During operational conditions superelastic materials can undergo complex biaxial states or strain path changes leading to faster degradation of the superelastic properties compared to uniaxial fatigue. There are very little details known about the mechanisms involved.
The transformation behaviour of a commercial superelastic NiTi material was studied during uniaxial tension and load path changes by employing a novel miniaturized biaxial stage and cruciform shaped specimens . The transformation characteristics are revealed by performing experiments inside a scanning electron microscope where displacements can be mapped in-situ with a sub-micron resolution using HRDIC and by performing insitu x-ray diffraction at the MS beamline of the Swiss Light Source. Complementary EBSD analyses are performed before and after deformation.
The initial microstructure consists of large austenitic grains that are subdivided in bands containing nanosized sub-grains. The results show a strong correlation of the martensitic transformation with Schmid’s law during uniaxial loading (F1 or F2). However, evidence of non Schmid behavior is obtained by the appearance of the same variants when going from an uniaxial stress state F1 to the multiaxial stress state (F1F2). Furthermore, bands of nanograins transform collectively and produce high-strain bands suggesting the importance of grain interactions at the nanoscale .
 Experimental Mechanics, 57, (2017) 569.
 Acta Materialia 144, (2018) 874