Orientation Effect On the Inhomogeneous Stress-Induced Martensitic Transformation In NiTi Alloys

In-situ electron backscatter diffraction (EBSD) studies were carried out to investigate the orientation effect on the Lüders-like deformation in NiTi shape memory alloys. Four specimens were cut from a NiTi plate along directions 0, 30, 60 and 90^o deviate from the rolling direction (RD). The texture analysis showed that the grains orientations distribute dispersively along the loading axis. Uniaxial tensile tests showed that the martensitic transformation proceeded in form of Lüders deformation band (LDB) in all of the four specimens. A crystallographic model, based on the consideration of the grain orientation and transformation crystallography, was suggested to predict the nucleation and propagation of LDB. For the specimens studied, the preferred normal direction was <111>. The shear angle of the LDB to the loading axis was predicted to vary from 32.1/36.6^o in specimen with LD of <01-1> to 66.6^o in specimen with from for loading axis of <01-1> to for loading axis of <11-2>.