Fatigue Properties of Metastable Beta Ti-22Nb-6Zr (at%) Alloy for Load-Bearing Biomedical Applications

Metastable beta Ti-Nb-based alloys have attracted significant interest of the scientific community due to their outstanding suitability for load-bearing biomedical applications. In addition to high strength, low elastic modulus, good deformability and superior corrosion resistance, these alloys can exhibit biomimetic superelastic behavior at the human body temperature, if they are subjected to an adequate thermomechanical treatment (TMT).

Ti-22Nb-6Zr (at%) alloy was subjected to TMT comprising cold rolling (true strain of 0.3) and post-deformation annealing (PDA) at 450 (1h), 600 and 750°C (0.5h). The obtained specimens underwent strain-controlled tension-tension fatigue testing in the 0.2 to 1.5% strain range. The alloy structure and phase composition were studied before and after testing using XRD and TEM techniques.

It was found that for the smallest applied strain (0.2%), a well-developed partially-recovered dislocation substructure of b-phase with small quantity of a-phase (PDA at 450°C) led to the highest fatigue life (run-out at 10⁶ cycles). For higher strains (0.3…1.5%), a polygonized dislocation substructure of b-phase (PDA at 600°C) showed superior fatigue performance, due to the involvement of the reversible stress-induced b↔a’’ transformation and therefore superelasticity in the process of cyclic deformation.