Chapter 4: Imperfections in Solids Flashcards
Definition of Solidification
Result of Casting of Molten Material
Process of Solidification
Start with Molten Material (All Liquid)
Crystals grow until they meet each other
What are Grain Boundaries
Regions Between Crystals
Transition from lattice of one region to that of the other
Slightly disordered
Low Density in Grain Bountaries
Properties of Low Density in Grain Boundaries
High Mobility
High Diffusivity
High Chemical Reactivity
Grains can either be
Equiaxed (Same size in all directions)
Columnar (elongated grains)
Is there a possible perfect solid?
There is no such thing as a perfect crystal.
3 Types of Imperfections in solids
Point Defects
Line Defects
Area Defects
Types of Point Defects
Vacancy Atoms
Interstitial Atoms
Substitutional Atoms
Types of Line Defects
Dislocations
Types of Area Defects
Grain Boundaries
Vacancy Atoms
Distortion of Planes, empty space in structure
Self-Interstitial Atoms
“Extra” Atoms in structure, distortion of plane
Equilibrium Concentration
Varies with Temperature
Formula for Equilibrium Concentration
(Nv / N) = exp(-Qv/kT)
where Nv = # of Defects N = # of potential sites Qv = Activation Energy k = Boltzmann's Constant (1.38 x 10^-23 J/atom-K) (8.62 * 10^-5 eV/atom-K) T = Temperature
Equilibrium Vacancy Motion
Increasing temperature causes surface island of atoms to grow.
Due to equivalent vacancy concentration increasing through atom motion from crystal to the surface
W Hume-Rothery Rule for Imperfections in Metals
- Atomic Radius < 15%
- Proximity in periodic table
- Same crystal structures for pure metals
- Valency
Dislocations
Type of line defect
Slip between crystal planes results when dislocations move
Produce Permanent (plastic) deformation
Types of Dislocations
Edge Dislocation
Screw Dislocation
Edge Dislocation
Extra Half-plane of atoms inserted into crystal structure.
Perpendicular to dislocation line
Screw Dislocation
Spiral planar ramp resulting from shear deformation
Parallel to dislocation line
Prefferec Structures
Close-packed planes and directions
Comparison of crystal structures
FCC: Many close-packed planes/directions
HCP: Only one plane, 3 directions
BCC: None
Types of Planar Defects
Twin Boundary
Stacking Faults
Twin Boundary Defect
Essentially reflection of atom positions across twin plane
Stacking Faults Defect
For FCC: error in ABCABC stacking sequence
ABCABACBABC
Type of Surface Defect
Catalyst
Catalyst Defect
Increases the rate of chemical reaction without being consumed
Active sites on catalysts are normally surface defects
Properties of grain boundaries through optical microscopy
Imperfections
More susceptible to etching
May be revealed as dark lines
Change in crystal orientation across alloy
Formula for Calculating # of grains at 100x magnification (N)
N = 2^(n - 1)
where n = ASTM grain size number
