Transformations - Kinetics Flashcards
What equation gives the rate of diffusion?
D = D_0 exp(-Q/RT)
What is a metastable phase?
A non-equilibrium phase that only exists because the rate of diffusion is low
Define recrystallisation
The generation of a new grain structure at high temperature drive by a reduction in dislocation density
What is recrystallisation often used for?
Softening work-hardened alloys; can also be used to improve strength and ductility if small grains produced
Describe and epxlain a plot of fraction recrystallised against the logarithm of time for a recrystallisation process
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.
What is the Avrami equation and what does it describe?
It describes the fraction of new grains formed as a function of time: X = 1 - exp(-kt^n)
What is recrystallisation time?
The time needed to accomplish some arbitrary fraction of recrystallisation (typically 50%) at a given temperature
What equation describes recrystallisation time at a given temperature?
The Arrhenius equation: Aexp(Q/RT)
How recrystallisation time and rate related?
They reciprocals of one another
What is recrystallisation temperature?
The temperature required to achieve some fraction of recrystallisation in some arbitrary time
What factors influence recrystallisation temperature?
Degree of work hardening
Solute content
Particles (i.e. from precipitate strengthening)
What is undercooling?
Bringing a material’s temperature beneath its solidification temperature
Why is undercooling important for phase transformations, if the rate of diffusion is lower for lower temperatures?
Undercooling provides the driving force for the transformation - the maximum transformation rate is typically seen at some compromise between diffusion rate and subcooling
Why are large grains undesirable?
They have a produce a lower grain boundary surface area (reducing strength) and cracks can follow their boundaries (prone to fracture)
What does time-temperature transformation plot depict?
Fraction of material transformed on temperature-against-time axes
What is dispersion strengthening?
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.
What does age hardening require in terms of component solubility?
It requires that the solute is fully soluble at high temperatures but comes out of the solution at lower temperatures
What are the three steps required by age hardening?
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
What is quench sensitivity?
The propensity of an alloy to precipitate its solute
Give three microconstituents of the iron-carbon system
Pearlite, bainite, martensite
What is the reaction mechanism of the formation of martensite?
Diffusionless transformation
What is another term for a metastable phase?
“Microconstituent”
What is the diffusing species in the formation of bainite?
Carbon (NOT iron)
At what temperatures does finer pearlite form? Why?
Finer pearlite forms at lower temperatures because the diffusion distance is smaller - this manifests itself as higher strength
Describe the formation of martensite
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
Why is bainite strong?
Because its plate-like structure has many boundaries
Why is bainite tough?
It has an intricate microstructure that deflects cracks
Why is the bainite bay lower than the pearlite bay on the time-temperature transformation diagram for steel?
Because the formation of bainite requires a larger driving force than the formation of pearlite
Describe a typical production process for a structural steel
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.
What characterizes a high-alloy steel?
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.
Which elements give stainless steel its corrosion-resistance?
Chromium and nickel - they form an oxide on the outside of the steel, as well as providing strength via solute strengthening
What is patented steel wire?
Fine pearlitic steel that has been drawn into a wire
What is the criticl cooling rate?
The rate at which cooling any faster will form martensite
