Effect of Niobium and Oxygen Contents on Microstructure and Mechanical Properties of α+β-type Ti-(5–25)Nb-(0.5–1)O (mass%) Alloys for Biomedical Applications

The microstructures, mechanical properties, and biocompatibilities of α+β-type Ti-Nb alloys with a wide range of Nb and oxygen contents were investigated. Ingots of Ti-(5–25)Nb-(0.5–1)O (mass%) alloys were prepared by arc melting and hot forging. The forged bars were heat-treated at 873–1373 K for 3.6 ks followed by quenching. The volume fraction of equiaxed α-phase (f_α) in the alloys was determined experimentally. The Ti-5Nb-yO and Ti-(15–20)Nb-yO alloys contained α'- and α"-martensite, respectively, while the Ti-10Nb-yO alloys contained both α'- and α"-martensite. Therefore, changing the Nb content alters the α'/α" ratio. The addition of oxygen increased the distribution coefficient of Nb and accelerated Nb distribution in the β-phase, which increased the boundary temperature for the formation of α'- and α"-martensite in the Ti-5Nb-yO alloys. The Ti-5Nb-(0.5–0.75)O alloys with f_α value of 0.5 had higher elongation and strength and a lower elastic modulus than Ti-6Al-4V alloy. Ion elution from the Ti-5Nb-(0.5–0.75)O alloys into 0.1 M NaCl–0.1 M lactic acid solution was comparable to that of a Ti-6Al-4V extra low interstitial (ELI) alloy. The Ti-5Nb-(0.5–0.75)O alloys did not exhibit cytotoxicity, indicating their excellent biocompatibility. Thus, we propose that Ti-5Nb-(0.5–0.75)O alloys are suitable for use as low-cost α+β-type Ti alloys for biomedical applications.