Nickel-titanium B19’ martensite is a strongly plastically anisotropic material with only one available slip system, which is the [100](001) M slip. Despite this, B19’ martensite polycrystals can be homogeneously plastically formed, reaching up to very high plastic strains. The absence of other slip systems is compensated by plastic twinning, in particular by the frequently appearing irreversible (20-1) M twins. However, these twins act on the same (010) M lattice plane as the plastic slip, and thus, do not seem to be a very suitable complement to the slip in terms of the Von Mises criterion. In fact, exactly the same strains as by the (20-1) M twins can be achieved also by the [100](001) M slip itself, and thus, a question arises, whether they can be understood as plastic twins in the conventional sense.

Great attention has been recently paid to the investigation of plastic deformation in NiTi. Experimental investigations of the mechanical response of NiTi polycrystalline samples within a broad stress-strain-temperature state space have revealed a complex response involving martensitic transformation, reorientation, and plastic deformation processes. The interactions between them result in complex coupled phenomena, such as transformation-induced plasticity, martensite stabilization through plastic deformation, and micro-strain heterogeneity induced by plasticity. Plastic deformation in NiTi not only generates irrecoverable strain at the macroscale, but it also induces substantial strain heterogeneity in the microstructure. This heterogeneity significantly affects the functional properties and may open up new technology pathways for designing sophisticated products. Tailored constitutive models that can reproduce the response in complex loading scenarios can be extremely beneficial.

This paper presents an extension of a well-established constitutive model for NiTi covering both reversible (elastic, martensitic transformation, martensite reorientation) and irreversible (plastic) deformation mechanisms. Besides the inclusion of mechanisms of plastic deformation in both austenitic and martensitic phases in an independent manner, the model also newly captures more complex coupled phenomena of martensitic transformation and plastic deformation, such as transformation-induced plasticity, stabilization of martensite by plastic deformation, or plasticity-induced microstrain heterogeneity leading to functional fatigue. Despite a large number of different mechanisms involved in the model, which is reflected by a considerable number of internal parameters introduced for the description of the evolving microstructure of the material, the model still brings a basic, simple phenomenological understanding of the coupled transformation-plasticity proceeding in NiTi. After successful implementation to FEM software, the model provides new possibilities for simulations of NiTi components' behavior and processing.