Özet:

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. It is well known that superelastic NiTi SMAs exhibit localized deformation during uniaxial tensile loading. During the stress-induced martensitic transformation (SIMT), macroscopic deformation occurs only in the meso-scale interface regions between the fully martensitic and the 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 behavior and local deformation characteristics 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 material behavior has a clear directional dependency and that the orientation/texture effect is reduced with decreasing width/thickness ratio of specimen. Furthermore, it is shown that the martensite/austenite interface formed in localized deformation has different angles to the load direction for different sample geometries and orientations and that the interface angle changes continuously 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.

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