A stainless steel is a ferritic alloy, in general, with a low carbon content and a Cr content equal to or greater than 10.5%. To study the phase transformations and heat treatments undergone by these steels, a phase diagram is used, as shown in the following figure:
In this diagram, there is a sigma phase of tetragonal structure, whose occurrence I have read should be avoided. But, in the notes I am reading, the reason does not appear. Why should the sigma phase be avoided?