Transformations - Phase Diagrams Flashcards
What is a phase diagram?
A tool to understand the state of a material at a given temperature and composition
What is a phase?
A homogeneous portion of a system that has uniform physical and chemical characteristics
What variables affect which phases are present in a given material?
Composition, temperature and pressure
What is an unary system?
A system in which only one component is present
What is a phase boundary on a phase diagram?
A line denoting the temperature/composition at which a material which transition to another phase
What is the region bounded by a phase boundary called?
A phase field
What is the Gibbs phase rule?
P + F = C + N P: Number of phases present F: DOF C: Number of elements/compounds (components) N: Number of parameters(T, p)
What do phase diagrams show?
What phase(s) should be present if the system is at equilibrium
What is an isomorphous phase?
A phase in which components are fully dissolved in one another
What does a cooling curve show?
A cooling curve shows temperature against time
How do the cooling curves of pure metals differ from those of alloys?
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.
What is a liquidus?
A phase boundary between a mush and a liquid
What is a solidus?
A phase boundary between a mush and a solid
Why does dissolving nickel in copper create a stronger material than copper alone?
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)
How can a phase diagram be used to determine the composition of constituent phases of a phase at a given temperature and %wt?
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
How is it possible to determine the fractions of each phase in a two-phase field?
Use the lever rule - the fraction of one phase is given by the fraction of the lever on the other side of the pivot
When describing solidification of a two-phase field, what characteristics should you focus on?
Transition temperature
Composition of phases
Change in phase fraction with temperature
What is a solvus?
A phase boundary separating a solid single phase field (e.g. α) from a mixed phase field (α + β)
What is the general form of a eutectic reaction?
α + β ⇌ L
What is a eutectic reaction?
The transformation of a liquid into two solids
What is the common feature of the triple point of water and the eutectic point of steel?
Both are invariant points - it is not possible to alter any state parameter without changing what phases are present
What can be said of the solidus through a eutectic point, and the phases at either end of this line?
It is a straight horizontal line, with the phases present in the eutectic at either end
Why is the eutectic point frequently exploited in the production of casting alloys?
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)
What is precipitation?
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.
Why does a eutectic form a lamelar structure upon solidification?
Eutectic solidification requires the formation of two phases simultaneously - for this to happen, extensive solute diffusion must occur. A lamelar structure enhances solute diffusion.
How can the phase fractions in a eutectic be determined?
Use the lever rule along the invariant line
What is the term for an alloy above eutectic composition?
Hypereutectic - the lamelar eutectic microconstituent exists, however it is supplemented by a primary phase
What is the term for an alloy below eutectic composition?
Hypoeutectic
How would you determine the fraction of a phase present at hypoeutectic composition?
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
Why are casting alloys the shit?
> 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
How are compounds of a strictly defined stoichiometry shown on a phase diagram?
They are represented by a vertical line
What is a peritectic reaction?
The transformation of a solid and a liquid to a new solid phase:
L + α ⇌ β
What is brass known for?
Having a low coefficient of friction, being quite corrosion resistant, and does not produce sparks when struck
What is a eutectoid?
A solid form of eutectic:
α ⇌ β + γ
What does polymorphic?
A change in structure without a change in composition
At what temperature is iron a liquid?
> 1538°C
Through what type of reaction does δ-ferrite form?
Via a peritectic reaction - by transforming from a liquid to a liquid and a solid to a new solid phase
At what temperature must steel be heated to form austenite?
727 degrees centigrade
Why is room-temperature steel heated to its austenite phase for forging?
Because austenite is ductile
What is unit cell of austenite?
Face-centered cubic
What are the material properties of ferrite-α?
Good ductility, modest strength, high toughness, ferromagnetism
What is cementite?
Iron-carbide
What are the material properties of cementite?
Very hard, very brittle
Why does carbon make iron stronger?
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
At what wt% carbon and temperature is the iron-carbon system eutectoid?
0.76 wt%, 727 degrees centigrade
Describe the room-temperature structure of steel at 0.76 wt% C
Pearlitic - a lamelar structure of interleaving layers of alpha-ferrite and cementite
What is a microconstituent?
A part of a material’s microstructure that has an identifiable structure, possibly consisting of more than one phase
Give two examples of hypoeutectoid alloys
Mil and medium carbon steel
Where does primary ferrite start to form as a hypoeutectoid steel alloy cool?
At grain boundaries
What applications are hypereutectoid steels used for?
Tool steels, surgical instruments - anything where hardness is desirable and brittle failure is not a major issue
