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.
