Effect of cold-drawn-induced amorphous phase on microstructure and pseudoelasticity properties of nanocrystalline NiTi alloys
Qihang Shen, Yu Zong, Yang Wang, Huadong Miao, Junsong Zhang, Jiang Jiang, Xiaobin Shi
Abstract
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.