Engineering - Partial Solubility TEDs

So far you should have looked at Solid Solution Alloys and their Thermal Equilibrium Diagrams's, as well as Eutectic Alloys.

The third type of alloy is a Partial Solubility Alloy. Metals that alloy is this way dissolve in solid solution only to a certain extent. Their T.E.D. is like a combination of a Solid Solution Alloy and a Eutectic alloy. The alloy we will use as an example is the Lead - Tin T.E.D. Because this type of alloy is more difficult to understand we will take it in sections.

A maximum of approximately 20% Tin can be dissolved, (remain in Solid Solution), in Lead up to 183°C. Above this temperature the amount of Tin that can dissolve decreases. A maximum of about 3% Lead can be dissolved in Tin. The lines that show this area on a T.E.D. are called Solvus Lines.

What we end up with, are essentially 3 different types of solid metals. On the left between the Solidus Line and the Solvus Line we have , (alpha), metal, a Solid Solution alloy of Lead and Tin. On the right between the Solidus Line and the Solvus Line we have , (beta), metal, another Solid Solution of Lead and Tin. In between the two Solvus Lines and the Solidus Line we have a Eutectic mixture of Lead and Tin, just like in a Eutectic T.E.D.

Now we will look at the completed Partial Solubility T.E.D.

Again we have a Eutectic Point present, where the alloy changes from a liquid to a solid without going through the pasty stage. The pasty areas comprise of , alpha, (the first), metal and liquid, to the left of the Eutectic Point, and , beta, (the second), metal and liquid to the right. Again the further the alloy is from the Eutectic alloy the larger are the grains of the respective metal.

We will take the example of the 90% Lead - 10% Tin alloy to explain more clearly how to read this type of T.E.D.

• At 400 °C the alloy is totally liquid.
• At 300°C, after crossing the Liquidus line, the alloy is a mixture of alpha solid solution grains in liquid.
• At 200°C, after crossing the Solidus Line, the alloy is solid solution alpha metal.
• At 100°C, after crossing the Solvus Line, the alloy changes it crystal grain structure to grains of Lead in Eutectic.