Superelastic Ti-Zr-Nb alloy for spinal rods: technology, microstructure, texture and functional properties

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. 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 technology which can reach two objectives simultaneously: a) obtaining the required semi-products (long-length bar stock for spinal rods), and b) providing the best combination of functional properties of these semi-products via the formation of an adequate material microstructure.

In this study, a novel combinations of radial shear rolling (RSR) and rotary forging (RF) 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. Evolutions of the microstructure, crystallographic texture, mechanical properties, and functional fatigue behavior of the fabricated bar stock were monitored along the technological route. Combined hot TMT leads to the formation of a dynamically polygonized substructure of β-phase with a homogeneous across-the-section grain size distribution (d≈34-43 µm). In this structural state, the alloy demonstrates an outstanding functional fatigue resistance. TMT including cold RF and post-deformation annealing results in the formation of the well-developed polygonized substructure in β-phase with nano- to submicrometer-sized subgrains. A beneficial combination of high value of strength-to-modulus ratio with stable functional fatigue behavior was achieved in this structural state.