Experimental and theoretical investigations of the stress-induced twinning/detwinning in the martensite phase of a FSMA system

Ferromagnetic shape memory alloys (FSMAs) have the largest magnetic field induced strain below austenite–martensite transformation temperatures. The martensitic phase has structural twins that reorient under applied magnetic field or stress. To understand this behavior, stress–strain measurements were made at different temperatures on a typical FSMA material, NiFeAl. A thermodynamic constitutive model based on Clausius–Duhem inequality, was used to theoretically determine the stress resulting from an applied stress at each temperature. In this paper, we studied twinning/detwinning in the martensite under applied stress only. Thus, the free energy of the system has a sole contribution from the mechanical energy, which, in turn, depends on the elastic moduli pertaining to the elastic and twinning strains. This volume fraction of martensite phase, both for the field-preferred and stress-preferred directions, and magneto-mechanical driving force, was determined and correlated with the experimental results. Finally, the model was found to fit well with data in the premartensitic region too.

► We present a model for stress-induced twinning/detwinning in the martensite phase of a FSMA. ► We then quantitatively compared the theoretical fit to the actual experimental data. ► It was found to be applicable even in the mixed region just above the austenite finish temperature.