Evolution of martensitic microstructures in nanocrystalline NiTi wires deformed in tension

An experimental method allowing for reconstruction of martensite variant microstructures evolving during tensile thermomechanical loading test on nanocrystalline NiTi wire is introduced. The method is based on the determination of the location, size and orientation of all martensite variants and interfaces within a selected polycrystal grain using post mortem selected area electron diffraction with dark field image analysis in TEM. Martensitic microstructures in the NiTi wire deformed up to the end of reorientation (transformation) plateau up to ∼7% strain at room temperature (100°C), respectively, were found to contain single domain (001) compound twinned martensite filling whole grains of the deformed wire. This observation was rationalized by theoretical treatment of strain accommodation in fiber textured NiTi wire deformed in tension predicting such singular microstructural state in grains of NiTi wire deformed up to 6.74% strain. Upon subsequent unloading and stress free heating above the A_f temperature, this martensitic microstructure retransforms back to the parent austenite yielding recoverable strains typical for NiTi wires (∼6%) accompanied by very small unrecovered strain (∼0.6% - 1.5% depending on the test temperature). On further tensile loading up to 15% strain, plastic deformation of oriented martensite proceeds by coordinated (100) and (20$\overline{1}$) deformation twinning in martensite assisted by [1 0 0](0 0 1) dislocation slip giving rise to characteristic martensitic microstructures containing deformation bands frequently forming wedges within a single martensite lattice. Upon subsequent unloading and stress free heating above the A_f temperature, this martensitic microstructure transforms to {114} twinned austenitic microstructure yielding very large recoverable strains (∼10%) accompanied by large unrecovered strains (∼5%). It is claimed that the dislocation slip assisted (100) and (20$\overline{1}$) deformation twinning in martensite renders NiTi excellent combination of strength and deformability and leads to refinement of austenitic microstructure accomplished via introducing {114} austenite twins into it.