Ternary Ti-Zr-Nb and quaternary Ti-Zr-Nb-Ta shape memory alloys for biomedical applications: Structural features and cyclic mechanical properties

Ti-18Zr-14Nb, Ti-18Zr-13Nb-1Ta, Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta (at%) shape memory alloys were obtained by vacuum arc remelting and subjected to different thermomechanical treatment (TMT) schedules. The transmission electron microscopy analysis revealed that a TMT, which consisted of cold rolling with a true logarithmic strain of e = 0.3 and post-deformation annealing at 600 °C (30 min), led to a subgrained structure formation with a size of structural elements of 200-300 nm and to a room temperature superelastic behavior. The X-Ray diffraction (XRD) analysis showed that a gradual replacement of Nb content by its Ta equivalent decreases the impact of alloying elements on the β-phase stabilization phenomenon and suppresses the ω-phase formation: at room temperature, Ti-18Zr-14Nb and Ti-18Zr-13Nb-1Ta alloys contain mainly parent β-phase, while Ti-18Zr-12Nb-2Ta and Ti-18Zr-11Nb-3Ta alloys contain considerable amount of α”-martensite. The crystallographic resource of recovery strain for all the alloys exceeds 5% and reaches 6%, for the Ti-18Zr-11Nb-3Ta alloy. The fatigue life of the Ti-18Zr-13Nb-1Ta alloy is almost twice as high as that of the Ti-18Zr-14Nb alloy. During multi-cycle mechanical testing, the following regularity appeared for all the alloys tested: during 10-15 first loading-unloading cycles, the mechanisms of plastic deformation and stress-induced martensite formation and stabilization prevail, while after that, the β⇄α” martensitic transformation mechanisms become dominant, and the alloys' behavior turns from elasto-plastic into superelastic.