Binary System with Incongruent Melting

Steve Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin-Green Bay

Here we have a ternary system where components A and B form an intermediate compund AB that melts incongruently. Just like any other ternary system, we can imagine wrapping the binary phase diagrams A-AB-B, B-C and C-A around a triangular prism.

There will be four regions within which either A, AB, B or C will form first but - the signature of incongruent melting - AB plots outside the AB field.

If the author doesn't label AB, and all too often they don't, you'll have to figure out where it is from its composition.

In the case of binary incongruent melting, there are four zones:

  • A forms first, then AB
  • A forms but is dissolved, then AB forms, and finally B
  • AB forms, then B, just like a simple eutectic.
  • B forms, then AB, just like a simple eutectic.

Since C is completely independent of A and B, it doesn't affect their chemistry at all. The diagram consists of four zones of final composition:

  • A, AB and C
  • A forms but is dissolved. The final composition is AB, B and C
  • AB, then B and C.
  • B, then AB and C.

 
The top surface of the phase diagram looks like this. Arrows indicate the direction of decreasing temperature.
The final composition of the solid falls into two regions as shown here.

The cases shown here are pretty straightforward. In the field C-B-AB, for example, we form C first. The remaining melt moves away from C until it hits the B-C cotectic, and B forms. The melt them moves away from both B and C until it hits the AB-B-C junction.

Now what? At the triple point we begin to form AB. But the melt can't form AB and simultaneously move toward AB on the plot. So the melt stays at the triple junction, crystallizing AB, B and C until it solidifies.

The domains inside the AB field look a bit odd but are not at all mysterious. In the AB field, as AB forms directly from the melt and the melt composition moves straight away from AB. In the case AB-B-C, the melt hits the AB-B cotectic and moves down it until it hits the triple point, where C begins to form as well.

In the case AB-C-B, AB begins to form and the melt composition moves radially away from AB until it encounters the AB-C cotectic. It travels down the cotectic until it hits the triple point, where B begins to form as well.

The portion of the diagram here will have AB and C in the final solid. A will form, but it may or may not survive depending on which side of the AB-C line the composition plots on.

We can project the histories of the different zones as follows:

  1. A forms first. When the melt hits the A-AB cotectic, AB begins to form. Since this region is
  2. A forms first, then the melt hits the A-C cotectic and C begins to form. The melt hits the triple point and AB forms as well.
  3. A forms first, then the melt hits the A-C cotectic and C begins to form. The melt hits the triple point and AB forms as well.
  4. A forms first, then the melt hits the A-AB cotectic and AB begins to form. The melt hits the triple point and AB forms as well.

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Created 15 October 2009, Last Update

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