Phase Diagrams Flashcards
Define a phase
All parts of a material with the same atomic structure are a single phase
Define Solid Solution
A solid solution is a solid in which one or more elements dissolved such that they are homogeneously dispersed.
What is the constitution of an alloy
The constitution is described by:
- Phases present
- Weight/Atomic fraction of each phase
- Composition of each phase
Define the Gibbs Free Energy Function
G = U + pV - TS = H - TS
Define the equilibrium constitution
The equilibrium constitution is the state of lowest Gibbs free energy for a given composition at given temperature and pressure.
Define a Phase Diagram
A phase diagram (also known as an equilibrium diagram) is a diagram with the Temperature and Composition as axes. Remember that since we are slow cooling that the overall alloy composition never changes only the phases and their relative proportions (as governed by the lever rule)
Define the Liquidus and Solidus Phase Boundaries
The phase boundary that limits the bottom of the single-phase liquid field is the liquidus boundary and the phase boundary that limits the upper bound of the single phase solid field is the solidus boundary.
Describe the key features of the cooling curve for an alloy
For pure metals the cooling cover contains arrest points where the pure metal changes states. These arrest points correspond to the release of a latent heat. However, for an alloy solidification occurs over a range of temperatures (as it is a mixture) so there are no arrest points but the cooling rate slows as the alloy passes through phase boundaries as latent heat is being released.
What is unique about the Cu-Ni Phase Diagram
Cu-Ni can form a single phase solid solution at any composition at low temperature so there are no solvus boundaries
What phases are present either side of the solvus boundaries.
2 phases are present. Either side of true solvus boundaries will be single phase regions e.g Pb-rich and Sn-rich solids.
What is the eutectic point
The lower limit of the single phase liquid region formed by the intersection of two falling liquidus lines is known as the eutectic point. The eutectic point also gives the lowest temperature at which the 100% liquid is stable.
What is the phase constitution in single phase region
At a constitution point in a single phase region the phase composition is the composition of the alloy itself
In a two phase region how do we determine the phase composition
In a two phase region the phase compositions lie either side of a horizontal line that passes through the constitution point.
Define the Lever Rule
If the x-axis is in wt% we measure the the length of the tie line and the relative proportions of the phases is given by the length of each section divided by the total length of the tie line.
If the x-axis scale is in at% then we cannot use the lengths of the segments and instead we find the difference in at% and calculate the same ratios: a/l and b/l
Explain the origin of Intermediate Compounds
Thermodynamically, compounds form because the the particular combination of components is able to form as a single phase with a lower free energy than a mixture of two phases. These compounds have higher melting points as they are thermodynamically more stable.
Name some examples of uses of iron-carbon alloys
- Cast iron brake discs
- I-Beams
- Car Bodies
Name the three phases of Iron and at what temperature they are found
- Ferrite (alpha) is BCC is the low temperature form
- Austentite (gamma) is FCC is the medium temperature form
- Delta is BCC is the high temperature form
Outline the key features of the cooling curve for Iron
The cooling curve will feature arrests due to the release of latent heat as it undergoes phase transformations from delta to gamma and finally to alpha. The arrests on the cooling curve are much smaller than that seen for changes of state since the latent heat release is much smaller
How does the solubility of carbon in Iron vary with the microstructure of the Iron
In FCC the interstitial holes are larger than BCC leading to higher solubility of Carbon. Therefore the size of the interstitial spaces determines the extent to which iron dissolves carbon to form a single phase.
What is the composition of Cementite
Cementite is Iron Carbide (Fe3C). It can be seen a thin vertical strip on the right side of the Iron-Carbon phase diagram above 7 wt% C
What is a eutectoid point
The lower limit of a single phase solid field formed by the two falling phase boundaries intersecting in a V shape is called the eutectoid point
What is a Peritectic point
An inverted V meeting a horizontal line giving the maximum temperature at which the enclosed liquid phase(s) form.
Why is Iron Carbide metastable
The Iron Carbon diagram is not strictly an equilibrium diagram. Iron Carbide (Cementite) is in a metastable state. Thermodynamically, a true stable state would be a two phase mixture of Iron and Carbon (like Pb-Sn). However, in most cases Iron Carbide forms in preference to Carbon showing that the mechanism of the phase transformation takes priority over free energy minimisation.
Why do phase transformations take place
When temperature varies in a process the equilibrium condition of the material keeps changing. These phase transformations determine what phases are present and therefore the final microstructure and material properties.
Outline the mechanism of solidification of pure metals
- Homogenous Nucleation - the solid colonies spontaneously form and grow stable provided they reach a critical radius. Heterogeneous nucleation facilitates the process with solid nuclei forming more readily on the pre-existing solid.
- Each region grows by atoms transferring at the solid-liquid interface
- Growth continues until impingement occurs - which is a misfit in the atomic packing
- The individual crystallise remain identifiable by their grain boundaries.
Define Eutectic Solidification
A eutectic reaction is a three phase reaction by which on cooling a liquid transforms into two solid phases at a constant temperature
What is the microstructure of eutectic solidification
The two metals inter diffuse to generate solid regions in which each dominates. This occurs over small diffusion distances resulting in an intimate mixture of the two phases.
Why are the shape of the phases in a eutectic solid plate/needle-like
- Minimise diffusion distance resulting in an intimate mixture of the two phases
- Reduced surface energy allowing for faster growth
Define off-eutectic solidification
Off-eutectic solidification occurs when the composition of the alloy being cooled is not equal to the eutectic composition.
Describe the micro-structural changes that occur when slow cooling off-eutectic solids (use Pb-Sn as example)
- Start as a single phase liquid
- As the liquidus line is reached nuclei of Pb-rich solid appear
- This nucleation of rich solid in the liquid phase ends at the eutectic temperature
- At the eutectic temperature the remaining liquid undergoes eutectic solidification. (Lamellar Structure)
- As the alloy is cooled to room temperature, the Sn-rich solid precipitates out.
How can we visually distinguish off eutectic solidification
Off eutectic solidification results in large plate like structures. These structures are the remnants of the rich solid nucleating after passing through the liquid phase.
Define the eutectoid reaction
A eutectoid reaction is a three phase reaction by which, on cooling, a single phase solid transforms into two different solid phases at constant temperature.
