Characterization of the Superelastic and Structural Characteristics of β-Ti Alloys by Strain-Controlled Cycling after Thermomechanical Processing and Subsequent Ageing

In this comparative study, the structural and superelastic characteristics of two thermomechanically treated metastable Ti-Nb based (Ti-22Nb-6Zr) and Ti-Zr based (Ti-18Zr-14Nb and Ti-18Zr-13Nb-2Ta (at. %)) alloy systems were studied. To study the influence of room temperature storage on the functional properties of these two alloy systems, the alloys were subjected to a multistage testing routine consisting of four ten-cycle loading-unloading testing series alternated with three room temperature ageing periods (1, 5 and 20 days). Based on microstructure-properties relationships, it was shown that for each alloy system, the forward stress-induced martensitic transformation was essentially dependent on the material microstructure, whereas the subsequent reverse martenstic transformation was controlled by the material composition. The Ti-Zr based alloys demonstrated more stable functional behavior than their Ti-Nb based counterparts. More specifically Ti-18Zr-13Nb-2Ta, subjected to a combination of cyclic training alternated with room temperature ageing showed a significant improvement in superelastic behavior with small accumulated strains and narrow stress hysteresis.