Effect of cold-drawn-induced amorphous phase on microstructure and pseudoelasticity properties of nanocrystalline NiTi alloys

Amorphous structures can be induced by cold deformation in the NiTi shape-memory alloys, which transform into nanocrystalline structures after annealing. In this study, the effect of cold-drawn-induced amorphous phase on the microstructure and pseudoelasticity properties of nanocrystalline NiTi alloys was investigated. Ni-49.2 at.%Ti alloy samples were subjected to different cold-drawn deformations. Transmission electron microscopy was used to investigate the microstructure of the samples. Three types of microstructures: twin-related lamellae, nanocrystalline, and amorphous, were observed in the cold-drawn samples. The amorphous volume fractions of the samples were determined using differential scanning calorimetry. The results suggested that the amorphous volume fraction increased with increasing cold-drawn deformation. The pseudoelasticity properties of the samples with different cold-drawn deformations were investigated using tensile tests. Notably, 59.05% cold-drawn deformation, which induced approximately 39.1% volume fraction of the amorphous phase, was the turning point in the performance of the NiTi alloy. The stress hysteresis and upper plateau stress increased with increasing cold-drawn deformation when the cold-drawn deformation was lesser than 59.04%. Furthermore, the difference in stress hysteresis and upper plateau stress was not significant when the cold-drawn deformation was greater than 59.04% because a complete nanocrystalline microstructure could be obtained after annealing when the volume fraction of the amorphous phase reached 39.1% in the NiTi alloy.