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. Nonchemical 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. gamma -> h.c.p. epsilon 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. gamma -> b.c.c. alpha (b.c.t. alpha') 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 M-s (martensitic start temperatures) are reported and discussed. A comparison among different approaches for the prediction of M-s temperatures is carried out. (C) 2008 Elsevier Ltd. All rights reserved.