Engineering - TEDs - Eutectic Diagrams

So far we have looked at Solid Solution Alloys and their Thermal Equilibrium Diagrams's. Now we will look at Eutectic Alloys. Whereas a Solid Solution grain looks like a pure metal under a microscope, a Eutectic grain may look as if there are layers of both metals alternating. This is because Eutectic alloys, when solid are not totally soluble in one another.

Below you can see the T.E.D of Cadmium and Bismuth. This is a Eutectic alloy. The T.E.D. is made up in the very same way as for a Solid Solution alloy.

As you can see this T.E.D. looks quite different from the Solid Solution T.E.D. The main difference is caused by the Eutectic Point. Here the alloy changes straight from a liquid to a solid without going through the pasty phase. It should also be noted that the melting temperature of this alloy is well below the melting temperature of either of the two metals on their own.

As usual above the Liquidus Line the alloy is totally liquid, and the two metals are totally soluble in one another.

Note that the Solidus line is level. Eutectic alloys always solidify at the same temperature. This temperature is called the Eutectic Temperature.

The solid area can be broken up into 3 sections. Below the Eutectic Point the solid is a Eutectic grain mixture, (i.e. alternating layers of each metal). To either side of the Eutectic Point the solid is in the form of grains of one of the metals embedded in solid Eutectic. The closer the alloy is to 100% of either metal the larger are the grains of that metal. (For example, to the left of the Eutectic Point the grains of Cadmium increase in size the closer the alloy is to 100% Cadmium. The Cadmium grains are surrounded by solid Eutectic, and there are no grains of Bismuth present.

From the T.E.D. we can see that there are 2 pasty areas. To the left of the Eutectic Point there are grains of Cadmium in liquid, and to the right of the Eutectic Point there are grains of Bismuth in liquid. Again the closer the alloy is to 100% of either metal then the larger are the grains of that metal.