lecture 3

Question 1

Solidification:

Answer 1

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

Question 2

crystalline defect refers to

Answer 2

lattice irrgularity

Question 3

defect refers to

Answer 3

disruption of a crystalline order of a periodic material

Question 4

defect effect on energy of crystal

Answer 4

lower it and make crystal more stable

Question 5

types of defects

Answer 5

point vacancies interstitial self interstitial substitional line edge screw dislocation area external surface, grain boundaries

Question 6

interstices

Answer 6

space between atoms

Question 7

vacancies

Answer 7

create distrotion of planes and vacancies can move usually low in number and 1 vacancy in 10,000 atoms when heated

Question 8

self interstitial

Answer 8

less likely due to energy needed yo stuff in the atom causes distortion of planes

Question 9

solid solutions

Answer 9

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

Question 10

two outcomes of solid material solution

Answer 10

a + b
a+b + particles of a new phase (usually for a
larger amount of B)

Question 11

tell me further about the new phase in the secon dout come of the solid solution

Answer 11

Second phase particle
–different composition
–often different structure.

Question 12

To what degree atoms of solute dissolve in solvent : W. Hume – Rothery rules :

Answer 12

1. Δr (difference in atomic radius) < 15%
2. Proximity in periodic table
   * i.e. similar electronegativities
   * Large differences results in intermetallic compounds
3. Same crystal structure for pure metals
4. Valences
   * All else being equal, a metal will have a greater tendency to dissolve a
   metal of higher valence than one of lower valence

Question 13

line dislocation

Answer 13

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

Question 14

edge dislocation

Answer 14

1d extra HALF plane added to the crystal
It causes misalignment of the atoms planes around it’s edge
dislocation perpendicular to burger vector

Question 15

Screw Dislocation

Answer 15

• Crystal is “cut halfway through & then slide sideways”
• For screw dislocation, b is parallel to the dislocation line

Question 16

Interfacial (Area) Defects

Answer 16

external surface and grain boundary defects

Question 17

external surface defects

Answer 17

• Crystal structure terminates; surface atoms lack neighboring atoms

Question 18

Grain boundaries

Answer 18

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

Question 19

Observation of Grain Structure

Answer 19

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)

Question 20

microscopy

Answer 20

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

Question 21

Bulk & Volume Defects

Answer 21

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.