Chapter 4

Question 1

What are the types of imperfections?

Answer 1

• 0 Dimensional (point)
• 1 Dimensional (linear)
• 2 Dimensional (Planar/interfacial)
• 3 Dimensional (volume)

Question 2

Give 3 examples of point defects

Answer 2

• Vacancy Atoms
• Interstitial atoms
• Substitutional atoms

Question 3

Give an example of a linear defect

Answer 3

Dislocations

Question 4

Give 3 examples of planar/interfacial Defects

Answer 4

• Grain Boundaries
• Twin Boundaries
• Stacking Faults

Question 5

Give 3 examples of volume defects

Answer 5

1. Impurity inclusions
2. Pores
3. Cracks

Question 6

What is a vacancy?

Answer 6

• A point defect where there are vacant atomic sites in the structure

Question 7

What is an interstitual atom?

Answer 7

A point defect where an atom is added into the structure

Extra atoms are added between atomic sites.

Question 8

What is a substitutional atom?

Answer 8

A point defect where one atom is replaced with another different atom not orignally in the structure

Question 9

What is the equilibrium concentration for point defects formula?

Answer 9

Question 10

For Point Defects in Alloys, what are two outcomes if an impurity (B) is added to a host (A)

Answer 10

1. Solid solution of B in A (i.e. random dist. of point defects)
2. Solid solution of B in A plus particles of a new phase (usually for a larger amount of B)

Question 11

What are the weight percent and atom percent formulas?

Answer 11

Question 12

What are dislocations?

Answer 12

• They are line defects that produce permanent plastic deformation
• Are 1D defects around which atoms are misaligned
• Slip between crystal planes result when dislocations move

Question 13

What are the 2 Types of Linear dislocations?

Answer 13

1. Edge dislocation
2. Screw Dislocation

Question 14

What is an edge dislocation?

Answer 14

• Dislocation where an extra portion of a plane of atoms, or half-plane has been inserted into the structure.
• The edge terminates within the crystal
• The burgers vector is perpendicular to the dislocation line

Question 15

What is an screw dislocation?

Answer 15

• Spiral planar ramp resulting from shear deformation
• Burgers Vector is parallel to dislocation line

Question 16

What is a Burgers vector?

Answer 16

• A vector that denotes the magnitude and direction of lattice distortion associated with a dislocation

Question 17

Determine the Burgers vector in the following picture:

Answer 17

• To determine the vector, count around the edge of the structure.
• The number of times counted along one edge must equal the number of times on the edge opposite.
• The vector drawn from the final position to the initial is the burgers vector.

Question 18

Compare the crystal Structures of FCC, BCC AND HCP

Answer 18

Close packed planes and directions are preferred

FCC- Many close packed planes/directions
HCP: only one plane, 3 directions
BCC: none.

Question 19

What is a twin boundary?

Answer 19

A planar defect that is a reflection of atom positions across the twin plane

Question 20

What are Stacking faults?

Answer 20

• Planar defects where there is an error in the packing sequence
  e. g. ABCABABC
• There is no C in the second ABC
  e. g. ABCABCBABC
• There is a B which has been inserted.

Question 21

What is the relationship between twins and SFs?

Answer 21

If Stacking faults occur periodically and remove every second letter, it appears as if twinning has occured.

e.g

ABCABCABCABCABCABCABC

ABCABCABC | BACBAC

Question 22

Name some other interfacial defects

Answer 22

• External surfaces
• Grain boundaries
• Anti-phase domain boundaries
• Phase boundaries

Question 23

Which defect is the most important in determining the mechanical properties of a structure?

Answer 23

Line Defects

Question 24

What are mixed Dislocations?

Answer 24

• A dislocation that has both edge and screw components

Question 25

What are impurity iclusions in volume defects?

Answer 25

Impurities are precipitated into the structure

Question 26

What controls Vacancy Concetrations in point defects?

Answer 26

Temperature

Increasing T causes surface island of atoms to grow.
Why? The equil. vacancy conc. increases via atom motion from the crystal to the surface, where they join the island.

Question 27

What is the dislocation line?

Answer 27

• The line that extends along the end of the extra half-plane of atoms for an edge dislocation and along the center of the spiral of a screw dislocation.