Localized deformation of NiTi in tension

Superelastic NiTi alloy shows localized recoverable deformation in tensile test up to several percent strain  due to stress induced cubic to monoclinic martensitic transformation

Figure 1: Superelastic deformation of NiTi wire in tension proceeds in localized manner via propagation of transformation fronts

A specialty of superelastic NiTi among the martensitically transforming shape memory alloys is the that the stress induced martensitic transformation in tension does not proceed homogeneously on macroscopic level  but in a localized manner via propagation of Martensite Band Fronts /MBF/  extending  over thousands of grains and separating the austenite from the heavilly strained martensite. On the lattice level within individual polycrystal grains, true austenite/martensite phase interfaces move and transform the austenite to martensite.

The localized deformation of NiTi was thoroughly investigated in the literature in the last 30 years on wires, ribbons, bars and tubes using wide range of experimental methods (DIC, IR camera, Combined DIC-IR, SEM-DIC, EBSD, FEM modelling, x-ray diffraction). In spite of that the phenomenon is not completely understood.

This site provides animations/videos,  supplemental to a Report in Science Magazine, presenting new experimental evidence on the topology and internal stress field of the Martensite Band Front propagating in thin superelastic NiTi wire loaded in tension obtained by combination of 3D x-ray diffraction /3D-XRD/ and Digital Image Correlation /DIC/ experimental methods  complemented by simulation of the localized superelastic deformation by finite element  implemented SMA model.

1. DIC observation of localized deformation in tensioned NiTi ribbon
2. DIC observation of localized deformation in tensioned NiTi wire
3. Localized deformation of tensioned NiTi wire analyzed by 3D-XRD method
4. Scale transition to macroscopic internal stress in tensioned NiTi wire
5. FE simulation of localized deformation in tensioned NiTi wire
6. Martensite band front in tensioned NiTi wire
7. DIC observation of cyclic localized deformation of NiTi wire and localization of fatigue damage
8. 3D-XRD method
9. Download the 3D-XRD dataset

¹P. Sedmák at all, Science 353, 559-562 (2016).,