Transformations - Kinetics

Question 1

What equation gives the rate of diffusion?

Answer 1

D = D_0 exp(-Q/RT)

Question 2

What is a metastable phase?

Answer 2

A non-equilibrium phase that only exists because the rate of diffusion is low

Question 3

Define recrystallisation

Answer 3

The generation of a new grain structure at high temperature drive by a reduction in dislocation density

Question 4

What is recrystallisation often used for?

Answer 4

Softening work-hardened alloys; can also be used to improve strength and ductility if small grains produced

Question 5

Describe and epxlain a plot of fraction recrystallised against the logarithm of time for a recrystallisation process

Answer 5

A sigmoid graph - initially there are few new grains, and these are very small. As the process continues, these grains grow and consume old grains more quickly - additionally, new grains are continuing to nucleate. Eventually the new grains run out of old grains and the rate of recrystallisation decreases; however, grain size continues to grow.

Question 6

What is the Avrami equation and what does it describe?

Answer 6

It describes the fraction of new grains formed as a function of time: X = 1 - exp(-kt^n)

Question 7

What is recrystallisation time?

Answer 7

The time needed to accomplish some arbitrary fraction of recrystallisation (typically 50%) at a given temperature

Question 8

What equation describes recrystallisation time at a given temperature?

Answer 8

The Arrhenius equation: Aexp(Q/RT)

Question 9

How recrystallisation time and rate related?

Answer 9

They reciprocals of one another

Question 10

What is recrystallisation temperature?

Answer 10

The temperature required to achieve some fraction of recrystallisation in some arbitrary time

Question 11

What factors influence recrystallisation temperature?

Answer 11

Degree of work hardening
Solute content
Particles (i.e. from precipitate strengthening)

Question 12

What is undercooling?

Answer 12

Bringing a material’s temperature beneath its solidification temperature

Question 13

Why is undercooling important for phase transformations, if the rate of diffusion is lower for lower temperatures?

Answer 13

Undercooling provides the driving force for the transformation - the maximum transformation rate is typically seen at some compromise between diffusion rate and subcooling

Question 14

Why are large grains undesirable?

Answer 14

They have a produce a lower grain boundary surface area (reducing strength) and cracks can follow their boundaries (prone to fracture)

Question 15

What does time-temperature transformation plot depict?

Answer 15

Fraction of material transformed on temperature-against-time axes

Question 16

What is dispersion strengthening?

Answer 16

Also known as precipitate or age hardening, dispersion strengthening is a heat treatment technique used to increase the yield strength of material. It relies on changes in a solid solubility with temperature to precipitate fine particles that impede the movement of dislocations.

Question 17

What does age hardening require in terms of component solubility?

Answer 17

It requires that the solute is fully soluble at high temperatures but comes out of the solution at lower temperatures

Question 18

What are the three steps required by age hardening?

Answer 18

Solution treatment - heat the metal in the single phase field and dissolve the solute
Quench - cooling the metal quickly prevents the solute diffusing to form large particles (smaller particles have a greater strengthening effect) and traps the solute
Heat treatment - reheat the super saturated solid solution to some modest temperature where diffusion can occur: the solute then diffuses slowly to form many small precipitates

Question 19

What is quench sensitivity?

Answer 19

The propensity of an alloy to precipitate its solute

Question 20

Give three microconstituents of the iron-carbon system

Answer 20

Pearlite, bainite, martensite

Question 21

What is the reaction mechanism of the formation of martensite?

Answer 21

Diffusionless transformation

Question 22

What is another term for a metastable phase?

Answer 22

“Microconstituent”

Question 23

What is the diffusing species in the formation of bainite?

Answer 23

Carbon (NOT iron)

Question 24

At what temperatures does finer pearlite form? Why?

Answer 24

Finer pearlite forms at lower temperatures because the diffusion distance is smaller - this manifests itself as higher strength

Question 25

Describe the formation of martensite

Answer 25

Diffusionless - the driving force causes the FCC cell to be strained in shear towards a BCC cell; the carbon does not diffuse out of the structure, therefore martensite is a super saturated solid solution

Question 26

Why is bainite strong?

Answer 26

Because its plate-like structure has many boundaries

Question 27

Why is bainite tough?

Answer 27

It has an intricate microstructure that deflects cracks

Question 28

Why is the bainite bay lower than the pearlite bay on the time-temperature transformation diagram for steel?

Answer 28

Because the formation of bainite requires a larger driving force than the formation of pearlite

Question 29

Describe a typical production process for a structural steel

Answer 29

Quench a 0.35% C steel to form martensite, then heat treat it to allow some of the trapped carbon to form carbides. Heat-treating relieves some of the strain on the crystal lattice, making the steel tougher.

Question 30

What characterizes a high-alloy steel?

Answer 30

High concentrations of other elements e.g. Ni, Mo, V and Mn. These elements are expensive, but create a steel that is strong and tough.

Question 31

Which elements give stainless steel its corrosion-resistance?

Answer 31

Chromium and nickel - they form an oxide on the outside of the steel, as well as providing strength via solute strengthening

Question 32

What is patented steel wire?

Answer 32

Fine pearlitic steel that has been drawn into a wire

Question 33

What is the criticl cooling rate?

Answer 33

The rate at which cooling any faster will form martensite