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DoITPoMS Teaching & Learning Packages Phase Diagrams and Solidification Phase Diagrams and Solidification (all content)

Phase Diagrams and Solidification (all content)

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Contents

Main pages

Additional pages

Aims

On completion of this TLP you should:

  • understand the thermodynamic principles behind free-energy curves
  • understand how free-energy curves relate to equilibrium phase diagrams
  • be able to construct a binary phase diagram from cooling curves
  • be able to use phase diagrams to predict the composition and volume fraction of phases

Introduction

The phase diagram is a crucial part of metallurgy - it shows the equilibrium states of a mixture, so that given a temperature and composition, it is possible to calculate which phases will be formed, and in what quantities. As such it is very valuable to be able to construct a phase diagram and know how to use it to predict behaviour of materials.

The main theory behind phase diagrams is based around the latent heat that is evolved when a mixture is cooled, and changes phase. This means that by plotting graphs of temperature against time for a variety of different compositions, it should be possible to see at what temperatures the different phases form.

It is relatively easy to produce a rough binary phase diagram, as will be shown later in the package, but although it is quick to take readings for the top part of a phase diagram, it takes longer, and hence more sensitive equipment to monitor the changes that take place when a solid changes phase. A typical simple binary phase diagram is as follows:

Schematic phase diagram for a binary system

Where L stands for liquid, and A and B are the two components and α and β are two solid phases rich in A and B respectively. The blue lines represent the liquidus and solidus lines, which are relatively simple to measure. The red lines involve a solid-to-solid transition, and so require much more sensitive equipment.

However, there is also a lot of thermodynamic theory behind phase diagrams, which allows more problematic or more complex systems to be predicted, and this can lead to faster creation of phase diagrams, as it can take a long time to pick up all the stable phases in experiments, and there is not always the time available for such practical work.

A crucial point to remember is that a phase diagram should always display the equilibrium phases, and so with cooler temperatures, these are hard to attain due to kinetic problems. Even at higher temperatures, there may be problems of having enough time for the solid to fully equilibrate as the system is cooling.

Thermodynamics: Basic terms

Internal Energy, U

The internal energy of a system is the sum of the potential energy and the kinetic energy. For many applications it is necessary to consider a small change in the internal energy, dU, of a system.

dU = dq + dw = CdT - PdV

dq = the heat supplied to a system
dw = the work performed on the system
C = heat capacity
dT = change in temperature
P = pressure
dV = change in volume

At constant volume,

dU = CVdT

Enthalpy, H

Enthalpy is the constant pressure version of the internal energy. Enthalpy,

H = U + PV.

Therefore, for small changes in enthalpy,

dH = dU + PdV + VdP.

At constant pressure,

dH = CPdT.

Entropy, S

Entropy is a measure of the disorder of a system. In terms of molecular disorder, the entropy consists of the configurational disorder (the arrangement of different atoms over identical sites) and the thermal vibrations of the atoms about their mean positions. A change in entropy is defined as,

\[dS \ge \frac{{{\rm{d}}q}}{T}\]

For reversible changes, i.e. changes under equilibrium conditions,

dq = TdS

For natural changes, i.e. under non-equilibrium conditions,

dq < TdS

Gibbs free energy, G

The Gibbs free energy can be used to define the equilibrium state of a system. It considers only the properties of the system and not the properties of its surroundings. It can be thought of as the energy which is available in the system to do useful work.

Free energy, G, is defined as,

G = H - TS = U + PV - TS

For small changes,

dG = dH - TdS - SdT = VdP - SdT + (dq - TdS)

For changes occurring at constant pressure and temperature,

dG = dq - TdS

Therefore, dG = 0 for reversible (equilibrium) changes, and dG < 0 for non-reversible changes.

From this it is clear that G tends to a minimum at equilibrium.

The Helmholtz free energy, F, is sometimes used instead of G, and is the equivalent of G for changes at constant volume. It is defined as,

F = U - TS

Thermodynamics of Solutions

Consider a mechanical mixture of two phases, A and B. If this is then transformed into a single solution phase with A and B atoms distributed randomly over the atomic sites, then there will be,

  • An enthalpy change associated with interactions between the A and B atoms, ΔHmix
  • An entropy change, ΔSmix, associated with the random mixing of the atoms
  • A free energy of mixing, ΔGmix = ΔHmix - TΔSmix

Assume that the system consists of N atoms: xAN of A and xBN of B, where,

xA = fraction of A atoms and xB = (1 - xA) = fraction of B atoms

Enthalpy of mixing

In calculating ΔHmix it is assumed that only the potential energy term undergoes any significant change during mixing. This change arises from the interactions between nearest-neighbour atoms. Consider an alloy consisting of atoms A and B. If the atoms prefer like neighbours, A atoms will tend to cluster and likewise B atoms, so a greater number of A-A and B-B bonds will form. If the atoms prefer unlike neighbours a greater number of A-B bonds will form. If there is no preference A and B atoms will be randomly distributed.

Let wAA be the interaction energy between A - A nearest neighbours, wBB that for B - B nearest neighbours and wAB that for A - B nearest neighbours.

All of these energies are negative, as the zero in potential energy is for infinite separation between atoms.

Let each atom of A and B have co-ordination number z.

Therefore, the total number of nearest-neighbour pairs is Nz/2.

Probability of A - A neighbours = xA2

Probability of B - B neighbours = xB2

Probability of A - B neighbours = 2xAxB

For a solid solution the total interaction energy is,

Hs - Us = Nz/2 (xA2 wAA + xB2 wBB + 2xAxB wAB)

For pure A, HA = (Nz/2)wAA

For pure B, HB = (Nz/2)wBB

Hence the enthalpy of mixing is given by,

ΔHmix = Hs - (xAHA + xBHB) = (Nz/2)xAxB (2wAB - wAA - wBB)

We can define an interaction parameter

W = (Nz/2)(2wAB - wAA - wBB)

Therefore,

ΔHmix = WxAxB

If A-A and B-B interactions are energetically more favourable than A-B interactions then W > 0. So, ΔHmix > 0 and there is a tendency for the solution to form A-rich and B-rich regions.

If A-B interactions are energetically more favourable than A-A and B-B interactions, W < 0, ΔHmix < 0, and there is a tendency to form ordered structures or intermediate compounds.

Finally if the solution is ideal and all interactions are energetically equivalent, then W = 0 and ΔHmix = 0.

Entropy of mixing

Per mole of sites, this is

ΔSmix = kN (- xAlnxA - xBlnxB)

(the derivation of this result makes use of Stirling's approximation)

where N = Avogadro's number, and kN = R, the gas constant.

Hence,

ΔSmix = R (- xAlnxA - xBlnxB)

A graph of ΔSmix versus xA has a different form from ΔHmix. The curve has an infinite gradient at xA = 0 and xA = 1.

The free energy of mixing is now given by,

ΔGmix = ΔHmix - TΔSmix = xAxBW + RT (xA lnxA + xBlnxB)

For W < 0, ΔGmix is negative at all temperatures, and mixing is exothermic. For W > 0, ΔHmix is positive and mixing is endothermic.

Free energy curves

For any phase the free energy, G, is dependent on the temperature, pressure and composition.

Pure Substances

For pure substances the composition does not vary and there is little dependence on pressure. Therefore the free energy varies greatest with temperature.

The phase with the lowest free energy at a given temperature will be the most stable. The curves for the free energies of the liquid and solid phases of a substance have been plotted below. It shows that below the melting temperature the solid phase is most stable, and above this temperature the liquid phase is stable. At the melting temperature, where the two curves cross, the solid and liquid phases are in equilibrium.

Diagram of free energy curves

Solutions

Solutions contain more than one component and in these situations the free energy of the solution will become dependent on its composition as well as the temperature.

It is shown above that the free energy of mixing is:

ΔGmix = ΔHmix - TΔSmix = xAxBW + RT (xA lnxA + xBlnxB)

The shape of the ΔGmix curve is dependent on temperature . For the curve shown below the value of ΔHmix is positive, leading to a maximum on the curve at low temperatures. ΔGmix is always negative for low solute concentrations as the gradient of ΔSmix is infinite at xA = 0 and xA = 1.

Graphs of free energy of mixing at varying temperatures

At high temperatures there is a complete solution and the curve has a single minimum. At low temperatures the curve has a maximum and two minima. In the composition range between the two minima (denoted by the dashed lines) a mixture of two phases is more stable than a single-phase solution.

The free energy of a regular solid solution, ΔGsol, is the sum of the free energy of mixing ΔGmix and the free energy of fusion ΔGfus.

Free energy of fusion

When a liquid solidifies there is a change in the free energy of freezing, as the atoms move closer together and form a crystalline solid. For a pure component, this can be empirically calculated using Richard's Rule:

ΔGfusion = - 9.5 (Tm - T)

Tm = melting temperature
T = current temperature

ΔGfusion = 0 at the melting temperature of the component.
ΔGfusion < 0 below the melting temperature of the component.
ΔGfusion > 0 above the melting temperature of the component.

In an alloy, if both the liquid and solid solutions are ideal then ΔGfusion for the alloy can be interpolated between the values for the two components.

Now we can plot the free energy of a regular solid solution from the equation,

ΔGsol = ΔGmix + ΔGfusion

Graphs of free energy of fusion

Graphs of the free energy of fusion

Phase diagrams 1

Free energy curves can be used to determine the most stable state for a system, i.e. the phase or phase mixture with the lowest free energy for a given temperature and composition. Below is a schematic free-energy curve for the solid phase of an alloy.

Schematic free-energy curve for the solid phase of an alloy

The solid shown could either exist as a mixture or as a homogeneous solution of A and B. The figures below show that an alloy of composition C can exist in different configurations with differing free energies. In the first figure (below) the free energy of unmixed A and B is shown as the diagonal black line. The free energy of this mixture at composition C is shown as a red point.

Schematic free-energy curve for the solid phase of an alloy

The system can reduce its free energy by existing as a mixture of two phases

Schematic free-energy curve for the solid phase of an alloy

Though the system has reduced its free energy from that of the mixture, the most stable configuration for the system is a solid solution. This allows the free energy of the system to sit on the free energy curve.

Schematic free-energy curve for the solid phase of an alloy

For most systems there will be more than one phase and associated free-energy curve to consider. At a given temperature the most stable phase for a system can vary with composition. While the system could consist entirely of the phase which is most stable at a given composition and temperature, if the free energy curves for the two phases cross, the most stable configuration may be a mixture of two phases with compositions differing from that of the overall system. The total free energy of the system in any given two-phase configuration can be found by linking the two phases in question with a straight line on a free-energy plot.

Schematic free-energy plots for liquid and solid phases

Schematic free-energy plots for liquid and solid phases

Taking a line that is a common tangent to the two free-energy curves produces the lowest possible free energy for the system as a whole. Where the line meets the free energy curves defines the composition of each phase.

Schematic free-energy plots for liquid and solid phases

For positions where it is not possible to draw a common tangent between the two free-energy curves the system will sit entirely in the phase with the lowest free energy. The borders between the single- and two-phase regions mark the positions of the solidus and liquidus on the phase diagram.

Schematic free-energy plots for liquid and solid phases

When the temperature is altered the compositions of the solid and liquid in equilibrium change and build up the shape of the solidus and liquidus curves on a phase diagram.

Below, a binary system can be seen along with the free-energy curves for the liquid and solid phases at a range of temperatures shown on the phase diagram.

Schematic phase diagram Schematic free-energy curves for solid and liquid
Schematic free-energy curves for solid and liquid
Schematic free-energy curves for solid and liquid Schematic free-energy curves for solid and liquid Schematic free-energy curves for solid and liquid

Phase diagrams 2

The free-energy curves and phase diagrams discussed in Phase Diagrams 1 were all for systems where the solid exists as a solution at all compositions and temperatures. In most real systems this is not the case. This is due to a positive ΔHmix caused by unfavourable interactions between unlike neighbour atoms. As the temperature is reduced the ΔHmix term becomes more significant and the curve turns upward at intermediate compositions, resulting in a curve with two minima and one maximum as described earlier. A common tangent can then be drawn between the two minima showing that the system can reduce its free energy through existing as a mixture of two distinct phases.

The free energy of a system of composition Co can be minimised by existing as a mixture of two solid phases of composition C1 and C2:

Schematic free-energy curve for a mixture of two solid phases

This effect can result in a system which, though single-phase upon solidification, will separate into two solid phases on cooling (e.g. Cr-W).

Another possible result is that the free-energy curve for the liquid will intersect the up turned section of the free-energy curve for the solid before the temperature is high enough to induce the formation of a solid solution. As the temperature is increased, the free-energy curve for the liquid moves downward relative to the solid curve and reaches a position where it is possible to link two parts of the solid free energy curve and one part of the liquid free energy curve with a common tangent. At this temperature three phases are in equilibrium.

Here the system is at the eutectic temperature and three phases can be joined by a common tangent:

Schematic free-energy curves

This is known as the eutectic temperature. At this temperature there will be a composition which solidifies at a single temperature through the co-operative growth of the two solid phases. This is the eutectic composition. It is this composition which will exhibit the lowest melting point for the system.

At temperatures above that of the eutectic there will be two common tangents producing two two-phase regions at the same temperature. The two different solid phases are commonly labeled as α and β

Schematic free-energy curves

Eutectic systems therefore have a liquidus which contains a V to the eutectic point where it meets the eutectic invariant-reaction line.

Here is an example of a eutectic phase diagram. α and β are both solid phases.

Eutectic phase diagram

The two-phase solid region on the phase diagram will consist of a mixture of eutectic and either α or β phase depending on the whether the alloy composition is hypoeutectic or hypereutectic. The constitution of an alloy under equilibrium conditions can be found from its phase diagram. This will be discussed in a later section.

Interpretation of cooling curves

The melting temperature of any pure material (a one-component system) at constant pressure is a single unique temperature. The liquid and solid phases exist together in equilibrium only at this temperature. When cooled, the temperature of the molten material will steadily decrease until the melting point is reached.

At this point the material will start to crystallise, leading to the evolution of latent heat at the solid liquid interface, maintaining a constant temperature across the material. Once solidification is complete, steady cooling resumes. The arrest in cooling during solidification allows the melting point of the material to be identified on a time-temperature curve.

Cooling curve for a pure material

Most systems consisting of two or more components exhibit a temperature range over which the solid and liquid phases are in equilibrium. Instead of a single melting temperature, the system now has two different temperatures, the liquidus temperature and the solidus temperature which are needed to describe the change from liquid to solid.

The liquidus temperature is the temperature above which the system is entirely liquid, and the solidus is the temperature below which the system is completely solid. Between these two points the liquid and solid phases are in equilibrium. When the liquidus temperature is reached, solidification begins and there is a reduction in cooling rate caused by latent heat evolution and a consequent reduction in the gradient of the cooling curve.

Upon the completion of solidification the cooling rate alters again allowing the temperature of the solidus to be determined. As can be seen on the diagram below, these changes in gradient allow the liquidus temperature TL, and the solidus temperature TS to be identified.

Cooling curve for system of two components

When cooling a material of eutectic composition, solidification of the whole sample takes place at a single temperature. This results in a cooling curve similar in shape to that of a single-component system with the system solidifying at its eutectic temperature.

Cooling curve for material of eutectic composition

When solidifying hypoeutectic or hypereutectic alloys, the first solid to form is a single phase which has a composition different to that of the liquid. This causes the liquid composition to approach that of the eutectic as cooling occurs. Once the liquid reaches the eutectic temperature it will have the eutectic composition and will freeze at that temperature to form a solid eutectic mixture of two phases.

Formation of the eutectic causes the system to cease cooling until solidification is complete. The resulting cooling curve shows the two stages of solidification with a section of reduced gradient where a single phase is solidifying and a plateau where eutectic is solidifying.

Cooling curve for hypoeutectic or hypereutectic alloys

By taking a series of cooling curves for the same system over a range of compositions the liquidus and solidus temperatures for each composition can be determined allowing the solidus and liquidus to be mapped to determine the phase diagram.

Below are cooling curves for the same system recorded for different compositions and then displaced along the time axis. The red regions indicate where the material is liquid, the blue regions indicate where the material is solid and the green regions indicate where the solid and liquid phases are in equilibrium.

Series of cooling curves for the same system over a range of compositions

By removing the time axis from the curves and replacing it with composition, the cooling curves indicate the temperatures of the solidus and liquidus for a given composition.

Series of cooling curves for the same system over a range of compositions, with time axis replaced by composition axis

This allows the solidus and liquidus to be plotted to produce the phase diagram:

Series of cooling curves for the same system over a range of compositions, with time axis replaced by composition axis, resulting in phase diagram for system

Experiment and results

The simplest way to construct a phase diagram is by plotting the temperature of a liquid against time as it cools and turns into a solid. As discussed in Interpretation of cooling curves, the solidus and liquidus can be seen on the graphs as the points where the cooling is retarded by the emission of latent heat.

Method

An experiment can be performed to get a rough idea of a phase diagram by recording cooling curves for alloys of two metals, in various compositions. The alloy chosen for this example is bismuth-tin, both of which metals have low melting points, and so can be heated and cooled more quickly and easily in the lab. So that the experiment could be performed in a reasonable time, 11 compositions were used, from pure bismuth to pure tin in steps of 10%. All the compositions were measured in weight percent.

Heating apparatus

Crucible

(Click on image to view a larger version)

The apparatus pictured was set up, with the maximum temperature to be attained of around 300°C. The sample in the crucible cooled, and the temperature was recorded at regular intervals. In the case of the results produced here, readings were made every two seconds, using a computer. However, it is adequate to take readings every 15 seconds manually.

Results

The procedure was repeated for all 11 compositions, and the following results recorded:

Graph showing the temperature of several tin-bismuth alloys cooling over a period of time

These lines can be made clearer by spacing them along the time axis, so that the alloys with a higher bismuth content appear further to the right, as shown below.

Graph showing the temperature of several tin-bismuth alloys cooling over a period of time, lines displaced along the time axis

From these graphs it is possible to pick out the changes in gradient which allow a simplified phase diagram to be drawn.

On the diagram with the displaced time-temperature plots, the changed gradients, i.e. the parts where some of the liquid is solidifying, are coloured white. These show the top of the liquidus and the bottom of the solidus.

If these curves are now straightened out, and the colours kept, with the white representing the solidification, it is possible to construct a basic phase diagram, as was shown for an isomorphous system in Interpretation of cooling curves. This is shown below, where the white line is part of the phase diagram, constructed from the cooling curves, and the thin grey line is the actual phase diagram, found from many experiments over a long period of time.

Graph comparing phase diagram constructed from cooling curves with that found from many experiments

Analysis

It can be seen from the diagram above that the recorded phase diagram is roughly 15°C lower than it should be, and that some of the measurements of the liquidus are not in the expected places. The lowering of the diagram is due to the thermocouple being contained in a glass rod, rather than actually touching the alloy. The occasional unexpected liquidus temperatures are probably due to the compositions being slightly inaccurate.

It is also worth noting that in this projected phase diagram, it was not possible to draw in a proper solidus on the right hand side, as none of the compositions near pure bismuth showed evidence of a solidus. As such, a dotted line has been plotted as an estimate of where it would go.

The microstructure of the alloy changes with composition. This can be seen in scanning electron microscope (SEM) images taken from each of the compositions used in the experiment above.

Interactive Sn-Bi phase diagram and SEM images

The lever rule

If an alloy consists of more than one phase, the amount of each phase present can be found by applying the lever rule to the phase diagram.

The lever rule can be explained by considering a simple balance. The composition of the alloy is represented by the fulcrum, and the compositions of the two phases by the ends of a bar. The proportions of the phases present are determined by the weights needed to balance the system.

Diagram illustrating the lever rule

So,

fraction of phase 1 = (C2 - C) / (C2 - C1)

and,

fraction of phase 2 = (C - C1) / (C2 - C1).

Lever rule applied to a binary system

Schematic phase diagram for a binary system

Point 1

At point 1 the alloy is completely liquid, with a composition C. Let

C = 65 weight% B.

Point 2

At point 2 the alloy has cooled as far as the liquidus, and solid phase β starts to form. Phase β first forms with a composition of 96 weight% B. The green dashed line below is an example of a tie-line. A tie-line is a horizontal (i.e., constant-temperature) line through the chosen point, which intersects the phase boundary lines on either side.

Part of a phase diagram

Point 3

A tie-line is drawn through the point, and the lever rule is applied to identify the proportions of phases present.

Part of a phase diagram

Intersection of the lines gives compositions C1 and C2 as shown.

Let

C1 = 58 weight% B

and

C2 = 92 weight% B

So,

fraction of solid β = (65 - 58) / (92 - 58) = 20 weight%

and

fraction of liquid = (92 - 65) / (92 - 58) = 80 weight%

Point 4

Part of a phase diagram

Let

C3 = 48 weight% B

and

C4 = 87 weight% B

So

fraction of solid β = (65 - 48) / (87 - 48) = 44 weight%.

As the alloy is cooled, more solid β phase forms.

At point 4, the remainder of the liquid becomes a eutectic phase of α+β and

fraction of eutectic = 56 weight%

Point 5

Part of a phase diagram

Let

C5 = 9 weight% B

and

C6 = 91 weight% B

So

fraction of solid β = (65 - 9) / (91 - 9) = 68 weight%

and

fraction of solid α = (91 - 65) / (91 - 9) = 32 weight%.

Modern uses

Phase diagrams are not just an abstract construction - they have applications in the real world, in deciding which compositions to use.

A major use of eutectics, or near eutectics is in solder. In plumbing, solder is used to join copper pipes together, producing a waterproof seal. For many years a lead-tin alloy has been used, as this has a low melting point, especially at eutectic. However, although a low melting point is sought, it is useful to be able to move the pipes around slightly when they are in place and the solder is solidifying. This means that a eutectic should not be used, as although it has the lowest melting point for the alloy system, it will all solidify at once, leaving little room for error. Instead it is useful to use an alloy whose composition deviates slightly from that of the eutectic, so that the solidification will take longer, making the solder easier to use, despite the higher temperatures, and so resulting in a better join.

Electrical solder uses a similar alloy to join parts of an electronic circuit together. In the case of a standard electrical solder, the eutectic is used, as high temperatures are to be avoided, and it is useful for the solder to solidify all at once.

In more modern soldering applications, such as a ball grid array which joins the chip to some circuit boards, the eutectic is still used. However, there are also situations where a slightly off-eutectic can be used. If there are several processing steps, it is useful to start off with a higher melting point alloy, and only use the eutectic in the final soldering stage. This allows the previous solders to stay in place when the heating takes place on the later stages.

Modern solders have moved away from lead-based alloys, because of environmental considerations, and been replaced with new alloys. In the case of plumbing, there is a tendency to use plastic piping instead of copper piping, so there is less need for solder in this industry. In the electronics industry, lead-tin is being replaced by copper-, tin-, and silver-based systems, which have less environmental problems, but can still be used to create low melting points and flexibility which the lead-tin systems provide. Lots of the modern research on solders relates to alternative systems, and characterising them for use. Part of this characterisation involves the production of phase diagrams to allow good choice of composition for the right properties.

In recent times, it has become necessary to mix several elements in order to improve properties of the materials. It is therefore useful to create phase diagrams which involve three elements, called "ternary" diagrams. These are more complicated than two element "binary" phase diagrams, but allow improved optimisation, and hence can produce better results.

Further considerations

Other methods for constructing phase diagrams

Although the easiest way to investigate phase transformations is by the use of time temperature cooling curves, there are many ways to investigate these changes. This is most helpful when observing solid to solid phase changes, as these take longer to reach equilibrium, and so the cooling of a material must take place at a slower rate in order to get accurate results. Unfortunately, as the rate of cooling decreases, it gets harder to detect the latent heat being released, and so other methods must be sought.

The thinking behind most of the methods is the same; as the material changes phase, there will be changes in its physical properties. As such, the phase change can be detected by observing a property as the temperature is reduced. Examples of this are density, electrical or thermal resistance, optical properties, Young's modulus and damping efficiency. Another frequently used measure is that of interplanar spacings in the crystalline phases, which can be measured using X-ray diffractometry.

These techniques have been used to map out phase diagrams, but they are always time consuming, especially during the solid transformations.

Dangers in interpretation of phase diagrams

An essential point to remember in phase diagrams is that during normal or fast cooling, results may not be as expected in the diagram. Both the theory and the experiments to construct phase diagrams rely on the assumption that the system is in equilibrium, which is rarely the case, as this only occurs properly when the system is cooled very slowly. In order to reach full equilibrium, the solute in the solid phases must stay completely uniform throughout the cooling. However, in most systems, if the system is not cooled quickly, the phase diagram will give fairly accurate results. In addition, near the eutectic, the results become even closer to the phase diagram, as the liquid solidifies at nearly the same time.

The non equilibrium conditions can sometimes be of benefit however, as microstructures at higher temperatures in a phase diagrams may sometimes be preserved to lower temperatures by fast cooling, i.e. quenching, or unstable microstructures may occur during fast cooling which can be useful when hardening an alloy.

Summary

In this package, the theoretical background to phase diagrams has been shown, as well as a method for constructing part of a diagram. Explanation has been given of how to use a phase diagram, and how it applies to real systems, and to understanding solidification.

It should now be appreciated that phase diagrams are a valuable resource in predicting behaviour of alloys during solidification, and the microstructures which will be produced.

However, there should also be an understanding that in normal cooling conditions although phase diagrams are generally fairly accurate, they are not always exact, and if diffusion is slow, there may be unexpected results.

Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

  1. What is the effect on the shape of the free-energy curve for a solution if its interaction parameter is positive?

    a Produces a curve which has one minimum
    b Produces a curve with no minimum and one maximum
    c Produces a curve which contains a maximum at low T
    d Produces a curve which contains a maximum at high T

  2. In terms of interatomic bonding, what does a negative interaction parameter represent?

    a That A-A and B-B bonds are more favoured
    b That A-B bonds are more favoured
    c That A-A bonds are more favourable than B-B bonds

  3. Cooling curve for a binary system:

    Cooling curve for a binary system

    a One phase
    b Two phases
    c Three phases
    d Four phases

  4. What is a hypoeutectic alloy?

    a An alloy which has solute content greater than that of the eutectic.
    b An alloy which has a solute content lower than that of the eutectic.
    c An alloy whose solute content is such that it contains no eutectic.
    d An alloy whose final microstructure is wholly eutectic.

  5. Look at the following Al-Si phase diagram. (Place the mouse over the diagram to determine the temperature and composition at any point.)

    For an Al 5 wt% Si alloy what will be the composition of the solid in equilibrium with the liquid at 600C?

    a 1 wt%
    b 2 wt%
    c 9 wt%
    d 13 wt%

  6. Look at the following Cu-Al phase diagram. (Place the mouse over the diagram to determine the temperature and composition at any point.)

    What will be the relative weight fraction of CuAl2 (θ) in a Al 15wt% Cu alloy at its eutectic temperature?

    a 66.6%
    b 33.3%
    c 50%
    d 75%

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

  1. Using the following data, calculate the volume fraction of the beta phase and eutectic at the eutectic temperature, for an alloy of composition 75 wt% Ag. Assume equilibrium conditions.

    At eutectic temp:

    eutectic composition = 71.9 wt% Ag
    maximum solid solubility of Cu in Ag = 8.8 wt% Cu
    density Ag = 10 490 kg/m3
    density Cu = 8 920 kg/m3

  2. Composition vs temperature phase diagrams exist for the combinations of three elements A, B and C (i.e. the three phase diagrams A-B, A-C and B-C). How might they be arranged in three-dimensional space to construct a "ternary" phase diagram for a system containing A, B and C?

Open-ended questions

The following questions are not provided with answers, but intended to provide food for thought and points for further discussion with other students and teachers.

  1. Under what conditions could the compositions of the phases present differ from that predicted in the phase diagram?

Going further

Books

  • Porter, D. A. and Easterling K, Phase Transformations in Metals and Alloys, 2nd edition, Routledge, 1992.
  • Smallman, R. E, Modern Physical Metallurgy, Butterworth, 1985.
  • John, V, Understanding Phase Diagrams, Macmillan,1974.

Websites


Stirling's approximation

Stirling's approximation is:

ln N! = NlnN - N, for large N

The entropy,

S = k lnw

where w is the number of possible configurations for a system.

For a mechanical mixture w = 1 as the only arrangement is A atoms on A sites and B atoms on B sites.

For a solid solution of A and B containing xAN A atoms and xBN B atoms the value of w is calculated as follows

  N!

{xAN}!{(1 - xA)N}!

Assuming that the thermal entropy of the system remains unchanged when A and B go into solution

  ΔSmix =
k ln   N!

  -k ln1
{xAN}!{(1 - xA)N}!
    = k [ln N! - ln {xAN}! - ln {(1 - xA)N}!]
    = k [ N ln N - N - xAN ln xAN + xAN - (1 - xA)N ln (1 - xA)N + (1 - xA)N]
    = kN [ln N - 1 - xA ln xAN + xA - (1 - xA) ln (1 - xA)N + (1 - xA)]
    = kN [ln N - xA ln {xAN} - (1 - xA) ln {(1 - xA)N}]
    = kN [ln N - xA ln xA - xA ln N - (1 - xA) ln (1 - xA) - ln N + xA ln N
    = kN [- xA ln xA - (1 - xA) ln (1 - xA)]
    = kN [- xA ln xA - xB ln xB]

Academic consultant: Lindsay Greer (University of Cambridge)
Content development: Matt Charles, Jacqui Capes and Andrew Cockburn
Photography and video: Brian Barber and Carol Best
Web development: Dave Hudson

This TLP was prepared when DoITPoMS was funded by the Higher Education Funding Council for England (HEFCE) and the Department for Employment and Learning (DEL) under the Fund for the Development of Teaching and Learning (FDTL).

Additional support for the development of this TLP came from the Armourers and Brasiers' Company and Alcan.