Multiscale modeling of twinning mechanism in polycrystalline 26at%Nb-Ti shape memory alloy

This article investigates the twinning mechanism in a polycrystalline 26at%Nb-Ti shape memory alloy, developed as a biocompatible alternative to NiTi. After metallurgical and mechanical characterization, tensile tests reveal that only the {332} $\langle$113$\rangle$ twinning system is active. EBSD observations show that the twin fraction increases with strain, tends to saturate, but remains strongly correlated with applied stress (highlighting a threshold of about 390 MPa). A multiscale thermodynamic model, based on Gibbs free energy density and a Boltzmann-type statistical law, is proposed to describe the evolution of twin fraction as a function of multiaxial stress. Model parameters are identified from uniaxial tests and validated through dedicated shear experiments. The simulated results accurately reproduce the experimental twin fractions highlighting the model's relevance and robustness.