Formation of nanostructures in thermomechanically-treated Ti–Ni and Ti–Nb-(Zr, Ta) SMAs and their roles in martensite crystal lattice changes and mechanical behavior

The processes of structure formation in Ti–Ni and Ti–Nb–Zr, Ti–Nb–Ta shape memory alloys under thermomechanical treatment were studied. The thermomechanical treatment comprised cold rolling with true strains from e = 0.25 to 2 and post-deformation annealing in the 300–900 °C temperature range. The differences in these processes between the two groups of alloys were evaluated in terms of their effect on nanostructures formation. The main conclusions are as follows: nanostructures created by thermomechanical treatment are quite useful, as they show a radical improvement of shape memory alloy functional properties. A specific nanostructure (nanocrystalline structure, nanosubgrained structure or their mixture) should be privileged considering other structural and technological factors such as mechanical damage, formation of a brittle second phase, etc., as well as application requirements.

► Severe cold rolling of Ti–Ni SMA creates a mixed amorphous-nanocrystalline structure, whereas severe cold rolling of Ti–Nb-(Zr,Ta) SMA does not imply material amorphization.
► Annealing of the severely deformed Ti–Ni alloy leads to a nanocrystalline structure formed by nanosize grains of two different origins: issued from recrystallization of the amorphous material and resulted from deformation-induced grain refinement.
► Annealing of the moderately deformed Ti–Nb-(Zr,Ta) alloys leads to the nanosubgrain structure in the material. Both nanocrystalline and nanosubgrain structures are actually effective for radical improvement of Ti–Ni and Ti–Nb-(Zr,Ta) SMA functional properties.