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The aim of the present work was first to develop and validate a test bench that simulates the in vitro conditions to which the biomedical implants will be actually subjected in vivo. For the preliminary application assessments, the strain-controlled fatigue tests of biomedically pure Ti and Ti-Nb-Zr alloy in simulated body fluid were undertaken. The in situ open-circuit potential measurements from the test bench demonstrated a strong dependence on the dynamic cycling and kind of material under testing. The results showed that during fatigue cycling, the passive oxide film formed on the surface of Ti-Nb-Zr alloy was more resistant to fatigue degradation when compared with pure Ti. The Ti-Nb-Zr alloy exhibited prolonged fatigue life when compared with pure Ti. The fractographic features of both materials were also characterized using scanning electron microscopy. The electrochemical results and the fractographic evidence confirmed that the prolonged functional fatigue life of the Ti-Nb-Zr alloy is apparently ascribable to the reversible martensitic phase transformation.
INTRODUCTION
Among the metallic biomedical materials, the pure Ti, Ti-6Al-4V, and Ti-Ni alloys have been investigated extensively for their diverse applications in the orthopedic and dental fields. Nevertheless, for the long-term survival of indwelling implants, the hypersensitivity of nickel and toxicity of vanadium has become an issue of widespread concern.1 On the other hand, although pure Ti (also known as CP Ti) has excellent biocompatibility, its applications are limited because of its poor mechanical properties.1,2 Therefore, to circumvent these challenging problems, rigorous endeavors have been undertaken to explore the new generation of Ni- and V-free biomedical alloys.2-6 In this context, superelastic beta titanium alloys seems conducive to future implant applications.2-11
Nevertheless, during service, the implants are commonly subjected to cyclic fatigue under the corrosive physiological conditions of the human body.2,5,12 In fact, the destructive nature of the corrosion-assisted fatigue cracking and overall fatigue degradation can incapacitate the long-term performance of implants.12-15 Therefore, it is vital to investigate the functional fatigue life under physiological conditions. Despite all its practical relevance, the specific pseudo-physiological test bench for evaluating the functional fatigue life of the superelastic beta titanium alloys and corresponding in situ electrochemical results revealing the impact of cycling on the corrosive and passive behavior for this kind of alloys are sparse in the literature and hence...