Novel sub-grain structures in B2 of a cold-rolled TiNi shape memory alloy with unique property

Plastic deformation significantly alters the microstructure and properties of TiNi shape memory alloys. This work investigates the formation of a unique herringbone-like sub-grain structure with extended twin boundaries in the B2 phase of a cold-rolled TiNi_50.8 (at. %) and its impact on the superelasticity of the alloy. Detailed electron back-scatter diffraction (EBSD) analysis reveals abundant sub-grain areas in different orientations distinct from the matrix grain after multiple passes in cold-rolling deformation. Notably, unique band and herringbone structures emerge within the sub-grain areas, with all the boundaries identified as coincident site lattice (CSL) boundaries. Further analysis reveals spontaneous CSL boundary generation following periodic <011>_B2 and <001>_B2 tilt axes perpendicular to the normal direction of the deformed surface in [100]_B2 oriented grains, forming a characteristic quadruple junction within the herringbone structure, distinct from the single [011]_B2 tilt axis identified in the band structure. Transmission electron microscopy (TEM) identifies B2 twin relations corresponding to the special CSL Σ boundaries, while dark-field and high-angle annular dark-field (HAADF) images show B19′ martensitic nanodomains decorating the twin boundaries. This extended twin boundary structure enhances drastically cyclically stable superelasticity characterized by much more limited functional fatigue, 27 % lower modulus and 76 % larger recoverable strain as compared to the solution-treated sample after 100 training cycles. Our study provides new insights into the microstructure evolution and enhancement of the superelastic properties of TiNi SMAs by cold-rolling, offering a novel approach to optimize the functionality of SMAs by introducing special sub-grain structures with martensitic-nanodomains-nested B2 twins through simple cold-rolling process.