Size Effect of Shape Memory Nanoparticles Studied by Constructing Size-Stress-Temperature Phase Diagram

Recently, nanoscale shape memory alloys (SMAs) have attracted special attention because of their unique characteristics. In particular, previous studies have focused on the phenomenon that the signal of shape memory behavior disappears below the critical size. In this study, we suggested a novel method to examine the size effect of shape memory alloys at the nanoscale by fabricating free-standing nanoparticles based on liquid-liquid phase separation in Ni-Ti-Gd system. We systematically performed thermal analysis and <i>in situ </i>mechanical tests in scanning electron microscope to understand martensitic phase transformation behaviors and carefully compared with the data of molecular dynamic simulation to analyze the mechanical behavior of the nanoparticles. Interestingly, the isotropic polycrystalline NiTi-based shape memory alloy particles show size-dependence of 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 to fabricate isotropic polycrystalline shape memory alloy nanoparticles and novel guidelines for studying the size effect of shape memory alloys. We also expect that the results provide an effective guideline for constructing the size-stress-temperature phase diagram, which can be used as an indicator that can determine the other one when two of the three factors, transition temperature, stress, and size are determined. Finally, we expect the study can accelerate the design and practical uses of shape memory alloys not only through composition but also through size.