Tailoring NiTi-Based Shape Memory Alloy Characteristics Based on the Extrinsic Size Effect

Nanoscale shape memory alloys (SMAs) have recently attracted special attention because of their unique shape recovery characteristics. In particular, previous studies have focused on the phenomenon of the signal of shape memory behavior disappearing below the critical size. However, examining the deformation and phase transformation behaviors of SMAs at a small scale, considering the extrinsic size effect, has been challenging due to issues such as ion damage formed during focused ion beam sampling. Thus, in this study, we tried to investigate the extrinsic size effect of polycrystalline NiTi-based SMAs by overcoming the limitations of the conventional experimental method. In particular, we suggest a novel fabrication method of NiTi-based free-standing SMA nanoparticles based on a liquid-to-liquid phase separation and selective leaching in Ni-Ti-Gd system, which is inspired by reported processing techniques of the bulk metallic glass. We utilized the melt-spinning technique to obtain liquid-to-liquid phase-separated microstructure. Due to the gradient in the cooling rate across the entire specimen, a droplet structure with various sizes was formed, ultimately producing particles in the size range of 100-1600 nm after selective leaching. The extrinsic size effect of obtained particles is then examined by conducting in-situ mechanical tests on individual particles within an electron microscope. Then, we carefully compared the experimental results with the results of molecular dynamics simulation to analyze the mechanical behavior of the nanoparticles. Interestingly, the isotropic polycrystalline NiTi-based shape memory alloy particles show size dependence on recoverable strain, which shows decreasing recoverable strain as the particle size increases until the critical size. The origin was a size-dependent transition temperature studied by molecular dynamics simulation. This study provides a theoretical basis for fabricating isotropic polycrystalline shape memory alloy nanoparticles and novel guidelines for studying the size effect of shape memory alloys, excluding any unintended effects such as ion beam damage. We also expect that the results will provide an effective guideline for tailoring shape memory alloy characteristics based on the extrinsic size effect, especially the constructed size-stress-temperature phase diagram. Finally, we expect the study to accelerate the design and practical uses of shape memory alloys not only through composition but also through size.