Roles of preliminary thermal routes and β→ω transformation mechanisms in formation of ωiso-phase in beta-Ti-Nb-Zr SMA

The effect of preliminary thermal route conditions, specifically cooling and heating rates, and subsequent aging temperatures and times, on the β → ω iso transformation features is studied in a Ti-22Nb-6Zr (at.%) shape memory alloy in a polygonized substructural state. Three preliminary thermal routes before aging at 300 °C or 375 °C for 5 or 180 min were applied: (1) water cooling to RT followed by fast heating; (2) slow cooling to RT followed by fast heating and (3) slow cooling to RT followed by slow heating. The evolution of phase composition and in-grain lattice distortion rated as in-grain lattice misorientation was studied using EBSD and XRD techniques. To estimate the contribution of diffusion effects to the ω iso -phase formation, the Zr diffusion path was calculated, and to evaluate the effect of ω -phase precipitation on mechanical and functional behavior, static and cyclic tensile tests were performed. The preliminary thermal route significantly affects subsequent ω iso -phase precipitation. A two-scale inhomogeneity of lattice distortions was detected after preliminary TMT. Subsequent aging eliminates macroinhomogeneity in the initial two-scale lattice distortion and leads to a single-scale microinhomogeneous in-grain lattice distortion. Comparison of the semi-quantitative XRD estimated changes in the ω iso -phase amount with the calculated Zr diffusion paths shows that the diffusion path alone cannot explain the observed peculiarities of ω iso -phase precipitation. Thus, both shear and diffusion contributions should be considered in the β → ω iso transformation. Heat treatment routes including any slow steps and/or prolonged high-temperature aging cause pronounced ω -phase precipitation and embrittlement, whereas fast cooling-heating with short aging leads to the appearance of the stress-induced β → α″ martensitic transformation plateau and manifestation of the transformation-induced plasticity (TRIP) effect and does not lead to pronounced embrittlement.