Effect of Cold Drawing and Annealing in Thermomechanical Treatment Route on the Microstructure and Functional Properties of Superelastic Ti-Zr-Nb Alloy

In this study, a superelastic Ti-18Zr-15Nb (at. %) alloy was subjected to thermomechanical treatment, including cold rotary forging, intermediate annealing, cold drawing, post-deformation annealing, and additional low-temperature aging. As a result of intermediate annealing, two structures of & beta;-phase were obtained: a fine-grained structure (d & AP; 3 & mu;m) and a coarse-grained structure (d & AP; 11 & mu;m). Cold drawing promotes grain elongation in the drawing direction; in a fine-grained state, grains form with a size of 4 x 2 & mu;m, and in a coarse-grained state, they grow with a size of 16 x 6 & mu;m. Post-deformation annealing (PDA) at 550 & DEG;C for 30 min leads to grain sizes of 5 & mu;m and 3 & mu;m, respectively. After PDA at 550 & DEG;C (30 min) in the fine-grained state, the wire exhibits high tensile strength (UTS = 624 MPa), highest elongation to failure (& delta; & GE; 8%), and maximum difference between the dislocation and transformation yield stresses, as well as the highest superelastic recovery strain (& epsilon;(SE)(r) & GE; 3.3%) and total elastic + superelastic recovery strain (& epsilon;(el+SE)(r) & GE; 5.4%). Additional low-temperature aging at 300 & DEG;C for 30-180 min leads to & omega;-phase formation, alloy hardening, embrittlement, and a significant decrease in superelastic recovery strain.