Effect of forging temperature on the structure, mechanical and functional properties of superelastic Ti-Zr-Nb bar stock for biomedical applications

Abstract

Ti-Zr-Nb shape memory alloys exhibit a unique combination of properties that make them suitable for bone implants: low Young’s modulus, superelastic behavior, superior corrosion resistance, and non-toxicity of all the constitutive elements. In this study, superelastic Ti-19Zr-14Nb (at.%) alloy was subjected to a combination of radial shear rolling at 900°C and rotary forging in a temperature range from 500 to 700°C to form long-length bar stocks for bone implants fabrication. Features of the grain structure, phase composition, mechanical and functional properties of the long-length bar stocks were analyzed using light microscopy, X-ray analysis, as well as during mechanical and functional tests. It was shown that after radial shear rolling at 900°C, a heterogeneous grain structure was formed over the cross-section of the bar stock, and this structure was inherited after the subsequent rotary forging at 500°C. With an increase in the forging temperature, the structural heterogeneity is eliminated and the grain size increases, while the hardness and strength characteristics of the material decrease. After rotary forging at 700°C, the alloy manifests the best combination of structural, mechanical, and functional characteristics. In this state, the long-length bar stock demonstrates a homogeneous grain structure with a certain fraction of a dynamically polygonized substructure of β-phase, a satisfactory strength (UTS ≈ 580 MPa), a low Young’s modulus (E ≈ 35 GPa), the high difference between dislocation and phase yield strength (Δσ ≈ 280 MPa), and a relatively large amount of superelastic recovery strain (εrSEmax ≈ 3.1%).