Chapter 4: Imperfections in Solids Flashcards

(29 cards)

1
Q

Definition of Solidification

A

Result of Casting of Molten Material

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2
Q

Process of Solidification

A

Start with Molten Material (All Liquid)

Crystals grow until they meet each other

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3
Q

What are Grain Boundaries

A

Regions Between Crystals

Transition from lattice of one region to that of the other

Slightly disordered

Low Density in Grain Bountaries

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4
Q

Properties of Low Density in Grain Boundaries

A

High Mobility

High Diffusivity

High Chemical Reactivity

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5
Q

Grains can either be

A

Equiaxed (Same size in all directions)

Columnar (elongated grains)

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6
Q

Is there a possible perfect solid?

A

There is no such thing as a perfect crystal.

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7
Q

3 Types of Imperfections in solids

A

Point Defects

Line Defects

Area Defects

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8
Q

Types of Point Defects

A

Vacancy Atoms

Interstitial Atoms

Substitutional Atoms

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9
Q

Types of Line Defects

A

Dislocations

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10
Q

Types of Area Defects

A

Grain Boundaries

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11
Q

Vacancy Atoms

A

Distortion of Planes, empty space in structure

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12
Q

Self-Interstitial Atoms

A

“Extra” Atoms in structure, distortion of plane

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13
Q

Equilibrium Concentration

A

Varies with Temperature

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14
Q

Formula for Equilibrium Concentration

A

(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
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15
Q

Equilibrium Vacancy Motion

A

Increasing temperature causes surface island of atoms to grow.

Due to equivalent vacancy concentration increasing through atom motion from crystal to the surface

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16
Q

W Hume-Rothery Rule for Imperfections in Metals

A
  1. Atomic Radius < 15%
  2. Proximity in periodic table
  3. Same crystal structures for pure metals
  4. Valency
17
Q

Dislocations

A

Type of line defect

Slip between crystal planes results when dislocations move

Produce Permanent (plastic) deformation

18
Q

Types of Dislocations

A

Edge Dislocation

Screw Dislocation

19
Q

Edge Dislocation

A

Extra Half-plane of atoms inserted into crystal structure.

Perpendicular to dislocation line

20
Q

Screw Dislocation

A

Spiral planar ramp resulting from shear deformation

Parallel to dislocation line

21
Q

Prefferec Structures

A

Close-packed planes and directions

22
Q

Comparison of crystal structures

A

FCC: Many close-packed planes/directions

HCP: Only one plane, 3 directions

BCC: None

23
Q

Types of Planar Defects

A

Twin Boundary

Stacking Faults

24
Q

Twin Boundary Defect

A

Essentially reflection of atom positions across twin plane

25
Stacking Faults Defect
For FCC: error in ABCABC stacking sequence | ABCABACBABC
26
Type of Surface Defect
Catalyst
27
Catalyst Defect
Increases the rate of chemical reaction without being consumed Active sites on catalysts are normally surface defects
28
Properties of grain boundaries through optical microscopy
Imperfections More susceptible to etching May be revealed as dark lines Change in crystal orientation across alloy
29
Formula for Calculating # of grains at 100x magnification (N)
N = 2^(n - 1) where n = ASTM grain size number