The superelastic anisotropy in a NiTi shape memory alloy thin sheet

NiTi shape memory alloys are used in practice in their polycrystalline form and usually consist of strong textures. The present research aims to understand the anisotropy in the stress–strain curve of a superelastic NiTi thin sheet. Tensile tests were conducted along various directions between rolling and transverse directions. It was observed for the first time that, although the shape of the stress–strain curve is strongly orientation dependent, the shape recovery strain which is as high as 9% is nearly orientation independent. Through texture and crystallographic analysis, the most favorably oriented martensite variants with respect to each deformation direction were determined and the corresponding theoretical transformation strains were calculated. The relation between the theoretical transformation strain and the shape of the stress–strain curve was further analyzed. Results show that the magnitude of the plateau-strain in the stress–strain curve is determined by the formation of the most favorably oriented martensite variants, while the magnitude of shape recovery strain which is independent of the plateau-strain is determined by the reverse transformation of both favorably and less-favorably oriented martensite variants. Two approaches were taken to determine the most favorable martensite variants and the corresponding theoretical transformation strains. One is based on the resulted transformation strain according to austenite–martensite lattice correspondence, and the other is based on the Schmid factor. Interestingly, both methods have led to the same sets of martensite variant hence the same values of theoretical transformation strain. This is discussed based on recent observations of the stress-induced martensite in NiTi.