Introduction
In this section we will look at two main topic – namely alloying and phase diagrams. We will look at how alloying is used to improve the properties of metals, and also how how phase diagrams may be used to determine the phases present in an alloy system and hence the properties of an alloy system.
Thanks to Dr Gareth Bradley from Perth College UHI for all his help with these materials.
Alloying
Pure metals tend to have good electrical and thermal conductivity and corrosion resistance, but they are usually of low strength. To improve their strength and hardness, or other properties such as their corrosion resistance if applicable, other elements are added to the molten metal during the production process, a process known as alloying. Arguably the best known alloy is steel.
Steel is an alloy of iron and carbon, the addition of carbon increasing the strength and hardness of the iron. Many steels contain other elements as well. If chromium is added to steel in sufficient quantities stainless steel is produced, this having good corrosion resistance. Other elements such as vanadium, molybdenum and nickel may also be added. Elements such as copper, magnesium and zinc are added to aluminium to improve its properties in a variety of ways.
When two liquid metals or materials are mixed together and allowed to solidify one of four things may happen:
- They are totally soluble in each other and solidify to form a solid solution for example copper and nickel:
- They are completely insoluble in each other and remain completely separate (aluminium and silicon).
- They are partially soluble in each other, i.e. depending on how much is added they may form two different solid solutions (lead and tin) or a solid solution and a compound (iron and carbon).
- They may form a chemical compound known as an intermetallic (copper and aluminium).
Phase Diagrams
Phase, or equilibrium, diagrams are used to determine the microstructure (and amount) of a material that will form when alloying elements are added. The properties, such as hardness, strength, ductility etc. of a metal depend on the type of microstructure present, and hence a phase diagram can be used to give an indication of the properties of an alloy.
H2O Phase Diagram & Cooling Curve
If you have undertaken the unit DT9P 34 Thermofluids then you will probably have already encountered the concept of phases of materials and how temperature and pressure affect them. These two diagrams show two ways that the phase of water can be shown – one just looking at temperature (cooling curve); the other involving pressure as well (phase diagram).
As can be seen in this phase diagram, At different pressures and temperatures different phases exist.
For example, at 1atm pressure H2O will be liquid between 0 and 100 C.
Gareth Bradley, Perth College UHI
This cooling curve however shows how H2O moves between the phases as temperature changes if pressure is constant.
At 100°C and atmospheric pressure steam starts to condense, both steam and water coexist, and the temperature remains constant for a short period of time. This is known as the arrest point. Once all the steam has condensed the water again starts to cool. A similar process occurs at 0°C.
Copper-Nickel Phase Diagram and Cooling Curve
If we look at these diagrams, we can see that a similar process to H2O cooling occurs with metal alloy systems, although the arrest temperature (which is when the alloy fully solidifies) covers a range and depends on the composition of the alloy.
Copper – Nickel Phase Diagram
Looking at the phase diagram in more detail then we can see the following:
- Liquidus line: above the liquidus line the alloy is a liquid.
- Solidus line: below the solidus line the alloy is a solid solution (in this case nickel, Ni, dissolved in copper, Cu,) known as the a phase.
- In between the liquidus and solidus lines both the liquid and a (solid solution) phase coexist.
Gareth Bradley, Perth College UHI
Activity
Looking at the Copper – Nickel Phase diagram, attempt to answer the following questions:
- For a 30 Wt% Ni/70 Wt% Cu alloy at a temperature of 1350 °C how many phases are present and what are they?
- For a 30 Wt% Ni/70 Wt% Cu alloy at a temperature of 1130 °C how many phases are present and what are they?
- For a 30 Wt% Ni/70 Wt% Cu alloy at a temperature of 1220 °C how many phases are present and what are they?
- What is the melting point of copper (Cu)?
- What is the melting point of nickel (Ni)?
- For a 60 wt% Cu/40 wt% Ni at what temperature is the mixture a liquid?
- At what temperature is the alloy in iii. fully solidified?
Solution:
- 1 phase, liquid
- 1 phase, α (solid)
- 2 phases, α and liquid
- 1085 °C
- 1455 °C
- 1280 °C
- 1240 °C
Lead – Tin Phase Diagram
As with the copper – nickel phase diagram, we can see the following:
- Liquidus line: above this line is a single phase (liquid) with a composition the same as that of the alloy.
- Solidus line: below the solidus line the phase(s) are in the solid state. More than one phase may exist.
- Solvus lines: indicate the maximum concentration with regards to the solid solution.
- Above the liquidus line the composition will be 90 wt% lead (Pb) and 10 wt% tin (Sn). (wt means weight)
- Once the liquidus line is reached crystals of the α phase will start to form.
- When the solidus line is reached solidification will be complete. The solid at this stage will consist of the α phase.
- On reaching the solvus line the α phase is saturated with Sn and a solid state reaction will occur and a second phase, β, will form.
Gareth Bradley, Perth College UHI
Eutetic Point
Where the liquidus and solidus lines meet is known as the eutectic point. This represents the lowest melting point for an alloy system. In the diagram, it is the lowest point of the liquid phase.
Gareth Bradley, Perth College UHI
If an alloy has a composition equal to the eutectic composition on cooling from the liquid it will form a metal consisting of crystals of A and B forming what is known as the eutectic microstructure that is frequently laminated in nature, as shown in the second diagram:
Hypo- and Hyper - Eutetic Composition
A hypoeutectic alloy has a composition that is less than the eutectic composition. One cooling solid A will start to form when the liquidus line is crossed. As only A is forming the composition of the remaining liquid must have more B in it. As the temperature drops more solid is formed and the remaining liquid becomes more and more rich in B. The actual composition follows the liquidus line. Eventually the composition of the remaining liquid will equal the eutectic composition and the remaining liquid will solidify with a eutectic microstructure.
The white arrow in the first diagram shows what is happening in this process.
Gareth Bradley, Perth College UHI
A similar process to that of a hypoeutectic composition occurs for a hypereutectic composition, except here solid B forms first, meaning that the remaining liquid becomes richer and richer in A. Again, the white arrow in the second diagram shows the process.
Gareth Bradley, Perth College UHI
Definitions related to phase diagrams
Alloys are usually made by melting the components and mixing them together. A binary alloy (one that contains only two elements) can take on one of four forms:
- a single solid solution (e.g. Cu-Ni alloy):
- two separated, essentially pure, components (e.g. Al-Si):
- two separated solid solutions (e.g. Pb-Sn);
- a chemical compound together with a solid solution (e.g. Fe-Fe3C).
Iron-Carbon Phase Diagram
Finally, to show how complex phase diagrams can become, here is the iron-carbon phase diagram. In practice, it is comparatively easy to use, the amount of carbon present and the temperature will be known, allowing the user to simply read off the information they need.
