Features of Nanosubgrained Structure in Deformed and Annealed Ti-Ni SMA

The nanostructures (NS) formed by post-deformation annealing (PDA) in cold rolled (CR) Ti-Ni SMAs can be graded as follows: a nanograined structure (NGS), a nanosubgrained structure (NSS), and a mixed NS+NG structure. This NS gradation is essential for the understanding of transformation and deformation behavior of SMAs since it underlines different permeability of grain and subgrain boundaries for the movement of interphase boundaries and dislocations. The structure after moderate CR is highly-dislocated, while the dislocation substructure of intermediately or severely deformed alloy already exists in a form of nano-sized subgrains and NG and amorphous structures. The mentioned differences indicate further development of the dynamical processes of substructure formation, when the dynamic recovery stage transforms to the dynamic polygonization stage during deformation. The low-temperature PDA after CR results in the NSS formation: as wide fields of subgrains (initial moderate CR, below e=0.5), submicron-sized NSS regions alternating with NGS regions (intermediate CR, e=0.5-1.5), and rare nano-sized islands in NGS-matrix (severe CR, above e=1.5). The NSS formed either after intermediate and severe CR or after PDA do not differ significantly from each other. It means that dynamic polygonization developed during CR is followed by static polygonization during PDA when subboundaries in the NSS become more perfect, while the subgrain growth is limited. In the case of moderate CR, the subgrains form as a result of only static polygonization of highly dislocated substructure during PDA. The special TEM experiments aimed to distinguish between the NS and NG structures are described.