lecture 3 Flashcards
Solidification:
Solidification: result of casting of molten material
* 2 steps
* Nuclei of solid phase start to form
* Nuclei grow to form crystals – when crystal boundaries meet
form grain structure
swe get impurities as material freeze
crystalline defect refers to
lattice irrgularity
defect refers to
disruption of a crystalline order of a periodic material
defect effect on energy of crystal
lower it and make crystal more stable
types of defects
point vacancies interstitial self interstitial substitional line edge screw dislocation area external surface, grain boundaries
interstices
space between atoms
vacancies
create distrotion of planes and vacancies can move usually low in number and 1 vacancy in 10,000 atoms when heated
self interstitial
less likely due to energy needed yo stuff in the atom causes distortion of planes
solid solutions
solid material solutions which are made up of the host solvent and the impurity solute usually will be either interstitial or substitional which can cause small disturbance in the surrounding lattice
two outcomes of solid material solution
a + b
a+b + particles of a new phase (usually for a
larger amount of B)
tell me further about the new phase in the secon dout come of the solid solution
Second phase particle
–different composition
–often different structure.
To what degree atoms of solute dissolve in solvent : W. Hume – Rothery rules :
- Δr (difference in atomic radius) < 15%
- Proximity in periodic table
* i.e. similar electronegativities
* Large differences results in intermetallic compounds - Same crystal structure for pure metals
- Valences
* All else being equal, a metal will have a greater tendency to dissolve a
metal of higher valence than one of lower valence
line dislocation
Are line (one-dimensional) defects
* Result from thermal stresses, mechanical deformation, rapid cooling
from liquid to solid
* Permanent deformation of crystalline materials is through movement of
dislocations
edge dislocation
screw dislocation
edge dislocation
1d extra HALF plane added to the crystal
It causes misalignment of the atoms planes around it’s edge
dislocation perpendicular to burger vector
Screw Dislocation
- Crystal is “cut halfway through & then slide sideways”
- For screw dislocation, b is parallel to the dislocation line
Interfacial (Area) Defects
external surface and grain boundary defects
external surface defects
- Crystal structure terminates; surface atoms lack neighboring atoms
Grain boundaries
Regions where grains (crystals) join. Because adjacent grains (crystals)
have different orientations then some disorder at boundaries. Atomic
bonding disrupted. More reactive chemically.
- transition from lattice of one
region to that of the other - slightly disordered
- low density in grain
boundaries - high mobility
- high diffusivity
- high chemical reactivity
Observation of Grain Structure
Grains, grain boundaries can be observed in metals/ceramics:
1. Section and Grind sample
2. Polish (diamond paste)
3. Etch (chemical attack) occurs preferentially depending upon
crystal orientation and in grain boundaries
4. View Microstructure with optical microscope or Scanning
Electron Microscope (SEM)
microscopy
For higher resolution need higher frequency
Electron (SEM)
* Wavelengths 3 pm (0.003 nm)
* (Magnification up to 1,000,000X)
* Atomic resolution possible
* Electron beam focused by magnetic lenses
Bulk & Volume Defects
Larger defects such as cracks, pores (holes), other stuff.
These are usually formed or introduced during casting or material
fabrication.
They are not really seen as being part of the atomic or crystal structure
but as defects introduced during a particular forming process, eg.
