For Fe-Cr-C steels to calculate a metastable phase formation diagram where only carbides are allowed to form that exhibit the same Fe/Cr ratio as the matrix. Since the martensite formation is a diffusionless transformation, we have now calculated an upper bound for the marteniste start temperature: Only below the blue line it will be possible to form martensite! For multicomponent systems it would also be possible to allow one element to be mobile while the others are not, e.g. This line is integrated in blue into the phase diagram: Martensite is a body-centered tetragonal crystal structure (BCT), that is similar in Gibbs energy to the BCC phase but less stable (otherwise it would be present in the phase diagram). The result is a single line where the Gibbs energies of the austenite (FCC) and ferrite (BCC) are equal.
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