Synergistic Size–Surface Effects on Martensitic Transformation of Shape Memory Alloy Nanorods for Micro/Nanoelectro-Mechanical Systems

Abstract

Martensitic transformation (MT) of shape memory alloys (SMAs) deserves promising applications in various smart devices including micro/nanoelectro-mechanical systems owing to the material’s large reversible deformation from cyclic MT. However, such systems always suffer from critical fatigue problems under cyclic loadings with microdamage accumulations closely related to hysteresis energy and residual strain. Here, the synergistic effects of the sample size and surface composition on the MT of SMA nanorods are systematically investigated by combining global responses and atomic evolutions based on molecular dynamics. It is found that the surface energy plays a resisting role in MT and the resistance increases with decreasing sample size, which in turn increases the critical stress, hysteresis, and residual strain. Intriguingly, the nucleated austenite–martensite transition zone during MT can be influenced by surface composition, and an ultralow hysteresis and residual strain with little size-dependence can be achieved in the nanorod with the surface composed of alternate Ni and Ti atoms. The low energy density of the transition zone indicates better microscopic lattice compatibility and smaller dissipations during MT. This reveals that the surface modification is promising for modulating the size effect and improving the reversibility and the fatigue resistance of the superelastic SMA nanostructures.