In situ synthesis of TiB2-NiTi-TiB2 sandwich structure for tailoring Ti2Ni to synergistically optimize strength, ductility, and superelasticity of NiTi wires

The brittle Ti₂Ni phase induces crack initiation and causes premature failure in NiTi alloys. This study demonstrates that introducing 0.2 wt% B element into NiTi alloy promotes in-situ formation of layered TiB₂, forming a semi-coherent TiB₂-NiTi-TiB₂ "sandwich" structure, which effectively suppresses the generation and growth of Ti₂Ni particles at subsequent grain boundaries. The semi-coherent interface between TiB₂ and the matrix, bonded through stacking faults, significantly strengthens the NiTi alloy, allowing it to withstand greater stresses during the elastic deformation stage. Furthermore, this interface structure enhances the continuity of reverse martensitic transformation in alloy wires, mitigating local stress concentration and optimizing superelasticity while maintaining excellent ductility. However, excessive B (0.5 wt%) triggers detrimental blocky TiB₂ formation, exacerbating stress concentrations. These findings indicate that precise control of B addition can achieve ideal TiB₂-Ti₂Ni coupling, providing a novel strategy for synergistically enhancing strength, ductility, and superelasticity in NiTi wires.