Phase transformation in superelastic NiTi polycrystalline micro-tubes under tension and torsion - from localization to homogeneous deformation

Recent tension-torsion test results on nano-grained NiTi shape memory alloy micro-tubes are reported in this paper. It is discovered that: (1) during uniaxial tensile loading, the stress-induced transformation in the micro-tubes is realized by the initiation and growth of a macroscopic spiral martensite band with a quite sharp austenite-martensite (A-M) interface; (2) during loading by torsion (pure shear), the stress-strain curve exhibits monotonic hardening, the stress-induced transformation is axially homogeneous throughout the whole tube and the transformation strain is much smaller than that under tension; (3) under tension-torsion combined loading with an increasing shear/tension stress ratio, there is a gradual change in the deformation mode from localization and propagation (under pure tension) to the homogeneous deformation (under pure shear). The tube surface morphology observation indicates that, during this gradual change, the A-M interface thickness increases as its evolution is recorded. The test results demonstrate that there is a strong anisotropy and loading path dependence of the tube responses in both kinematics and kinetics. Possible underlying physical mechanisms are analyzed and implications for future theoretical modelling are also discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.