Shear-induced amorphization in nanocrystalline NiTi micropillars under large plastic deformation

Shear-induced amorphization is a phenomenon observed in polycrystalline NiTi shape memory alloys under severe plastic deformation, but the underlying mechanism of such a microstructural change has remained elusive. To study the isothermal large-strain plastic deformation behavior and the associated mechanisms, uniaxial compression is performed on nanocrystalline NiTi cuboidal micropillars with initial grain sizes of 35 and 110 nm. It is found that the micropillars demonstrate high deformability with plastic strains up to 110% via shear-induced amorphization. It is shown that plastic strain localization in shear bands of the 35 nm-grain-size sample prompts notable amorphization up to 90% and crystal refinement down to 5 nm. High-resolution transmission electron microscope and molecular dynamics simulations reveal that the shear-induced amorphization starts from the local martensite phase near grain boundaries via accumulation of crystalline defects. The amorphization can lead to a reduction in the crystal size and an increase in the plastic flow stress. The large plastic deformation of the nanocrystalline NiTi micropillars via shear-induced amorphization is enabled by the suppression of crack nucleation through dynamic recovery of nanovoids. Our work provides new perspectives for producing crystalline-amorphous nanostructures in shape memory alloys at small scale.