Experimental investigation on phase transformation of superelastic nickel-titanium microtubes

The superelastic behavior of polycrystalline nano-grained NiTi shape memory alloy micro-tube under uniaxial tension and tension-torsion are reported in this paper. Firstly, the nominal tensile stress-strain curve of the micro-tube during superelastic deformation is recorded. Both direct surface observation and observation by using a special surface coating show that the deformation of the tube is via the nucleation and propagation of macroscopic stress-induced martensite band. It is also found that the martensite nucleates in the form of a spiral lens-shaped narrow band that inclines at about 33° to the plane of the cross section of tube when the stress reaches the peak of the stress-strain curve. The spiral band grew via gradual increase in both width and length of the band and finally merged into a single cylindrical band. The subsequent deformation of the tube is realized by the growth of this cylindrical martensite band. Several other deformation features of the tube are also observed and the results are discussed and compared with the theoretical analysis in this paper. The tension-torsion test results further revealed that: (1) During uniaxial tensile loading the deformation (via stress-induced transformation) of the micro-tube is realized by the initiation and growth of the spiral martensite band with quite sharp A-M interface; (2) During loading by torsion (pure shear) the stress strain curve exhibits monotonic hardening and the stress-induced transformation is homogeneous throughout the whole tube; (3) Under tension-torsion combined loading, with increasing the shear/tension stress ratio there is a gradual change in the deformation mode from localization and propagation (in pure tension) to the homogeneous deformation (in pure shear). The surface morphology of the tube and the macroscopic A-M interface thickness evolution are recorded. The results are analyzed by Eshelby's inclusion theory and the obtained theoretical results on the band orientation and shape agree quite well with experiments.