Nanostructuring Ti-Alloys by HPT: Phase Transformation, Mechanical and Corrosion Properties, and Bioactivation

Due to their unique properties, titanium (Ti) and Ti-alloys are particularly suitable for biomedical devices. Ti has a high specific strength and low Young's modulus (reducing stress shielding), high corrosion resistance, and superior biocompatibility. However, Ti's moderately low Young's modulus (100-110 GPa) is still considerably higher than to bones (5-30 GPa). The /3-Ti phase, whose elastic modulus is closer to the bone, can be kept by increasing the contents of non-toxic /3 stabilizing elements. Besides stress shielding and corrosion resistance, adjusted bioactivity (bone-bonding ability) is another primary prerequisite for implants that can be improved by ultrafine-grained (UFG) microstructures and surface modification (anodization and acid+alkaline treatment). UFG by HPT also enhances wear resistance and mechanical properties. Representative alloys (Ti-6Al-7Nb (TAN), Ti-13-Nb-13Zr (TNZ), and Ti-35Nb-7Zr-5Ta (TNZT)) and cp-Ti, were presented in this overview. Samples started with different phases and morphologies. Deformation by HPT induced phase transformation in the alloys, which depended on the amounts of a or /3 stabilizers, the strain rate, applied loads, and starting phases and a morphologies. Grain sizes were reduced to about 120 nm. Mechanical properties depended mainly on the number of grain boundaries and their nature and different phases, sizes, and strengths. Young's modulus diminished when the /3 was increased. Polished surfaces and cp-Ti presented similar corrosion resistance, improved by surface treatments, which reached maximum protection in anodized samples processed by HPT. After bioactivity tests, different growth rates for various processing conditions and alloys were observed, the highest for the TNZ alloy, and improved after HPT processing.