Superelastic anisotropy and meso-scale interface regions in textured NiTi sheets

Abstract

Typical manufacturing processes of NiTi shape memory alloys (SMAs) involve drawing and rolling operations leading to crystallographic texture and superelastic anisotropic behavior. Therefore, an in-depth understanding of directional material properties and underlying mechanisms may help to guide the production process to create suitable textures for applications requiring different mechanical properties. During stress-induced martensitic transformation (SIMT), macroscopic deformation occurs only in the mesoscale interface regions between fully martensitic and fully austenitic regions. In case of localized deformation, the detailed characterization of these transition regions is therefore of crucial practical importance. In this study, the effects of crystallographic texture and specimen geometry on mechanical/localized deformation behavior of polycrystalline NiTi superelastic sheets were systematically analyzed. Local surface strain fields are recorded during deformation by digital image correlation (DIC) in order to characterize interface regions in detail. The results show that the mechanical behavior has a clear directional dependency, and that the orientation/texture effect is reduced with decreasing width/thickness ratio of specimen. For different sample geometries and orientations, martensite/austenite interface has different angles which changes during SIMT. The results provide new insights into the interaction of crystallographic texture, SIMT characteristics, superelastic anisotropy and specimen geometry which is essential for engineering applications of NiTi.