Lecture 10-11

Question 1

Where is the eutectic point on a phase diagram?

Answer 1

The V that is created where the composition of the material has a lower melting point than originally

Question 2

Eutectoid reaction is

Answer 2

where one solid phase forming two other solid phases

Question 3

Solvus lines separate the

Answer 3

single phase alpha and beta regions with the two phase alpha + beta region

Question 4

Lamellae are formed when what soldifies

Answer 4

Eutectic alloys

Question 5

A lamellae is

Answer 5

grains made up of alternating layers of alpha and beta

Question 6

Where on the phase diagrams are hypoeutectic alloys found?

Answer 6

to the lower end from the eutectic point

Question 7

Where on the phase diagrams are hypereutectic alloys found?

Answer 7

to the upper end from the eutectic point

Question 8

3 step and result of microstructure evolution for a hypoeutectic alloy

Answer 8

1. Primary alpha phase with state beginning in liquidus
2. Primary alpha phase begins to solidify
3. Eutectic microstructure/lamellae layers form around the primary alpha phase

Result; alpha + beta layers surrounding the alpha solids

Question 9

Similarly to a pure substance, an alloy with eutectic composition solidifies…

Answer 9

at one temperature until it has changed state fully

Question 10

Similarly to a standard alloy, an alloy with hypereutectic or hypoeutectic composition solidifies…

What happens at the eutectic temp?

Answer 10

over a temperature range depending on composition.

at the eutectic temp the remaining liquid solidifies

Question 11

Strengthening means to increase the

Answer 11

yield and tensile strength of a metal

Question 12

Four main methods of strengthening for alloys:

Answer 12

1. Increased number of dislocations
2. Presence of grain boundaries
3. Presence of solute atoms
4. Presence of second phase particles

Question 13

Two main methods of strengthening for pure metals:

Answer 13

1. Increased number of dislocations
2. Presence of grain boundaries

Question 14

Work hardening is aka

Answer 14

cold working

Question 15

Work hardening is

Answer 15

plastically deforming a metal

Question 16

Work hardening pro and con

Answer 16

Increases strength, BUT reduces ductility

Question 17

Work hardening takes place during which processes, name 5

Answer 17

• Rolling
• Forging
• Drawing
• Extrusion
• Forming

Question 18

An elongated grain structure is called

Answer 18

anisotropic

Question 19

If a metal is stressed into the plastic region and the load is removed the —- —– is recovered, but permanent —— has occurred

Answer 19

elastic strain, deformation

Question 20

As the number of dislocations in a metal increased, the dislocations become

Answer 20

harder to slip due to neighbouring grains impeding the slip

Question 21

σ y = σ o + k y X d -1/2

What does each letter signify?

Answer 21

• σ o and k y are constants for the material
• σ y is the yield stress
• d is the avg grain diameter

Question 22

In Solid Solution Strengthening what is preferred about the property of the solute atoms? Why?

Answer 22

A large difference in the size of the atoms to disrupt the uniform layers

Question 23

Four pros and two cons for solid solution strengthening?

Answer 23

Pros:
- increase in yield strength
- increase in tensile strength
- increase in hardness
- increase in creep resistance

Cons:
- reduced ductility
- reduced electrical conductivity

Question 24

Dispersion strengthening to strengthen a material is when

Answer 24

there are second phase particles or regions in amongst the matrix which reduces the ease of slippage of dislocations

Question 25

What is an improved form of dispersion hardening?

Answer 25

Age hardening (precipitation)

Question 26

Pro about age hardening?

Answer 26

second phase particles are more highly dispersed

Question 27

Two preferred properties of the precipitate particles used in dispersion strengthening. Why for each?

Answer 27

- Precipitate particles should be small and numerous -> maximise interference with slip - Precipitate particles should be round rather than needles -> avoid stress raising

Question 28

What is not used in dispersion strengthening to make age hardening an enhanced process?

Answer 28

Heat treatment

Question 29

What is over ageing and how can it come about

Answer 29

Particles grow too large and become less effective at preventing dislocation slip Higher ageing temps and and longer times

Question 30

Why can't precipitation hardened alloys be used in high temp environments?

Answer 30

The second phase particles tend to re-dissolve or grow in size and the strengthening mechanism is lost

Question 31

Annealing is

Answer 31

the process used to increase the ductility of a metal, typically after cold working. basically heating

Question 32

Three stages of annealing are

Answer 32

- Recovery - Recrystallisation - Grain growth

Question 33

Two cons of annealing on metals:

Answer 33

- if the metal is to operate in high temperatures, the metal will anneal slowly becoming weaker and more prone to yielding - certain metal joining processes heat the local area which can cause local recrystallisation and grain growth so strength in this area is reduced drastically

Question 34

Recovery in annealing is

Answer 34

(aka stress relief anneal) the process where the original electrical and thermal properties are returned by low temp heating. It does not change the mechanical properties of the metal though

Question 35

Annealing is not a ----- process for metals.

Answer 35

strengthening

Question 36

Recovery in annealing helps the dislocations in the metal to do what?

Answer 36

Helps them arrange themselves to minimise the interactions they have other dislocations

Question 37

Annealing does not distort the

Answer 37

grain structure

Question 38

Recrystallisation is

Answer 38

the process by which a higher temperature is applied for a longer time

Question 39

Recrystallisation allows the distorted grains in the alloy to be replaced by a set of

Answer 39

strain free, smaller grains

Question 40

Recrystallisation reduces the number of ----- and the metal's ----- .

Answer 40

dislocations, strength

Question 41

4 things the recrystallisation temperature depends on:

Answer 41

- Amount of cold work already completed - Original grain size - Alloy composition and melting temp - Annealing time

Question 42

The opposite of grain size strengthening is

Answer 42

grain growth

Question 43

Annealing to a higher temperature after crystallisation is the process of

Answer 43

grain growth

Question 44

Grain growth reduces

Answer 44

the number of grain boundaries there are in the metal

Question 45

As the grain size of the metal ----, the strength of the metal -----.

Answer 45

increases, decreases