Comparative study of structure formation and mechanical behavior of age-hardened Ti-Nb-Zr and Ti-Nb-Ta shape memory alloys

This work sets out to study the peculiar effects of aging treatment on the structure and mechanical behavior of cold-rolled and annealed biomedical Ti-21.8Nb-6.0Zr (TNZ) and Ti-19.7Nb-5.8Ta (TNT) (at%) shape memory alloys by means of transmission electron microscopy, X-ray diffractometry, functional fatigue and thermomechanical testing techniques. Dissimilar effects of aging treatment on the mechanical behavior of Zr- and Ta-doped alloys are explained by the differences in the omega-phase formation rate, precipitate size, fraction and distribution, and by their effect on the alloys' critical stresses and transformation temperatures. Even short-time aging of the TNZ alloy leads to its drastic embrittlement caused by overaging. On the contrary, during aging of the TNT alloy, formation of finely dispersed omega-phase precipitates is gradual and controllable, which makes it possible to finely adjust the TNT alloy functional properties using precipitation hardening mechanisms. To create in this alloy nanosubgrained dislocation substructure containing highly-dispersed coherent nanosized omega-phase precipitates, the following optimum thermomechanical treatment is recommended: cold rolling (true strain 0.37), followed by post-deformation annealing (600 degrees C, 15-30 min) and age-hardening (300 degrees C, 30 min) thermal treatments. It is shown that in TNT alloy, pre-transition diffraction effects (diffuse reflections) can mask the beta-phase substructure and morphology of secondary phases. (C) 2015 Elsevier Inc. All rights reserved.