From shear bands to kink bands: exploring the plastic deformation mechanisms in NiTi crystalline-amorphous nanocomposites

NiTi crystalline-amorphous nanocomposites unite the merits of both phases to deliver a combination of superelasticity, high strength, and large deformability. In this study, we employ shear-induced amorphization to tailor the fraction and spatial arrangement of amorphous phase in the crystalline-amorphous nanocomposites. Micromechanical testing reveals a non-monotonic dependence of deformability on amorphous fraction: either excessive amorphous fraction or local distribution of low-fraction amorphous phase promote pronounced local plastic deformation in discrete shear bands that precipitate premature fracture. Conversely, an intermediate amorphous fraction delocalizes plasticity, where the evenly distributed crystals arrest unstable shear band propagation. Micropillars containing nano-laminar crystals and amorphous phase achieve a superelastic strain of 4.3 %, a yield strength of 2.2 GPa, and a compressive true strain exceeding 120 % without catastrophic failure. The nano-laminar crystals function as ductile barriers to shear band propagation, leading to strain distribution in the form of diffuse kink bands. In such circumstances, the high deformability is achieved through progressive shear-induced amorphization within the crystalline regions and stable shear deformation within the amorphous phase. These findings highlight the potential of deliberately engineering crystalline-amorphous nanostructures to realize property synergy.