Chapter 4: Imperfections in Solids

Question 1

Definition of Solidification

Answer 1

Result of Casting of Molten Material

Question 2

Process of Solidification

Answer 2

Start with Molten Material (All Liquid)

Crystals grow until they meet each other

Question 3

What are Grain Boundaries

Answer 3

Regions Between Crystals

Transition from lattice of one region to that of the other

Slightly disordered

Low Density in Grain Bountaries

Question 4

Properties of Low Density in Grain Boundaries

Answer 4

High Mobility

High Diffusivity

High Chemical Reactivity

Question 5

Grains can either be

Answer 5

Equiaxed (Same size in all directions)

Columnar (elongated grains)

Question 6

Is there a possible perfect solid?

Answer 6

There is no such thing as a perfect crystal.

Question 7

3 Types of Imperfections in solids

Answer 7

Point Defects

Line Defects

Area Defects

Question 8

Types of Point Defects

Answer 8

Vacancy Atoms

Interstitial Atoms

Substitutional Atoms

Question 9

Types of Line Defects

Answer 9

Dislocations

Question 10

Types of Area Defects

Answer 10

Grain Boundaries

Question 11

Vacancy Atoms

Answer 11

Distortion of Planes, empty space in structure

Question 12

Self-Interstitial Atoms

Answer 12

“Extra” Atoms in structure, distortion of plane

Question 13

Equilibrium Concentration

Answer 13

Varies with Temperature

Question 14

Formula for Equilibrium Concentration

Answer 14

(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

Question 15

Equilibrium Vacancy Motion

Answer 15

Increasing temperature causes surface island of atoms to grow.

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

Question 16

W Hume-Rothery Rule for Imperfections in Metals

Answer 16

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

Question 17

Dislocations

Answer 17

Type of line defect

Slip between crystal planes results when dislocations move

Produce Permanent (plastic) deformation

Question 18

Types of Dislocations

Answer 18

Edge Dislocation

Screw Dislocation

Question 19

Edge Dislocation

Answer 19

Extra Half-plane of atoms inserted into crystal structure.

Perpendicular to dislocation line

Question 20

Screw Dislocation

Answer 20

Spiral planar ramp resulting from shear deformation

Parallel to dislocation line

Question 21

Prefferec Structures

Answer 21

Close-packed planes and directions

Question 22

Comparison of crystal structures

Answer 22

FCC: Many close-packed planes/directions

HCP: Only one plane, 3 directions

BCC: None

Question 23

Types of Planar Defects

Answer 23

Twin Boundary

Stacking Faults

Question 24

Twin Boundary Defect

Answer 24

Essentially reflection of atom positions across twin plane

Question 25

Stacking Faults Defect

Answer 25

For FCC: error in ABCABC stacking sequence

ABCABACBABC

Question 26

Type of Surface Defect

Answer 26

Catalyst

Question 27

Catalyst Defect

Answer 27

Increases the rate of chemical reaction without being consumed

Active sites on catalysts are normally surface defects

Question 28

Properties of grain boundaries through optical microscopy

Answer 28

Imperfections

More susceptible to etching

May be revealed as dark lines

Change in crystal orientation across alloy

Question 29

Formula for Calculating # of grains at 100x magnification (N)

Answer 29

N = 2^(n - 1)

where n = ASTM grain size number