Chapter 4 Flashcards
What is solidification?
Result of casting of molten material
What are the two steps to solidification?
- nuclei of the solid phase form
- crystals grow until their boundaries meet each other– the crystals become grains
What do you have to start with in the process of solidification?
start with a molten material– all liquid
What are the two types of grains after solidification?
Equiaxed: roughly all the same dimension in all directions
Columnar: grains elongated in one direction
What is the function of a grain refiner?
Added to make smaller, more uniform, equiaxed grains
Where is it most common to see equiaxed grains/ columnar grains in a container when cooling?
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
What are grain boundaries?
Regions between grains (crystals)
What are four properties of grain boundaries?
- Crystallographic misalignment across a grain boundary
- Slight atomic disorder
- high atomic mobility
- high chemical reactivity
What makes a grain boundary small or high?
the radius in between
Is there such a thing as a perfect crystal?
no
What is a crystalline defect?
Lattice irregularity with dimensions on the order of an atomic diameter
Are crystal imperfections bad?
No, it depends on the application
What are the three types of imperfections?
- point defects (0 dimensional)
- linear defects (1 dimensional)
- interfacial defects (2 dimensional)
What are the different types of point defects?
3 types
Vacancies
Interstitial atoms
Substitutional impurity atoms
What is the type of linear defects?
dislocations
What is the type of interfacial defects?
Grain Boundaries
What are the properties of point defects in metals?
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
What is the relationship between vacancies and temperature?
Vacancies increase with temperature
What could happen to a material when temperature increases?
Surface area increases because there are more vacancies
What are the two outcomes if impurity B atoms are added into a solid? What can often be a deciding factor?
- substitutional solid solution
- interstitial solid solution
atom size can be a deciding factor
- if B is small relative to A it will want to be interstitial
What can happen to a solid solution B in A?
there is a different composition
What are the four conditions for formation of substitutional solid solutions? What are these rules called?
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
What are dislocations? What are the two types of linear defects?
One-dimensional defects around which atoms are misaligned
- Edge dislocation
- extra half-plane of atoms
inserted in a crystal structure - b perpendicular to dislocation
line
- extra half-plane of atoms
- Screw dislocation
- spiral planar ramp resulting from shear deformation
- b parallel to dislocation line
What is the Burger’s vector?
measure of lattice distortion
Where is the line of dislocation?
Where the atoms aren’t even
What happens to dislocations when stresses are applied?
- Dislocations move
- Permanent (plastic) deformation results from dislocation motion
Why do some dislocations look like they were done in steps?
It’s easier to dislocate something in steps
What’s another name for interfacial defects?
Planar defects
What are the three types of planar defects?
- Twin Boundaries
- mirror reflections of atom positions on one side of twin plane to the other - Stacking faults
- occur when there’s an error in the stacking sequence
ex: FCC: ABC ab ABC - Grain Boundaries
How many planar defects are there?
3
stacking, grain, twin
What is a catalyst?
Something that increases the rate of a chemical reaction without being consumed
Where do catalytic reactions normally occur?
Surface Defect sites
What are the four parts of the single crystals used in automotive catalytic converters?
Ledge, step, terrace, adatom
What are some examples of materials with large grain sizes?
- large single crystal of quartz
- diamond
- Si
What is Optical Microscopy?
It uses light up to 2000X magnification
- polishing removes surface features
- etching changes reflectance
What’s the relationship between grain boundaries and etching?
- Grain boundaries are more susceptible to etching
- after etching, grain boundaries appear as dark lines
What is polarized light used for?
- metallographic scopes often use polarized light to increase contrast
- also used for transparent samples such as polymers
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
What are the three kinds of microscopy?
- Optical microscopy– uses light
- Electron Microscopy– electron beams
- Scanning Tunneling Microscopy (STM)– microprobe tapers for a single atom
What is Scanning Tunneling Microscopy?
- Surface atoms imaged used a microprobe that tapers to a single atom at its tip
- Surface atoms can be rearranged by pushing them into the desired position using the probe
What are the dislocation types?
- Edge, screw, and mixed
Up to what resolution can optical microscopy provide?
0.1μm
Is optical or electron microscopy better?
Electron has higher magnifications and better resolution
