Thermal Equilibrium Diagrams

Contain information about changes that take place in alloys

Alloys

• Alloys are combinations or mixtures of elements.

• Metals are alloyed to improve on properties of pure metals such as hardness, strength, corrosion resistance, etc.

• Alloys may behave differently when combined, some mix easily while others will only be soluble to a limited extent. Thermal Equilibrum diagrams seek to interpret how alloys combine.

Solid Solution Alloys

Two metals are completely soluble in each other. Solid solution alloys are normally:

Substitutional solid solution or,

Interstitial solid solution. Copper-nickel alloy is a

solid solution .

Cooling curveThis shows how a pure metal cools from a liquid to a solid. A graph of temperature and time shows that copper is fully liquid at 1500C. As temperature falls to 1083C, copper begins to change into a solid. As this solid-liquid change happens, the temperature does not change (the latent heat of fusion is given up keeping the temperature constant)

Cooling curve graphs of Copper and Nickel

Thermal Equilibrium diagram for Copper-nickel alloy drawn from cooling curves.

Parts of Copper-nickel diagram

Solidus Line – all points below this line are in solid form.

Liquidus Line – all points above this line are in liquid form.

Between the solidus and liquidus lines, all points are in a mixture of liquid and solid (pasty) form.

Composition of phases at a particular temperature (Lever Law)

Ratio = Mass of Solid = AB

Mass of Liquid BC

= 65-60 = 5

60-48 12

Eutectic alloy

The two metals are completely soluble in the liquid state but are completely insoluble in the solid state.

Cadmium-Bismuth is an example of a eutectic alloy. The phase equilibrum diagram is compiled in the same

way as the solid solution alloy but looks completely different.

The eutectic point (140C) is the lowest melting point of the alloy. The alloy changes directly from a liquid to a solid without going through a pasty stage at this point.

Cadmium-bismuth (eutectic alloy) phase equilibrum diagram.

Partial Solubility

When one metal dissolves in another to a limited extent.

Lead and tin is a partial solubility alloy. The solvus lines give an indication of the amount of

one metal that will dissolve in the other metal. Soft solders are based on lead-tin alloys with tinmans

solder (used for electronics) being the eutectic alloy. Plumbers solder has 70% lead with the pasty phase allowing the joint to be shaped.

Partial solubility thermal equilibrum diagram of lead and tin

Intermetallic compounds

This is a chemical compound formed between at least two elements. Iron carbide, or cementite, is a compound of iron and carbon. It is hard and brittle.

The Allotrophy of Iron

Iron can exist in more than

one form. Up to 910 degrees, alpha

iron is in BCC form. Between 910 and 1390

degrees, gamma iron exists in FCC form.

Above 1390 degrees, delta iron returns to BCC form.

Iron Carbon Equilibrum Diagram

Steel part of the Iron Carbon Equilibrum Diagram

Eutectoid point is a solid to solid change point where solid Austenite changes into solid Pearlite.

Microstructures on steel part of the Iron Carbon Equilibrum Diagram