A Thermodynamic Model And Application Of Martensite Variants Rearrangement Behavior For Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory alloys (FSMAs) are a potential application of ferromagnetic functional material, which exhibit advantages such as high ferromagnetism, huge output strains, high response frequencies etc. The shape memory effect (SME) of FSMAs can be controlled not only by a thermal field or a stress field, but also by a magnetic field. The theoretical studies and numerical simulations for FSMAs are conducted, aiming at their mechanical-magnetic coupling behaviors of martensite variants rearrangement and huge output magnetic field-induced strain behaviors caused by applied both of transversal magnetic field and axial compressive stress.Firstly, the total strain of FSMAs is derived from the Eshelby equivalent inclusion method and the Mori-Tanaka averaged scheme during the martensite variants mechanical-magnetic coupling rearrangement process, by treating the growth variant as inclusion phase and the decrease variant as parent phase. Secondly, the thermodynamic model based on thermodynamic principle is proposed to describe the mechanical-magnetic coupling behaviors during the martensite variants rearrangement process. Furthermore, a simple form thermodynamic resistance formula for martensite variants rearrangement process is proposed, which has the mechanical significant. And the correlation coefficients can be measured by experiment. Thus, a thermodynamic balance equation is established for the thermodynamic resistance formula and the generalized thermodynamic driving force leading to martensite variants twin boundary motion.The proper internal state variables, which combine the micromechanics theory with the thermodynamic principle, are chosen to describe the microstucture change during the martensite variants rearrangement process. The above method realizes to describe the mechanical-magnetic coupling rearrangement behaviors of the martensite variants. The evolution process of the three internal state variables and the magnetization are numerical simulated to describe the microstructure change of martensite variants. The results show that, the model can give the good predictions on the mechanical-magnetic coupling behaviors during the martensite variants rearrangement or inverse rearrangement process of the FSMAs under either the loading of magnetic field or compressive stress. The model can well predict the critical field for the start and end of the martensite variants rearrangement process and the critical stress for the start and end of the inverse rearrangement process. Lastly, the model discusses the mechanism and the applied field conditions of how the martensite variants rearrangement process could not happen, partly happen or completely happen.