Martensitic transformation in crystal-amorphous superlattices of NiTi shape memory alloy

Shape memory alloys (SMAs) exhibit unique thermo-mechanical properties arising from reversible martensitic transformation. Incorporating a coherent second phase has emerged as an effective strategy for tailoring these transformations and improving functional performance. However, identifying a suitable second phase that is thermodynamically compatible with the base martensitic matrix while simultaneously enhancing thermo-mechanical properties remains a significant challenge. Crystal-amorphous superlattices (CAS), in which the second phase is derived from the base material itself, provide an attractive alternative. However, the influence of partial amorphization of the base martensitic material on martensitic transformations and the resulting thermo-mechanical behavior remains largely unexplored. In this work, large-scale molecular dynamics simulations were performed to investigate both temperature- and stress-induced martensitic transformations in CAS-NiTi with different crystalline phase fractions. Our simulations reveal that the incorporation of an amorphous phase fundamentally modifies the transformation pathway, producing an initial continuous (second-order-like) martensitic transformation followed by the conventional first-order transformation. Furthermore, the amorphous phase enhances the reversibility of the martensitic transformation, reducing the thermal hysteresis from 275 K for fully crystalline NiTi to 95–110 K for CAS-NiTi systems. Simultaneously, the elastic modulus and the critical stress for stress-induced martensitic transformation increase by approximately 60–90%, depending on the crystalline phase fraction. These improvements originate from heterogeneous nucleation at crystal-amorphous interfaces, retained austenite that facilitates the reverse transformation, and the mechanical constraint imposed by the amorphous phase. Our findings highlight crystal-amorphous superlattices as a promising microstructural design strategy for tailoring the thermo-mechanical performance of SMAs.