Thermodynamic extrapolation and martensite-start temperature of substitutionally alloyed steels

The martensite-start temperature M_s of any steel can, in principle, be calculated by balancing the driving force for nucleation ∆F _Ms ^γ→α′ against the available chemical free energy change accompanying the transformation from austenite to martensite. Calculation of the latter quantity involves the extrapolation of the free energy surfaces of austenite and ferrite into temperature regimes where they are not in thermodynamic equilibrium; several methods are available for this purpose. An advanced method of this kind is based on the Lacher and Fowler and Guggenheim formalisms, and was first applied to steels by Aaronson et al. This was subsequently corrected by Shiflet et al. and was recently modified by Bhadeshia in order better to represent martensitic transformations.