Recent experimental findings from thermomechanical testing and microstructural analysis of NiTi polycrystalline shape memory alloy have brought a qualitatively new picture of plastic deformation mechanisms of this most common shape memory material [1]. In particular, they challenge the widely accepted paradigm that austenite is a phase more susceptible to dislocation slip and plastic deformation in NiTi than B19' martensite, despite the fact that only a single [100](001)M dislocation slip system has been observed to be active in martensite.
In this contribution the irreversible plastic forming of B19’ martensite is discussed within the framework of continuum mechanics. We will show that if the [100](001)M plastic slip and the martensite reorientation between two variants sharing the (010)M plane are active at the same time, a novel deformation mechanism that combines kink banding and twinning arises, resulting in new types of deformation bands – ‘kwink bands’ [2]. Through formation of kwink bands, the monoclinic lattice of martensite can achieve macroscopically homogeneous plastic strains (at stress levels well below the yield stress of austenite at the given temperature), despite having only one available slip system.  We will then discuss the consequences of kwinking on the mechanical performance of irreversibly strained NiTi, such as the grain refinement and hardening, the strong stabilization of martensite, and the usual appearance of austenitic (41̄1)P twin bands after the reverse transformation.

[1] Šittner P., Sedlák P., Seiner H., Sedmák P., Pilch J., Delville R., Heller L., Kadeřávek L., On the coupling between martensitic transformation and plasticity in NiTi: Experiments and continuum based modelling (2018) Progress in Materials Science, 98, pp. 249 – 298.
[2] Seiner H., Sedlák P., Frost M., Šittner P., Kwinking as the plastic forming mechanism of B19′ NiTi martensite (2023) International Journal of Plasticity, 168, art. no. 103697.