A model of phase transformation kinetics for ferromagnetic shape memory alloys

As a new kind of magnetic smart material, ferromagnetic shape memory alloys (FSMAs) can be controlled not only by a thermal field and/or a stress field, but also by a magnetic field. The strain of FSMAs in the magnetic field is induced for the phase transition or reorientation in material. Based on the energy dissipation of thermodynamics on the twinned interface of martensite variants, a theoretical model is suggested in the present paper to describe the behavior of phase transformation of FSMAs. Considering a specific free Helmholtz energy, a generalized thermodynamic driving force for martensitic variant reorientation is introduced into the model. Mass-change of martensite is introduced by considering the thermodynamic flux associated with the driving force. The thermodynamic equations of phase transformation are obtained on the assumptions that a threshold function depending on driving force reaches a certain value when the dissipation reaches a maximum for the process are employed. Finally, the transformation kinetics model is reduced to the I-D condition and some numerical simulations are carried out. The results of the mass fraction of martensitic variant depending on the driving force, stress and strain, as well as magnetic field are obtained.