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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