Crystalline-amorphous architectures modulate martensitic transformation and mechanical response in NiTi

Abstract NiTi shape memory alloys (SMAs) exhibit outstanding pseudoelasticity, yet the strongly first-order nature of their martensitic transformations (MTs) produces autocatalytic strain avalanches and large hysteresis that limit controllability and energy efficiency. Recent experiments have shown that severe plastic deformation or irradiation can induce partial amorphization of NiTi, forming crystalline-amorphous (C+A) composite microstructures whose cyclic responses evolve from conventional flag-shaped pseudoelasticity toward a more linear superelastic behavior. In this work, phase-field simulations are employed to elucidate how C+A architectures influence MT and mechanical response in NiTi SMAs. Two representative two-phase microstructures are considered: (i) spinodal-like bicontinuous networks and (ii) lamellar multilayers. By systematically varying volume fraction, domain size, and interface inclination, we demonstrate that the critical transformation stress can be tuned over a wide range (220-2000 MPa), the hysteresis can be reduced from ~500 MPa to nearly zero, and the apparent superelastic modulus can be adjusted between 42.6 and 55.1 GPa. These results establish quantitative microstructureproperty relationships for C+A composite and reveal how elastic confinement from the amorphous regions modifies the MT pathways. The findings highlight C+A architecture design as a viable strategy for engineering NiTi SMAs with tailored damping capacity, reduced hysteresis, and programmable superelasticity.