Atomistic Simulations on Phase Transformation and Deformation Behaviors of Shape-Memory Alloys at the Nanoscale

Martensitic phase transformation has received great interest owing to its broad academic and technological relevance. A representative class of materials utilizing the martensitic phase transformation is shape-memory alloys (SMAs). SMAs are widely used in many applications by virtue of their unique shape-memory effect and superelasticity, which can be controlled by reversible temperature- and stress-induced phase transformations, respectively. As applications of SMAs recently entered the arena of smart materials for micro- and nano-electromechanical systems (MEMS/NEMS) and nano-composite materials with exceptional mechanical properties, their distinctive phase transformation and deformation behaviors at the small-scale have attracted particular attention. For example, a noticeable common characteristic of phase transformations in nanoscale SMAs is an over-stabilization of the austenite phase with respect to the martensite phase. If the system size approaches the nanometer scale, a decrease of the transformation temperature is commonly observed for various kinds of SMAs regardless of the boundary conditions, e.g., nanocrystalline SMAs, shape-memory nano-precipitates embedded in a stiff non-transforming matrix, and freestanding shape-memory nanoparticles.<br/>In this study, detailed mechanisms of the phase transformation and deformation behaviors at the nanoscale are investigated using atomistic simulation techniques such as the molecular dynamics. For this purpose, interatomic potentials capable of reproducing the martensitic phase transformation were developed based on the modified embedded-atom method (MEAM) model. As a fitting method of potential parameters, the force-matching method based on the density functional theory (DFT) calculation was used. The resulting interatomic potentials reproduce accurately the temperature- and stress-induced martensitic phase transformations of NiTi SMAs as well as various fundamental physical properties. Subsequent molecular dynamics simulations verified that developed potentials can be successfully applied to provide insights into the atomic details of the phase transformation and deformation behaviors of various SMA-related materials.