Transformations - Phase Diagrams

Question 1

What is a phase diagram?

Answer 1

A tool to understand the state of a material at a given temperature and composition

Question 2

What is a phase?

Answer 2

A homogeneous portion of a system that has uniform physical and chemical characteristics

Question 3

What variables affect which phases are present in a given material?

Answer 3

Composition, temperature and pressure

Question 4

What is an unary system?

Answer 4

A system in which only one component is present

Question 5

What is a phase boundary on a phase diagram?

Answer 5

A line denoting the temperature/composition at which a material which transition to another phase

Question 6

What is the region bounded by a phase boundary called?

Answer 6

A phase field

Question 7

What is the Gibbs phase rule?

Answer 7

P + F = C + N
P: Number of phases present
F: DOF
C: Number of elements/compounds (components)
N: Number of parameters(T, p)

Question 8

What do phase diagrams show?

Answer 8

What phase(s) should be present if the system is at equilibrium

Question 9

What is an isomorphous phase?

Answer 9

A phase in which components are fully dissolved in one another

Question 10

What does a cooling curve show?

Answer 10

A cooling curve shows temperature against time

Question 11

How do the cooling curves of pure metals differ from those of alloys?

Answer 11

A pure metal does not change temperature whilst changing phase - energy is released (in the case of solidification) as heat. By contrast, an alloy’s temperature can continue to decrease during solidification.

Question 12

What is a liquidus?

Answer 12

A phase boundary between a mush and a liquid

Question 13

What is a solidus?

Answer 13

A phase boundary between a mush and a solid

Question 14

Why does dissolving nickel in copper create a stronger material than copper alone?

Answer 14

Nickel has a different atomic radius to copper, which creates strains in the atomic lattice that restrict dislocation movement; movement of dislocations is essential to plastic flow (and therefore yield)

Question 15

How can a phase diagram be used to determine the composition of constituent phases of a phase at a given temperature and %wt?

Answer 15

Draw a horizontal tie line through the point - the compositions where this line intersects the adjacent phase fields are the compositions of the constituent phases

Question 16

How is it possible to determine the fractions of each phase in a two-phase field?

Answer 16

Use the lever rule - the fraction of one phase is given by the fraction of the lever on the other side of the pivot

Question 17

When describing solidification of a two-phase field, what characteristics should you focus on?

Answer 17

Transition temperature
Composition of phases
Change in phase fraction with temperature

Question 18

What is a solvus?

Answer 18

A phase boundary separating a solid single phase field (e.g. α) from a mixed phase field (α + β)

Question 19

What is the general form of a eutectic reaction?

Answer 19

α + β ⇌ L

Question 20

What is a eutectic reaction?

Answer 20

The transformation of a liquid into two solids

Question 21

What is the common feature of the triple point of water and the eutectic point of steel?

Answer 21

Both are invariant points - it is not possible to alter any state parameter without changing what phases are present

Question 22

What can be said of the solidus through a eutectic point, and the phases at either end of this line?

Answer 22

It is a straight horizontal line, with the phases present in the eutectic at either end

Question 23

Why is the eutectic point frequently exploited in the production of casting alloys?

Answer 23

The eutectic temperature is lower than the melting points of pure metals, making eutectic compositions preferable for casting alloys as they will then require lower temperatures to cast (and therefore be cheaper to cast)

Question 24

What is precipitation?

Answer 24

The separation of a single phase (e.g. α) into two phases (e.g. α and β). Typically occurs during cooling - the single phases ejects excess solute, which forms the other phase.

Question 25

Why does a eutectic form a lamelar structure upon solidification?

Answer 25

Eutectic solidification requires the formation of two phases simultaneously - for this to happen, extensive solute diffusion must occur. A lamelar structure enhances solute diffusion.

Question 26

How can the phase fractions in a eutectic be determined?

Answer 26

Use the lever rule along the invariant line

Question 27

What is the term for an alloy above eutectic composition?

Answer 27

Hypereutectic - the lamelar eutectic microconstituent exists, however it is supplemented by a primary phase

Question 28

What is the term for an alloy below eutectic composition?

Answer 28

Hypoeutectic

Question 29

How would you determine the fraction of a phase present at hypoeutectic composition?

Answer 29

Apply the lever rule across between the primary phase field and the a vertical line through the eutectic - the phase fraction present will be equivalent to that given by the the opposite lever fraction summed with the product of the fraction of the phase present in the eutectic and the fraction that the eutectic constitutes overall

Question 30

Why are casting alloys the shit?

Answer 30

> Less heat is needed to melt the metal and casting is cheaper
Heat losses to the mold and surroundings are lower due a smaller temperature gradient

Question 31

How are compounds of a strictly defined stoichiometry shown on a phase diagram?

Answer 31

They are represented by a vertical line

Question 32

What is a peritectic reaction?

Answer 32

The transformation of a solid and a liquid to a new solid phase:
L + α ⇌ β

Question 33

What is brass known for?

Answer 33

Having a low coefficient of friction, being quite corrosion resistant, and does not produce sparks when struck

Question 34

What is a eutectoid?

Answer 34

A solid form of eutectic:

α ⇌ β + γ

Question 35

What does polymorphic?

Answer 35

A change in structure without a change in composition

Question 36

At what temperature is iron a liquid?

Answer 36

> 1538°C

Question 37

Through what type of reaction does δ-ferrite form?

Answer 37

Via a peritectic reaction - by transforming from a liquid to a liquid and a solid to a new solid phase

Question 38

At what temperature must steel be heated to form austenite?

Answer 38

727 degrees centigrade

Question 39

Why is room-temperature steel heated to its austenite phase for forging?

Answer 39

Because austenite is ductile

Question 40

What is unit cell of austenite?

Answer 40

Face-centered cubic

Question 41

What are the material properties of ferrite-α?

Answer 41

Good ductility, modest strength, high toughness, ferromagnetism

Question 42

What is cementite?

Answer 42

Iron-carbide

Question 43

What are the material properties of cementite?

Answer 43

Very hard, very brittle

Question 44

Why does carbon make iron stronger?

Answer 44

Solid solution strengthening - carbon atoms dissolve in the iron and take up positions between iron atoms (interstitial) - this distorts the crystal lattice, which impedes dislocation movement

Question 45

At what wt% carbon and temperature is the iron-carbon system eutectoid?

Answer 45

0.76 wt%, 727 degrees centigrade

Question 46

Describe the room-temperature structure of steel at 0.76 wt% C

Answer 46

Pearlitic - a lamelar structure of interleaving layers of alpha-ferrite and cementite

Question 47

What is a microconstituent?

Answer 47

A part of a material’s microstructure that has an identifiable structure, possibly consisting of more than one phase

Question 48

Give two examples of hypoeutectoid alloys

Answer 48

Mil and medium carbon steel

Question 49

Where does primary ferrite start to form as a hypoeutectoid steel alloy cool?

Answer 49

At grain boundaries

Question 50

What applications are hypereutectoid steels used for?

Answer 50

Tool steels, surgical instruments - anything where hardness is desirable and brittle failure is not a major issue