Tensile and Corrosion Properties of Anodized Ultrafine-Grained Ti-13Nb-13Zr Biomedical Alloy Obtained by High-Pressure Torsion

Abstract

Severe plastic deformation (SPD) is a popular group of techniques applied to achieve the nanostructuring of the metallic biomaterials and improvement of their mechanical characteristics. One of the most commonly used SPD methods is the high-pressure torsion (HPT) technique which enables the obtainment of the microstructure with small grains and high strength. In the present study, the influence of the plastic deformation and surface modification treatment on the tensile and corrosion properties of the Ti-13Nb-13Zr (wt%) alloy is investigated. In that purpose, the coarse-grained (CG) Ti-13Nb-13Zr (TNZ) alloy was subjected to the HPT processing by applying a pressure of 4.1 GPa with a rotational speed of 0.2 rpm and 5 revolutions at room temperature to obtain the ultrafine-grained (UFG) microstructure. The alloy microstructure before and after HPT processing was analysed using the scanning electron microscopy (SEM) and the X-ray diffraction (XRD). The homogeneity of the UFG TNZ alloy was determined by microhardness testing and microscopic observations. The nanotubular oxide layer on the surface of the TNZ alloy, both in CG and UFG condition, was formed by electrochemical anodization in 1 M H3PO4 + NaF electrolyte for 90 min. SEM analysis was used to characterise the morphology of the anodized surfaces, while energy dispersive spectroscopy was applied to determine the chemical composition of the nanostructured layers formed at the alloy surfaces. Mechanical properties of the TNZ alloy, before and after HPT processing and electrochemical anodization, were determined by tensile testing. After tensile testing, the fractographic analysis was conducted to identify the fracture mechanisms. The potentiodynamic polarization technique was used to determine the corrosion resistance of the alloy before and after plastic deformation and surface modification treatment. The obtained results showed that the alloy is reasonably homogeneous after the HPT processing. The XRD analyses reviled the presence of alpha ' and beta phases in the CG TNZ alloy microstructure, while the additional omega phase was detected in the microstructure of the UFG TNZ alloy. The HPT obtained alloy exhibits higher hardness and improved tensile properties than the alloy in the as-received CG condition, while the electrochemical anodization leads to a decrease of its mechanical properties. Both CG and UFG alloys show excellent corrosion stability in Ringer's solution. Moreover, electrochemical anodization leads to a decrease or an increase of the corrosion resistance of these materials, depending on the morphology of the formed nanotubular surface layers. The results indicate that the anodized CG TNZ alloy is characterized by a lower modulus of elasticity and better corrosion resistance properties than the anodized UFG TNZ alloy.