Defects

Question 1

What are the four classification of defects? And what do they consist of?

Answer 1

1. Point defects (0D)
   - Vacancy atoms
   - Interstitial atoms
   - Substitutional atoms
2. Linear defects (1D)
   - Dislocations
3. Planar defects (2D)
   - Grain boundaries
   - Stacking faults
4. Bulk defects (3D)
   - Pores
   - Inclusions
   - Cracks

Question 2

Describe what happens when dislocations move and name the two types of dislocation.

Answer 2

Slip between crystal planes result when dislocations move and plastic deformation is produced.

Edge and screw dislocations

Question 3

What are the strengthening mechanisms of metals?

Answer 3

Decreasing grain size

Solid solution strengthening

Precipitation hardening

Strain hardening (cold work)

Question 4

What is the impact of cold work on mechanical properties?

Answer 4

Yield strength increases

Tensile strength increases

Ductility decreases

Modulus (E) remains unchanged

Question 5

What is the formula for the number of vacancies in a volume?

Answer 5

Nv = Ne(-Qv/KbT)

where:

Nv - number of vacancies
N - number of atoms
e - natural exponential
Qv - activation energy required to produce vacancies
Kb - Boltzmann constant (= R/Na)
T - temperature in K

Question 6

Why are real materials not as strong as theoretical models

Answer 6

Due to the presence of defects

Question 7

What is the influence of grain size on material properties?

Answer 7

Metals that have small grains are relatively strong and tough at low temperatures.

Metals that have large grains have good creep resistance at relatively high temperatures.

Question 8

Describe the strengthening by solid solution alloying

Answer 8

Impurity atoms distort the lattice and generate lattice strains.

These strains can act as barriers to dislocation motion

Question 9

What is the reason for strain hardening

Answer 9

To increase the dislocation density with plastic deformation