Effects of preexisting dislocations on phase transition and deformation of NiTi shape memory alloy: An in situ multi-scale study

Phase transition and deformation in cold-rolled and heat-treated Ni 50 . 6 Ti 49 . 4 alloys are explored by in situ multi- scale measurements with simultaneous X-ray diffraction and optical digital image correlation. Macroscale stress-strain curves, mesoscale strain fields and microscale X-ray diffraction peaks are obtained. Electron backscatter diffraction and transmission electron microscopy are also conducted on samples before and after tension as complements. For cold-rolled samples, since martensite nucleation is strongly associated with the widely distributed preexisting dislocations, homogeneous martensite phase transition takes place, in contrast with the localized mode in heat-treated samples. For localized phase transition, the intense growth of martensite results in clear L & uuml;ders bands. The beginning and the end of phase transition plateau on the stress-strain curve are associated with the initiation and complete propagation of L & uuml;ders bands. For the homogeneous phase transition, broad growth of martensite leads obscure L & uuml;ders bands and uniform strain fields, without any phase transition plateau. Given the initial {110}(110) 110 ) and {111}(110) 110 ) sheet textures in the heat-treated samples, different martensite variants with different preferred orientations are activated with respect to the loading directions.