Abstract

First-generation medical titanium alloys like CP Titanium Grade 4 or Ti-6Al-4V (ELI) are important materials in medical engineering, especially in the field of dental implantology which can be explained by their physical, mechanical and chemical properties. A high strength helps to reduce the risk of implant or abutment breakage and allows smaller implant designs. A low Young’s modulus (<70 GPa) is needed to reduce stress shielding which might lead to bone resorption, gap formation between bone and implant, bacterial infections and total implant loss. However, the Young’s modulus of the first-generation implant alloys is still comparably high (100 – 115 GPa) so that second- and third-generation titanium alloys were developed.

One very promising third-generation titanium alloy, Ti-36Nb-2Ta-3Zr-0.3O (TNTZ-O possible phases: α and β - equilibrium phases, α" - martensite with low Young’s modulus, ω - athermal or equilibrium phase with high Young’s modulus), comprises a very low Young’s modulus at high strength in a cold-worked state. At present, this alloy is exclusively produced by powder metallurgy, deformation and annealing, limiting the dimensions of semi-finished parts to diameters of a few millimetres. Hence, only orthodontic wire applications are possible.

Previous studies on TNTZ-O showed that 90% cold working leads to a Young’s modulus of approx. 50 GPa and a strength of more than 1000 MPa. The β-annealed state has a Young’s Modulus of 55 GPa and a strength of 620 MPa. However, in these states, implant applications of TNTZ-O are not feasible as its Young’s modulus is too low. This can be explained by too large elastic deformations of the implant during the implant ingrowth phase resulting in overloading and dying of the newly formed bone cells and poor osseointegration. Furthermore, the 90% cold worked state exhibits non-linear elastic behaviour and an absence of work-hardening. The underlying mechanisms are still unclear. Possible deformation phenomena include stress-induced (reversible) α"-martensite formation, giant faults, dislocations, twins or dislocation channelling with a participation of ω-phase.

Therefore, in the present study, a conventional production route (PAM/CWI) including ingot melting (diameter <120 mm, length <2000 mm), deformation and annealing is developed. This includes the determination of suitable Niobium and Oxygen contents to allow optimisation of the mechanical properties by controlling the microstructure and the volume fractions of the phases (α, α", β). In particular, the impact of Niobium and Oxygen on the mechanical properties in both the cold-worked and β-annealed states and the resulting precipitation behavior of α and ω as well as the formation of the α"-phase were investigated.

It was discovered that α-phase is needed to raise the Young’s modulus from <50 GPa to between 60 and 70 GPa (i.e. the optimal region for dental implants). First, the as-cast material was deformed by rolling. Afterwards, a direct precipitation heat treatment of the α-phase led to equiaxed α-grains needed for good fatigue life. Further strengthening was achieved by precipitating fine dispersed secondary α-phase. The development of the conventional production route described here has shown very promising results so that future dental implant applications of TNTZ-O-alloy are feasible.