Microstructure and properties of superelastic Ti-18Zr-15Nb alloy subjected to combination of moderate/severe cold drawing and post-deformation annealing

Ti-18Zr-15Nb (at%) shape memory alloy was subjected to a thermomechanical treatment (TMT), combining cold drawing (CD) with moderate (e = 0.5/1.1), and severe (e = 1.9/3.3) true strains, and post-deformation annealing (PDA) at 500-600 degrees C to fabricate a wire for medical application. The phase composition, microstructure, crystallographic texture, mechanical and functional properties were studied. The combination of cold drawing with true strain e = 0.5 and PDA at 550 degrees C for 5 min showed remnants of the original deformed grains, inside of which there is apparently a polygonized substructure, and a partially recrystallized structure of beta-phase with a low amount of alpha-phase (grain size of D approximate to 8 mu m). With an increase in true strain, annealing temperature and holding time, a fully recrystallized structure and grain growth were observed. After TMT including the moderate cold drawing, a crystallographic texture with preferable orientation in the <101>(beta) direction was formed(.) As true strain increases to e = 1.9/3.3, the preferable orientation changes to the <210>(beta) direction; the intensity of this crystallographic texture increases as the true strain increases. Maximum difference between the dislocation and transformation yield stresses was observed after CD with true strain e = 1.1 and PDA at 550 degrees C for 30 min (Delta sigma approximate to 405 MPa). Elongation to failure about 20 % was observed after CD with a true strain of e = 0.5 and PD & Acy; at 600 degrees C for 30 min (D approximate to 15 mu m). During superelastic cyclic testing with 4 % applied strain after CD (e = 1.9-3.3) and PDA at 550 degrees C for 5-15 min (D approximate to 1-3 mu m) the alloy exhibits high values of superelastic (epsilon(se)(r) approximate to 1,8-2,1 %) and total elastic+superelastic (epsilon(el+se)(r) approximate to 3,3-3,5 %) recovery strains as well as sufficient maximum tensile stress (sigma(max) approximate to 441-472 MPa), and accumulated residual strain (epsilon(acc) approximate to 0.4-0.7 %).