Effects of Cr addition on Ti implant alloys (Ti-Cr/Ti-Al-V-Cr) to enhance corrosion and wear resistance

Due to their excellent biocompatibility, favorable strength-to-weight ratio and mechanical properties, Ti-based alloys are most commonly used for long-term implants in the human body. Nevertheless, low wear resistance and increased degradation due to corrosion under critical in vivo conditions impair the service life of these implants. This fact opens the potential for optimization, which can be exploited by chemical alloying with Cr. This study investigates the effect of Cr alloying on the mechanical, tribological, corrosion properties and cytocompatibility of cp Ti and TiAl6V4 alloys. Argon-arc melting was used to cast binary and quaternary specimens of varying Cr content (0.1, 0.2, 0.4, 1, 2, 4, 8, 10, 15 and 20 m%). After homogenization (1100 °C, 30 min), microstructures were characterized by means of XRD and EBSD and correlated with mechanical properties using hardness and compression tests. At up to 2 m% Cr, a martensitic α' microstructure is formed. A Cr content of 4 m% reveals two phase α' + β alloys. Alloying with ≥8 m% Cr results in complete β phase, whereas the significantly reduced fracture compression indicates the formation of metastable ω phase for Cr content of 8-10 m%. Based on XPS analysis, a change in the composition of the passive layers by incorporation of Cr₂O₃ and CrO_x is verified. These modified passive layers result in a reduction in corrosion current densities under mimicked severe inflammatory conditions (PBS with HCl and H₂O₂). In addition, the tribological behavior is significantly improved by a reduced wear rate for binary Ti-2/4Cr and quaternary TiAlV-4/8Cr alloys. Cell viability is not inhibited by Cr alloying, but reduced calcification is observed for all Cr modified specimens. These findings highlight the tremendous potential of Ti alloying with Cr for improved implant properties, with the alloy range of 2-4 m% Cr being the most suitable.