Investigation of the phase state of transformation shear bands in superelastic Ni-rich NiTi shape memory alloys with syn
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Investigation of the phase state of transformation shear bands in superelastic Ni-rich NiTi shape memory alloys with synchrotron diffraction W. W. Schmahl1), A. Baruj2), J. Khalil-Allafi1,3), H. Nebel1) 1) Fakultät für Geowissenschaften, Ruhr-University, D-44780 Bochum, Germany 2) Institut für Werkstoffe, Ruhr-University, D-44780 Bochum, Germany 3) Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran
ABSTRACT The deformation of superelastic NiTi due to the stress-induced transition from cubic B2 to monoclinic B19' phase is localised in a Lüders-band-like transformation shear bands (TSB). We investigate the phase state and texture in the bands in-situ using diffraction with high-energy synchrotron-radiation. Residual austenite is observed to persist up to the end of the superelastic plateau in the stress-strain curve. The deformation corresponds to a displacement of the boundary of the TSB. After a certain volume of the specimen has transformed when the boundary has passed by, there is no further change of the crystallographic state of this volume up to the end of the macroscopic stress-strain plateau. The variant orientation texture is different at the surface from that present in the bulk. INTRODUCTION Superelasticity and shape memory of NiTi alloys are related to the martensitic phase transition from the cubic B2 austenite to the monoclinic B19' martensite phase [1]. The phase transition can be induced by stress as well as by changes in temperature. Shaw and Kyriakides [2-4] demonstrated that the deformation causing the superelastic plateau in the stress-strain curve of flat tensile-test specimens is localized in Lüders-band-like transformation shear bands (TSB). The B2-B19' transformation nucleates at a critical stress in specific locations and forms macroscopic shear bands, which then propagate at constant stress through the sample when it is extended. Brinson, Schmidt and Lammering [5] investigated the effect thoroughly using reflected-light optical microscopy, and they showed that the transition from B2 to B19' is incomplete in the TSB. Moreover, they found that the surface relief hardly changes at all at a particular point in the TSB when the interface between TSB and the B2 matrix progresses with the macroscopic strain to the end of the superelastic plateau. Schmahl et al. [6] investigated the localized transformation behaviour using soft x-rays (wavelength near 1.7 Å) from a synchrotron source. They also found a significant amount of non-transformed material in the TSB, as indicated by about 10% of residual intensity of the B2 austenite (110) peak. Soft x-rays, including those generated by the usual laboratory x-ray sources used for x-ray diffraction (XRD) can penetrate only a few tens of nanometers into NiTi, and hence the information obtained by traditional XRD is related to a surface layer. Further, these experiments are necessarily limited to reflection geometries, such that only those lattice plains which are - more or less- parallel to the surface contribute to th
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