sigma phse, need some help plz..

hello I'm a student of Material eng and I'm writing a paper about the sigma phase, i found in a books i read 2 different explanations.
in one was written that Ni is inducer of the sigma phase but in another was written that is inhibitive the sigma phase, what is the right one??
another question i have is why the sigma phase prefer to precipitat from the ferrit and not from the austenit?
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Dina wrote:

Look for the ternary diagram Fe-Ni-Cr! The sigma phyase is stable for some temperatures and compositions.
Michael Dahms
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