Characterization of the Σ9(114)[110] Symmetric Tilt Grain Boundary in NiTi and Its Relationship to the Martensitic Transformation

Abstract

Shape memory alloys are used in many applications which require them to undergo numerous transformation cycles. Generally, an important property for use in such cyclic applications is a small hysteresis, which is linked to functional fatigue resistance. Leveraging microstructural features which promote martensite nucleation is one strategy to achieve reduced hysteresis. In the austenite phase of NiTi, the Σ9(114)[110] symmetric tilt grain boundary has been recognized as one such feature. We have performed a series of molecular dynamics simulations to characterize this grain boundary and its relationship to the martensitic transformation. Upon thermal equilibration, even above the transformation temperature, the grain boundary spontaneously forms a twinned martensite structure at its core, which serves as a nucleus during the martensitic transformation. When the grain boundaries are near one another, the energetic barrier to the transformation is reduced and a small hysteresis results. In polycrystalline microstructures, the added constraints lead to an expanded transformation window and retained austenite upon cooling. Based on these results, grain boundary engineering could be an effective strategy to produce shape memory alloys with improved performance in cyclic applications.