Deformation behavior associated with different initial phase structures was studied systematically in polycrystalline NiTi shape memory alloy tubes by tensile testing at different temperatures. The nominal stress-strain curves of the tubes from room temperature to 70°C were recorded. It was found that the deformation of NiTi tubes with initial structure of R-phase proceeded by R→M martensitic transformation, while the deformation of NiTi tubes with initial structure of M-phase proceeded by martensitic reorientation. The R→M martensitic transformation was realized by nucleation and growth of cylindrical martensitic bands, while no macroscopic martensitic band was observed during martensitic reorientation. Further, two-stage yielding, which is associated with the A→R and R→M phase transfo...[ Read more ]

Deformation behavior associated with different initial phase structures was studied systematically in polycrystalline NiTi shape memory alloy tubes by tensile testing at different temperatures. The nominal stress-strain curves of the tubes from room temperature to 70°C were recorded. It was found that the deformation of NiTi tubes with initial structure of R-phase proceeded by R→M martensitic transformation, while the deformation of NiTi tubes with initial structure of M-phase proceeded by martensitic reorientation. The R→M martensitic transformation was realized by nucleation and growth of cylindrical martensitic bands, while no macroscopic martensitic band was observed during martensitic reorientation. Further, two-stage yielding, which is associated with the A→R and R→M phase transformation, was observed in the stress-strain curves of NiTi tubes by increasing the testing temperatures. The surface profiles of the martensitic bands were also measured at different temperatures. By constraining the displacement of the specimens at both ends, it was demonstrated that the depth of the martensitic bands increased with the testing temperature.

The mechanical behaviors of the NiTi sheets with different orientations to rolling direction (RD) were studied in order to compare them with the results of the NiTi tubes. It was shown that the stress-strain response of the sheet specimens was greatly affected by the texture in the case of martensitic reorientation. A stress-plateau and higher shape recovery strain were resulted in the specimens tested at 0° & 45° to RD, while strain hardening and lower shape recovery strain resulted in the specimen tested at 90° to RD. The nucleation and growth of martensitic bands were observed in the specimens with the testing angle of 0° & 45° to RD, while no macroscopic martensitic band was nucleated in the specimen with the testing angle of 90° to RD. On the other hand, stress-plateaus and martensitic bands were observed in the specimens with different orientations (0°, 45° & 90° to RD) in the cases of A→M and R→M martensitic transformation. Moreover, higher recovery strain and lower transformation stress were observed in tension along the RD, while lower recovery strain and higher transformation stress were observed in tension along transverse direction (TD). Finally, it was revealed that the slope of the surface profile at the transition zone of the martensitic band nucleated during A→M martensitic transformation was steeper, and resulted in sharper band interface than the martensitic bands nucleated during R→M martensitic transformation and M→M martensitic reorientation.