Imperfections in crystals

Question 1

What is the process of solidification? (4)

Answer 1

• As material cools down, small crystallites appear out of molten material and act as nuclei sights of growth.
• Particular orientation with respect to each other
• As cooling proceeds the crystals get bigger and bigger, as thermal energy drops they can grow
• As they solidify it becomes grains as they grow they start to touch and random angles

Question 2

What types of defects arise in solids?

Answer 2

?

Question 3

Can the number and type of defects be varied

and controlled?

Answer 3

?

Question 4

How do defects effect material properties?

Answer 4

?

Question 5

Are defects undesirable?

Answer 5

?

Question 6

Why can diffusion easily occur in grain boundaries?

Answer 6

Since bonds are not at maximum strength they are easily broken, therefore diffusion can occur easily

Question 7

Why is the chemical reactivity high in grain boundaries?

Answer 7

As it’ll oxidise around the grain boundaries before the inner parts due to the bonds not being strong and there being gaps

Question 8

What is the weak point in polycrystalline materials?

Answer 8

the high-angle grain boundary

Question 9

What shape can grains become? (2)

Answer 9

equiaxed (roughly same size in all directions)

columnar (elongated grains)

Question 10

How do columnar grains occur in solidification?

Answer 10

in areas with less undercooling

Question 11

How do equiaxed grains occur in solidification?

Answer 11

shell of equiaxed grains due to rapid cooling (greater ∆T) near wall

Question 12

Which direction to the grains follow?

Answer 12

They grow in the direction of the heat flow

Question 13

What is grain refiner? (2)

Answer 13

• added to make more smaller, more uniform, equiaxed grains when they meet
• you can get more nuclei per unit volume

Question 14

What are characteristics of zone melting? (4)

Answer 14

• only a small part of the charge is molten
• material is added to molten region
• molten zone is advanced by moving the charge or the gradient
• axial temperature gradient is imposed along the crucible

Question 15

Why are imperfections sometimes bad?

Answer 15

as they can cause material failure such as metal fatigue which arises from the growth of cracks

Question 16

Why are imperfections sometimes good?

Answer 16

some imperfection can be used to stop cracks from growing eg. carbon impurities are put into iron to make steel, this strengthens the material by stopping the growth of cracks

Question 17

Why are imperfections sometimes good?

Answer 17

some imperfection can be used to stop cracks from growing eg. carbon impurities are put into iron to make steel, this strengthens the material by stopping the growth of cracks
or putting in crystallites

Question 18

Where does metal fatigue occur usually?

Answer 18

structures affected by waves and subject to a lot of pressure and decompression. Causing cracks to join up and meet up

Question 19

what are three types of defects?

Answer 19

point defects
line defects
area defects

Question 20

What are point defects?

Answer 20

one atom is out of place

Question 21

What are three types of point defects from atoms?

Answer 21

Vacancy atoms
interstitial atoms
substitutional atoms

Question 22

What are vacancy atoms?

Answer 22

an atom is missing

Question 23

What are interstitial atoms?

Answer 23

atoms not sitting on the right spot and actually are in between lattice sides

Question 24

What are substitutional atoms?

Answer 24

wrong atom in there

Question 25

What do vacancy atoms cause? (2)

Answer 25

a distortion of planes

and helps diffusion go more quickly

Question 26

What do interstitial atoms cause?

Answer 26

distortion of planes

Question 27

How does exponential dependence occur in a graph with defect concentration against temperature?

Answer 27

as we increase the temperature, then the number of vacancies will start to increase.
Like melting , before it becomes liquid It’ll have a lot of vacancies as we’ve given atoms enough energy to move out of the minimum energy configuration through the thermal energy applied

Question 28

What are the two outcomes if impurity B added to host A ?

Answer 28

Solid solution of B in A, creates random dist. of point defect
solid solution of B in A plus particles of a new phase (usually for a larger amount of B)

Question 29

What are two types of point defects in alloys?

Answer 29

substitutional solid solution

interstitial solid solution

Question 30

Give an example of what substitutional atoms do in alloys.

Answer 30

Copper replaces nickel atoms, copper atoms are smaller atoms than nickel

Question 31

Give an example of what interstitial atoms do in alloys.

Answer 31

Random distribution of carbon in Fe.

Copper is much more smaller than Fe and fits into the gaps between Fe atoms

Question 32

What is the new phase when a solid solution of B is added into A?

Answer 32

different composition, often different structure

Question 33

What is the new phase when a solid solution of B is added into A?

Answer 33

different composition, often different structure such as columns of atoms

Question 34

Why does increasing temperature cause surface island of atoms to grow?

Answer 34

the equilibrium vacancy concentration increases via atom motion from the crystal to the surface where they join the island.
The island grows/shrinks to maintain equilibrium vacancy concentration in the bulk

Question 35

What are 4 conditions for substitutional solid solution (rough indication of what alloys go together)?

Answer 35

1. ∆r (atomic radius) < 15%
2. Proximity in periodic table
   • i.e., similar electronegativities
3. Same crystal structure for pure metals
4. Valency
   • All else being equal, a metal will have a greater tendency
   to dissolve a metal of higher valency than one of lower
   valency

Question 36

What are dislocations in line defects? (3)

Answer 36

• one dimensional defects, a line of point defects creating misalignment
• slip between crystal planes result when dislocations move
• produces permanent (plastic deformation

Question 37

How are dislocations caused?

Answer 37

after tensile elongation pulling it apart creates slip steps

Question 38

What are two types of dislocations?

Answer 38

edge dislocation and screw dislocation

Question 39

What is edge dislocation?

Answer 39

extra half-plane of atoms inserted in a crystal structure, creaintg an extra step
b is perpendicular to dislocation line

Question 40

What is burger’s vector, b?

Answer 40

measure of direction and magnitude of lattice distortion

Question 41

How does edge dislocation occur?

Answer 41

requires successive bumping of a half plane of atoms.

bonds across the slipping planes are broken and remade in succession, creating a new line

Question 42

What is screw dislocation?

Answer 42

spiral planar ramp resulting from shear deformation

b is parallel to dislocation line

Question 43

How do dislocations effect the electrical properties of solids?

Answer 43

Can affect electrical properties as if electricity hits an atom out of place it can scatter, if there are a lot of dislocations atoms will scatter a lot.

Question 44

What are area defects?

Answer 44

crystallites are 3D and surface area is the grain boundaries

Question 45

What are the directions and planes among crystal structures (FCC, HCP, BCC)?

Answer 45

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

Question 46

What are planar defects in solids?

Answer 46

one case is a twin boundary (plane), essentially a reflect of atoms positions across the twin plane, making it weak

plane is split in half and one is almost perpendicular to the other