Structural characterization of Ti90Al6V4 alloy and sulphur segregation

Abstract

Ti90Al6V4 alloy is used in orthopaedic medical implant applications because of its biocompatibility, corrosion resistance and mechanical load resistance. We characterized the time evolution of the surface composition of TiAlV, the surface roughness and the microstructure, which influence the formation of a suitable tissue-implant interface. For this we used Auger electron spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffraction. The impurity diffusion was accelerated by annealing in ultra high vacuum. We found sulphur segregation from the bulk to the surface. The value of the activation energy of 150 kJ/mol allows us to conclude that diffusion at room temperature is too slow to influence the composition during the implant lifetime. The microstructure of the alloy is equiaxed at room temperature and is modified by annealing above 950°C with the appearance of a lamellar phase. In the diffractograms after the treatments performed in our study we always observed only the α-phase peaks. During annealing in air the surface oxide thickens and increases its vanadium content compared to the bulk alloy. This finding is relevant for high temperature applications, but also for the machining treatments on devices applied at room temperature since the possible surface heating during wear processes could modify the surface composition.