Rapid Stabilization and Linear Superelasticity Mediated by Semicoherent Interfacial Architecture in NiTi Shape Memory Alloys

NiTi shape memory alloys possess outstanding superelasticity and functional recoverability, but their widespread application is constrained by slow stabilization and fatigue accumulation during mechanical training, often requiring hundreds of cycles. In this work, we designed a high-stability NiTi composite by engineering grain-boundary chemistry and interface structure. Using low-energy ball milling followed by spark plasma sintering, a quaternary Ti–Ni–Si–C phase forms in situ and self-organizes into a continuous grid-like network along grain boundaries. This phase exhibits a semicoherent interface with the B2 NiTi matrix phase, featuring partial lattice matching and periodic misfit dislocations. As a result, the composite rapidly stabilizes after only five cycles and demonstrates linear superelasticity of 10,000 compression cycles at 2% strain, with progressively reduced hysteresis. Meanwhile, the introduction of the Ti–Ni–Si–C and TiC phases, along with refined Ni4Ti3 precipitates, doubles the hardness from ∼250 to ∼500 HV1. This interface-guided phase design provides a general strategy for developing NiTi-based materials with rapid training, enhanced fatigue resistance, and improved mechanical robustness for advanced actuation and structural applications.