Chapter 4 - Imperfections

Question 1

What are the three steps of the Solidification process of molten metal?

Answer 1

1. Nuclei develop the solid phase form
2. Crystals grow until their boundaries each other
3. crystals touch, and become grains.

Question 2

Is there such a thing as a perfect crystal?

Answer 2

No

Question 3

What are the two types of grains, What makes them unique?

Answer 3

equiaxed → roughly the same dimension in all directions
Columnar → grains elongated in one direction

Question 4

What usually creates Columnar grains?

Answer 4

slow cooling

Question 5

What creates equiaxed grains?

Answer 5

rapid cooling

Question 6

What is the difference between a high-angle grain boundary, and a small-angle grain boundary?

Answer 6

High angle: large angle of misalignment (greater than a couple of degrees)

Small angle: small angle of misalighnment

Question 7

What are the types of imperfections in Solids?

Answer 7

Point defects → Vacancy atoms, Intersitital atoms, Substitutional atoms, Self-interstitial atoms

Line defects → Dislocations

Area Defects → Grain Boundaries, Surfaces

Question 8

What are vacancy point defects?

Answer 8

Vacant atomic sites in a structure.

Question 9

What are self-interstitial point defects?

Answer 9

An additional atom that puts itself between atomic sites.

Question 10

What is this equation
𝑁𝑣 = 𝑁𝑒 [− 𝑄𝑣 /
(𝑘𝑇)]
do? What do the variables inside it represent?

Answer 10

Finds the number of defects.

𝑁𝑣 = # of defects
N = # of potential defect sites
𝑄𝑣 = Activation energy
𝑘 = Boltzmann’s constant (given in the formula sheet)
𝑇 = Temperature

Question 11

Are pure metals consisting of only ONE type of atom possible?

Answer 11

NOPE

Question 12

What are the three benifits of Alloying metals?

Answer 12

they can become more corrosive resistant, harder, and stronger.

Question 13

what is a solid solution?

Answer 13

when solute atoms are added to the host material, the crystal structure is maintained, and no new structures are formed.

Question 14

What are the two cases for when impurity atoms are added to a metal to create a solid solution?

Answer 14

Substitutional solid soln. → Where atoms around the same size take some of the host atoms’ places in unit cells
Interstitial solid soln. → a smaller atom fits within the void space of an atom.

Question 15

What are the conditions for solid solutions for substitutional Impurity? (W. Hume – Rothery rule)

Answer 15

1. The atomic radius must be within 15%
2. They must have similar electronegativities
3. Same crystal structure for pure metals
4. Similar Valency

Question 16

For substitutional Impurity, if all else is equal between 2 pure atoms, which one is more likely to dissolve?

Answer 16

If both valencies are the same distance apart, the one with the higher valency is more likely to dissolve.

Question 17

For the crystalline structure, BCC give the Coordination #, # of sites, and # of equivalent interstitial atoms in the unit cell for both Tetrahedral and Octahedral sites.

Answer 17

Tetrahedral → coordination number 4, 24 sites, and 12 equivalent tetrahedral atoms in the cell

Octahedral → coordination number 6, 18 sites, and 6 equivalent tetrahedral atoms in the cell

Question 18

For the crystalline structure, FCC give the Coordination #, # of sites, and # of equivalent interstitial atoms in the unit cell for both Tetrahedral and Octahedral sites.

Answer 18

Tetrahedral → coordination number 4, 8 sites, and 8 equivalent tetrahedral atoms in the cell

Octahedral → coordination number 6, 13 sites, and 4 equivalent tetrahedral atoms in the cell

Question 19

Place the interstitial sites in both FCC, and BCC atoms

Answer 19

Did you do it?

Question 20

In metallic materials is the concentration of interstitial impurity atoms high or low? why?

Answer 20

low because of the extremely high APF

Question 21

For ceramic materials what type of point defects can occur?

Answer 21

Because of the polarity of the ions and medium APF all types of point defects can occur. (vacancies, interstitials, and subsitiutial defects) In addition different defects can happen with the different ions as they have different sizes and valencies.

Question 22

For Ashby Maps, Rank the different types of compounds from low Young’s modulus (10^-3) and Density (0.1) to high (1000, and 12 respectively)

Answer 22

Polymers, metals, then ceramics (but ceramics and metals have same density, just different young’s modulus)

Question 23

What does this formula do?
𝐶1 = 𝑚1 / (𝑚1 + 𝑚2) 𝑋100

Answer 23

Calculates the wieght percent of a unit cell.

Where m is the wieght of the atom.

Question 24

What does 𝐶1′ = 𝑛𝑚1 / (𝑛𝑚1 + 𝑛𝑚2) 𝑋100 Do?

Answer 24

Calculates the atom percent of a unit cell.

where nm1 = number of moles of component

Question 25

What does this formula do?
𝜌𝑎𝑣𝑒 = 100 / [(c1 / p1) + (c2 / p2)]

Answer 25

Calculates the average density of unit cell

Question 26

What does this formula do?
A𝑎𝑣𝑒 = 100 / [(c1 / A1) + (c2 / A2)]

Answer 26

Calculates the average atomic weight of solid solution

Question 27

What are Dislocations?

Answer 27

Are one-dimensional defects around which atoms are misaligned

Question 28

What are edge dislocations?

Answer 28

An extra half-plane of atoms inserted in a crystal structure

Question 29

What is a screw dislocation?

Answer 29

A spiral planar ramp resulting from shear deformation

The lattice plane shifts by one layer or more

Question 30

What is burgers vector (b) facing in edge and screw dislocations

Answer 30

Edge dislocation = perpendicular line vector
Screw dislocation = parallel to line vector

Question 31

What are Phase boundaries?

Answer 31

The junction of two crystals which differ in lattice structure and composition.

Question 32

What are Twin boundaries?

Answer 32

one side of the crystal plane is positioned as a reflected version across a Twin plane

Question 33

What are stacking faults?

Answer 33

The sequence ABCABC becomes ABCABABC

Question 34

What are Dislocations? Give an example.

Answer 34

movement of a material when stress is applied

Permanent (plastic) deformation.