The development and characterization of new Ti-based implant alloys has received a continuously growing research effort in the last two decades. Main aspects are use of solely non-toxic elements as well as tailoring mechanical strength and biocompatibility, especially the Young’s modulus (E) to achieve minimization of “stress-shielding-effects”, higher reliability and faster healing processes. Understanding the complex behavior under cyclic mechanical loading is thereby essential to assess the full potential of this alloys while comparing them with actual implant alloys, like Ti-6Al-4V-(ELI) or cp-Ti.
For this purpose, S-N curve based load-controlled fatigue tests under tension-compression (R =-1) were realized in this study on a recently developed binary Ti-40Nb alloy with an ultralow Young’s modulus. The alloy was cast, normalized and thermomechanically treated (cold-rolling + annealing steps). An aged condition (E= 72 GPa) with nanometer-sized ω-precipitates in a fully recrystallized ß-matrix as well as a recrystallized condition (E= 60 GPa) without subsequent precipitation annealing was studied. Electrochemically polished surfaces minimized the influence of roughness related notch effects and served as ideal condition for high resolution EBSD investigations before and after cyclic loading.
Results show a fatigue range from σa = 225-325 MPa with a sensitive dependence on the ß-phase stability and thus, occurring deformation mechanisms. The lower fatigue strength of the recrystallized state is hereby characterized by a significant development of stress-induced α’’-phase due to cyclic loading without slip band formation. The Ti-40Nb alloy in the aged condition is strongly stabilized due to ω-precipitates, which consequently inhibit α’’-phase formation, leading to higher values of fatigue strength through effective precipitation hardening.
Additional tests under in-vitro conditions (Ringer solution, 37°C) to evaluate the influence of corrosion fatigue effects are currently carried out. Up to date no distinct differences compared to the tests in lab-air were observable, hence displaying the good corrosion fatigue resistance of this alloy.
This work is funded by the DFG - SFB/Transregio 79 under project M1.