I read a paper address that "The effect of elastic strain energy is neglected in the model since the precipitation of sigma phase is simulated only in base metals where there is no coherency between austenite/ferrite." I don't understand this, how can we know whether there is coherency between phases? thanks for your answering
It is not that simple. The boundary is not always incoherent. If the microstructure contains Widmannstatten ferrite, then there is a coherent, or semicoherent boundary between this ferrite and austenite.
The nature of a boundary between phases, of which one is the result of a precipitation process, depends on several factors, primarily on the difference in the free formation energy of both phases, on the grain boundary energies and on the rate of movement of the respective grain boundaries of the precipitated phase.
These physical factors, in case of steel, depend on the composition (carbon content) and the cooling rate.
For a more comprehensive reply and understanding of the physical principles, I suggest to read the book: Phase Transformations in Metals and Alloys, third edition, by D. Porter, K, Easterling and M. Sharif, CRC Press, especially page 299 onward.
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