Microstructural and mechanical properties, surface and electrochemical characterisation of a new Ti-Zr-Nb alloy for implant applications

A novel ternary Ti-15Zr-10Nb alloy with innovative microstructural and mechanical properties and very good long-term corrosion resistance in simulated physiological solutions was studied in this paper. Optical and scanning electron microscopy (SEM) observations revealed α + β biphasic Widmanstatten microstructure with characteristic interleaving lamella of α and β phases. Energy dispersive spectroscopy (EDS) measurements gave good compositional homogeneity with wt.% contents of 75 ± 5% for Ti, 15 ± 2% for Nb, and 10 ± 2% for Zr. The XRD pattern was indexed to a biphasic crystalline structure containing the hexagonal α phase with calculated lattice parameters of a = 2.92 Å, c = 4.63 Å and the cubic β phase with calculated lattice parameter of a = 3.19 Å. The alloy has very good mechanical properties: Young's modulus of 64 GPa, very close to that of the human bone; superior ultimate tensile strength, appropriate 0.2% yield strength and strain to fracture to those of Ti. The alloy native passive film has a thickness of 5.5 ± 0.5 nm and contains protective TiO2, ZrO2 and Nb2O5 oxides (X-ray photoelectron spectroscopy - XPS analysis). The cyclic voltammetry in Ringer's solutions indicated a nobler electrochemical behaviour of the alloy in comparison with Ti due to the beneficial influence of the alloying elements, which participate with their oxides to the compactness, thickening, reinforcement of the native passive film. The electrochemical impedance spectra were modelled with an electric equivalent circuit comprising two time constants: for inner, compact layer and outer, porous slightly inferior protective layer.