Plastic Deformation Mechanism and Size-Dependent Mechanical Behavior of NiTiNb Shape Memory Alloy Thin Strips Subjected to Medium-Temperature Rolling

In this study, a NiTiNb shape memory alloy (SMA) thin strip with an interpenetrating microstructure of NiTi matrix and β-Nb phase was fabricated via medium-temperature rolling. The phase constitution, microstructural evolution, and mechanical behavior of the rolled alloy were systematically investigated. The results indicate that the rolled NiTiNb SMA thin strip consists of B2 austenite and β-Nb phases, with no detectable B19′ martensite. Plastic deformation is primarily governed by dislocation slip. With increasing rolling reduction, the grains are progressively elongated along the rolling direction, gradually forming a layered structure. As the accumulated strain increases, continuous grain refinement occurs, with localized regions refined to the nanocrystalline scale, exhibiting an average grain size of approximately 36.3 nm. Meanwhile, dislocations continuously multiply and entangle, leading to an increase in tensile strength up to 992.3 MPa. However, when the strip thickness is further reduced to 0.15 mm, the tensile strength decreases to 859.6 MPa. The fracture surfaces of the thin strips are characterized by deep dimples, indicating a typical ductile fracture mode.