Synergistic regulation of microstructure and strength–ductility behavior in TiNi alloy by in situ reaction reinforcement with YB₆ and cold rolling

Near-equiatomic TiNi shape memory alloys still suffer from insufficient strength and ductility, which remains one of the major bottlenecks restricting their practical applications. In this study, a series of TiNi alloys with minor YB₆ additions were fabricated by vacuum arc melting. Combined with a multi-pass cold rolling/intermediate annealing process, the synergistic effects of the in situ reaction induced by YB₆ and plastic deformation on the microstructure, mechanical properties, and shape memory effect of the alloys were systematically investigated. DSC and XRD results indicate that all alloys undergo a one-step B2↔B19′ martensitic transformation, and the addition of YB₆ shifts the transformation temperatures to higher values overall. TEM observations confirm that YB₆ reacts in situ with the matrix to form elongated TiB₂ whiskers and NiY particles. Among them, TiB₂ forms a semi-coherent interface with the B19′ matrix, which is beneficial for stable interfacial bonding and efficient load transfer, thereby contributing to strengthening and toughening in combination with dispersion strengthening and grain refinement strengthening. When the YB₆ addition is 0.02wt.%, the average grain size is refined from 8.71 μm to 4.01 μm, the ultimate tensile strength increases from 504.0 MPa to 626.6 MPa, and the elongation after fracture improves from 5.6% to 8.07%. On this basis, after four-pass cold rolling with a total deformation of 60%, the grains become markedly fibrous and are further refined to 3.78 μm in the transverse direction. Meanwhile, a high density of dislocation cells and 001 Type I martensitic twins are formed, leading to a further increase in ultimate tensile strength to 745.7 MPa and elongation after fracture to 14.9%, with the fracture mode changing to typical ductile fracture. Shape memory tests show that the 0.02wt.% YB₆-added alloy exhibits shape recovery rates of 93.3% and 86.7% under pre-strains of 6% and 9%, respectively, maintaining excellent shape recovery capability while significantly enhancing both strength and ductility. The composite regulation strategy proposed in this work, namely the introduction of in situ reaction-derived reinforcing phases combined with multi-pass cold-rolling deformation, provides a feasible route for the synergistic optimization of strength, ductility, and functional properties in high-performance TiNi shape memory alloys.