Superelastic Ti-Zr-Nb Alloy for Spinal Rods: Technology, Microstructure, Texture, and Functional Properties

摘要

The interest in developing metastable Ni-free Ti-Zr-based shape memory alloys (SMA) for load-bearing biomedical applications experienced a marked increase over the last decade [1]. These alloys demonstrate a unique combination of low Young's modulus, superelastic behavior, which is close to the behavior of bone, and contain only non-toxic elements in their chemical composition. It is known that thermomechanical treatment (TMT) is an effective tool to form an adequate material microstructure in the required semi-products (long-length bar stock for spinal rods) providing the best combination of functional properties [2J. Optimizing the metal forming techniques as part of the TMT regime is a crucial task for materials science engineers and metallurgists. Significant progress in this field can be achieved using the radial shear rolling (RSR) technology and its combination with rotary forging (RF). Radial shear rolling is an advanced metal forming technique for the production of long-length bar stocks with diameters larger than 10-12 mm and with a well-organized structure. It has been shown that the dynamicaly polygonized dislocation substructure of β-phase with a 1-2 um subgrain size formed in Ti-18Zr-14Nb SMA as a result of hot RSR led to a superior functional fatigue resistance [3]. Applying after RSR the rotary forging technique, which consists in applying pulsing localized impacts on the bar stock surface, allows reducing the bar diameter to 3-8 mm, while ensuring a uniform deformation and a high surface quality. In this study, a novel combinations of radial shear rolling and rotary forging operations at different temperatures were applied to Ti-18Zr-14Nb (at. %) shape memory alloy with the objective to form a long-length bar stock for spinal rods fabrication.