resolution

Elements (15 decks)

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