Metastable Zr-Nb alloys for spinal fixation rods with tunable Young's modulus and low magnetic resonance susceptibility.

Good ductility, low magnetic susceptibility, and tunable Young's modulus are highly desirable properties for materials usage as spinal fixation rods. In this study, the effects of niobium content on the microstructure, magnetic susceptibility, and mechanical properties of Zr-xNb (13≤x≤23wt%) alloys were investigated. For the Zr-15Nb and Zr-17Nb alloys, a remarkable increase in Young's modulus was achieved due to the occurrence of deformation-induced ω phase transformation. This was the result of the competition of two factors associated with the Nb content: an increase of the stability of β phase and a decrease of the amount of athermal ω phase with increasing Nb content. When the Nb content was 15% or 17%, the amount of deformation-induced ω phase was maximum. Moreover, the magnetic susceptibility decreased with the deformation-induced β→ω phase transformation, and the Zr-17Nb alloy with apparent kink bands exhibited a smaller amount of springback than the Zr-15Nb alloy with {332} 〈113〉 mechanical twins. Furthermore, the ions released from the Zr-xNb alloys in accelerated immersion tests were at a very low level. The combination of low initial Young's modulus, and its remarkable variation induced by deformation, low magnetic susceptibility, good ductility, and smaller springback make the Zr-17Nb alloy a potential candidate for spinal fixation rods. STATEMENT OF SIGNIFICANCE: For the rods of spinal fixation devices, it is important but difficult to lower the springback for bending formativeness while keeping the low initial Young's modulus for biocompatibility and lower the magnetic susceptibility for postoperative examination simultaneously. In this study, Zr-17Nb alloy was successfully developed via deformation-induced ω phase transformation during loading, simultaneously meeting the abovementioned properties for spinal fixation rods.