6. Microstructure Flashcards
What is optical microscopy?
Looking at metals through a microscrope.
How do you reveal the microstructure?
THREE STEPS:
1) Polish material to mirror finish (i.e. remove macroscopic defects)
2) Use an etching agent that preferentially attacks defects or different phases
3) Let light scatter differently on different planes to reveal different arrangements.
Why does the etchant preferentially attack defects?
Defects are where the bond strength between atoms is weakest.
What does the etchant tend to attack preferentially?
Attacks different phases, different planes (which is rare) and grain boundaries.
Why is etching an art?
It’s really trial and error. There’s no science to it.
Can optical microscopy catch point and line defects?
Nope. Too small. Easier to do through electron microscopy.
MICROSTRUCTURE DESCRIPTIONS
Yay!
How can you describe the number of phases?
Color, size, shape, location
What are the ways you can describe a phase?
Color, size, distribution, shape, Volume fraction
What is the matrix phase?
The matrix which is either
1) the continuous phase (not broken up into globules)
2) the largest in volume
How can you quantify grain size?
The Heyn lineal intercept method! Length of line divided by number of intercepts.
What are the five possible types of grains you can see?
Cylindrical Equiaxed Lenticular Lamellar needle
What are the differences between the 3D and 2D versions of these shapes?
Cylinders can look like ellipses of different shape
Equiaxed looks like octagons.
lanellar will look like parallelograms or rectangles
Lenticular will look like ellipses
Needle
What do dendritic structures look like?
They can look really weird. Often circles
LEC 3-26
Yay!
What resolution does optical microscopy give you?
And electron microscopy?
Micrometer scale
Nanometer! Electon wavelegnth = 0.008nm when operated at 20kV
Electron diffraction giiives…?
Crystallography!
What are the advantages and disadvantages of SEM?
ADV:
- nanometer resolution
- Reveals crystallographic structure
- doesn’t require etching
- Can reveal point defects
DISADV:
- expensive
- takes hours
What are characteristic X-rays?
X-rays emitted when an electron drops from a higher orbit to a lower orbit.
These orbit drops happen during SEM, when an electron is ejected by the electron bombardment
What are composition maps?
Maps generated from scanning electron microscopes - they reveal the composition of a material
What sort of information can you acquire from composition maps?
Location of material (i.e. if they’re along grain boundaries)
Matrix phases vs continuous phases
If a material is in solid solution
How does TEM work?
You prepare a nanometer thick specimen, beam electrons at it, and look at the electrons transmitted.
What are the possible magnification scales of TEM?
20-100k typically
but 500-1000k possibly
Which electrons reveal crystallography and defects?
The forward scattered electrons
