Despite the fact that the formation of martensite is technically a non-equilibrium process, quantifying several aspects of athermal martensite was one of the earliest applications of the CALPHAD method (1). Being a shear transformation, martensite can also be generated by deformation, which is clearly a process even further removed from equilibrium but is of interest to fabricators of stainless and TRIP steels. Early attempts to define the role of thermodynamics in Deformation martensite could not be tested extensively, because the required input data includes values for DG (g-a) and the Stacking Fault energy (SFE), neither of which were available for commercial compositions or as a function of temperature.
With the advent of programmes such as JMatPro (2), it is now possible to re-examine the validity of such theories and see how far this leads to any new conclusions. This account starts with a brief recapitulation of the thermodynamics of the better known athermal transformation, and then moves on to outline the treatment of Olsen and Cohen (3) The key role played by the entropy of transformation and its variation with temperature will be demonstrated, and used to explain some the differences in the way the volume fraction of matensite varies with composition, deformation and temperature.
REFERENCES:
1. L.Kaufman and M.Cohen Prog.Met.Phys 1958 7 165
2. JMatPro website <www.jmatpro.co.uk>
3. G.B.Olson and Morris Cohen Met.Trans 1975 (6A) 791