Development of a low modulus Ti-Nb-Ta-O alloy for orthopedic applications
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Ti-Nb-Ta-Zr based β-Titanium alloys are currently projected as potential candidates for use in orthopaedic implants owing to their low elastic modulus and use of non-toxic elements. However, most of these alloys contain large amount of Ta and Zr, which increase the cost of these alloys. Moreover, bulk properties such as mechanical strength and surface properties such as wear resistance, fatigue resistance and osseointegration, for these alloys are still unsatisfactory. In the first part of this study, the microstructure and crystallographic texture evolution in β-annealed conditions and their effect on the mechanical and functional response of a new β Ti-34Nb-2Ta-0.5O (wt.%) alloy has been first studied and compared with those of a more popular Ti-34Nb-2Ta-3Zr-0.5O alloy (wt.%). The compositional differences, along with alterations in processing route, were found to affect the texture evolution by changing the slip activities in the two alloys. In the annealed condition, Ti-34Nb-2Ta-0.5O exhibits a low elastic modulus of 63 GPa, owing to its single β-phase microstructure. The presence of interstitial oxygen also imparts substantial solid solution strengthening. The mechanical and electrochemical properties were found to be affected by changes in composition and crystallographic texture. The Ti-34Nb-2Ta-0.5O alloy was further strengthened via aging treatment. A schedule for aging was developed in the present study where 30% increase in strength was obtained by formation of ultrafine α-precipitates in β-matrix. However, presence of α increased the modulus to 87 GPa. Moreover, after aging, the fretting wear resistance showed unexpected decrease due to lower ductility aided by accelerated tribocorrosion of the two-phase microstructure. Finally, the response of the Ti-34Nb-2Ta-0.5O alloy to surface processing was investigated by performing surface mechanical attrition treatment (SMAT) of the β-annealed and α+β aged microstructures. SMAT considerably increased the surface hardness of β-annealed material by forming dislocation substructures on the surface, which also reduced the wear rate after SMAT. In contrast, hardening from SMAT was marginal for aged samples due to deformation-induced coarsening of α. Nevertheless, this was beneficial for the wear behaviour as it improved the ductility by reducing the α + β interfaces. The corrosion resistance improved after SMAT due to formation of a stronger and denser passive layer induced by its defect structure. In addition, the cytocompatibility of the present alloy was found to be excellent in all condition, though no improvement was found after SMAT processing. Taken together, this study shows that surface treated low modulus Ti-Nb-Ta-O alloy can be a promising candidate for orthopedic implant applications.