Surface mechanical attrition treatment of low modulus Ti-Nb-Ta-O alloy for orthopedic applications.

Surface mechanical attrition treatment (SMAT) is recognized as a surface severe plastic deformation (SPD) method that is effective in improving the surface-dependent mechanical and functional properties of conventional metallic biomaterials. In this study, we aimed to systemically investigate the effect of SMAT on the physical, electrochemical, tribological and biological performances of a newly developed low modulus ß Ti-Nb-Ta-O alloy with two different microstructures, namely, single phase ß-treated and dual phase ß + α aged. The microhardness results showed considerable hardening for the ß-treated condition due to formation of deformation substructures; that was associated with increased corrosion resistance resulting from a stronger and denser passive layer on the surface, as revealed by Tafel polarization, impedance studies and Mott-Scottky plots. The wear volume loss during fretting in serum solution was found to decrease by 46% while friction coefficient decreased only marginally, due to presence of a harder and more brittle surface. In the ß + α condition of the alloy, minimal hardening was observed due to coarsening of the precipitates during SMAT. However, this also reduced the number of α-ß interfaces, which in turn minimized the tendency for galvanic corrosion resulting in lower corrosion rate after SMAT. Wear resistance was enhanced after SMAT, with 32% decrease in wear volume loss and 21% decrease in friction coefficient resulted due to improved ductility on the surface. The attachment and growth of osteoblasts on the alloys in vitro were not affected by SMAT and was comparable to that on commercially pure Ti. Taken together, these results provide new insights into the effects of surface SPD of low modulus ß- Ti alloys for orthopedic applications and underscore the importance of the initial microstructure in determining the performance of the alloy.

Role of aging induced α precipitation on the mechanical and tribocorrosive performance of a ß Ti-Nb-Ta-O orthopedic alloy.

A low modulus ß Ti-Nb-Ta-O alloy was subjected to heat treatment to investigate its phase stability upon aging. The resultant effect on the mechanical and functional properties was systematically evaluated. The aging of the ß-only microstructure, obtained by solutionizing and quenching, resulted in the formation of ultrafine α-precipitates with increasing order of size as the aging temperature increased from 400 °C to 600 °C. The variation in the size of α-precipitates effected the mechanical properties at the three different aging temperature. The highest hardening observed at 400 °C was associated with macroscopic embrittlement, whereas age softening was observed in samples aged at 600 °C due to coarsening of precipitates and softening of the ß-matrix. In contrast, aging at 500 °C resulted in about 32% increase in tensile strength from the ß-solutionized condition. As the samples aged at 500 °C showed optimum combination of mechanical properties among the aged samples, these were further characterized for their electrochemical, tribological and biological responses. The fretting wear studies showed that the wear rate of the solution-treated samples increased after aging due to the higher corrosion rate leading to a higher rate of tribocorrosive dissolution and formation of a transfer layer harder than that of solution treated sample. The Ti-Nb-Ta-O alloy supported the attachment and proliferation of osteoblasts similar to that on commercially pure Ti. Taken together, this work provides new insights into the preparation of next-generation Ti alloys for biomedical applications with high strength and low modulus through microstructural control induced by heat treatment.