Effect of cold work on martensitic phase transformation and mechanical properties of Ta nanofibers / NiTi shape memory alloy composite

Abstract

This work report a novel Ta nanofibers - reinforced NiTi shape memory composite exhibiting both high radiopacity and exceptional deformability. Herein, the composite was fabricated via compositional design, casting, forging, hot extrusion, warm drawing, and 88 % cold-drawing deformation followed by annealing. Microstructural analysis reveals that the diameter of Ta nanofibres decreases with increasing cold-drawing deformation. Upon tensile loading, the sample subjected to 80 % cold-drawing deformation followed by low-temperature annealing at 400 °C exhibits a high upper plateau stress of 670 MPa, attributed to the synergistic strengthening effect of the nanocrystalline NiTi matrix and the refined Ta nanofibres. The phase transformation behaviour after tensile deformation reveals that the B19′→B2 reverse transformation temperature of the deformed samples increases markedly during the initial heating cycle. During subsequent cooling and reheating of the DSC tests, the reverse transformation temperature of the deformed samples remains higher than that of the undeformed sample. In-situ XRD revealed that these phenomena are closely related to the internal stress coupling between the matrix and the plastic deformation of the Ta nanofibers. Furthermore, X-ray imaging confirmed that NiTiTa composites exhibit markedly enhanced radiopacity relative to commercial NiTi alloys. This work provides great potential for applications in the biomedical field, especially in materials for implants and diagnostic tools.