Structure and Superelasticity of Novel Zr-Rich Ti-Zr–Nb Shape Memory Alloys

Two novel superelastic Ti-41Zr-8(10)Nb alloys were studied and compared to the reference Ti-18Zr-15Nb alloy (all in at. %) in terms of their microstructure and mechanical properties. A thermomechanical treatment consisting of cold rolling and post-deformation annealing was applied to all the alloys to create conditions for their superelastic behavior at room temperature. X-ray diffraction analysis demonstrated considerably larger lattice distortions of both parent and martensitic phases in Zr-rich alloys that resulted in significantly higher theoretical limits of recovery strain in these alloys (~ 8.0%) as compared to the reference alloy (~ 5.5%). However, during room temperature fatigue testing, the novel alloys accumulated considerable residual strains and showed a relatively weak fatigue resistance caused by the presence of notable quantities of α″-phase at this temperature. Conversely, the reference alloy containing only β-phase at the temperature of testing and therefore, a more favorable phase composition at the testing temperature manifested a better superelasticy, and therefore, a better fatigue resistance. Nonetheless, an excellent combined effect of shape memory and superelasticity in Zr-rich alloys indicates that their room temperature superelasticity could be improved via an additional adjustment of their chemical composition.