Thermodynamics of martensitic transformations in the framework of the CALPHAD approach

Martensitic phases occur in a number of metallic materials, and are metastable in many systems. They have been of significant importance in steels for a long time, and have generated a great interest in Shape Memory Alloys (SMA) in recent years. This review describes a thermodynamic and kinetic approach for modelling martensitic phases in the framework of the CALPHAD approach.A general introduction of thermodynamics of the martensitic transformation is first given. Non-chemical terms are essential in the energetic balance, and they are discussed in detail. Differences between thermoelastic and non-thermoelastic martensitic transformations and their thermodynamic behaviour is also discussed. Modelling of martensites in the Ni–Ti system is then considered as an example of thermoelastic transformations (Shape Memory Alloys). Non-thermoelastic martensitic reactions are then discussed for the f.c.c. γ→γ→ h.c.p. εε transformation in the binary Fe–Mn and ternary Fe–Mn–Si systems, where magnetic contributions to the Gibbs free energy and their effect on martensitic transformations are examined. Finally, the f.c.c. γ→γ→ b.c.c. αα (b.c.t. α′α′) transformation in Fe-based alloys and martensitic steels is described as the most important technological application. Calculations of the driving force for martensitic formation and predictions of MSMS (martensitic start temperatures) are reported and discussed. A comparison among different approaches for the prediction of MSMS temperatures is carried out.