Kwink patterns in plastically formed NiTi martensite

The plastic forming mechanism of B19’ NiTi combines martensite reorientation and highly anisotropic plastic slip. As a result, unique regular patterns arise, that can be described as microstructures of twinning-assisted plastic kinks, or kinking-assisted plastic twins, which can be both shortened into a novel term ‘kwinks’. The kwinks are observed in nearly all plastically formed NiTi martensitic materials, ranging from soft ribbons with large grains to nano-grained cold-worked wires.

The talk will summarize the theoretical background for describing the kwinks at the continuum and atomistic length-scales, highlighting the importance of the kwinking process for the plastic formability of B19’, and giving a direct comparison to experimental observations. It will be shown that the frequently reported (20-1) crystallographic orientation of the kwinks results from continuum-level energy minimization, and thus, that the non-linear elasticity theory of martensite can be used as a proper tool for predicting and interpreting all observed patterns in irreversibly formed NiTi materials when extended by a simple crystal-plasticity framework. Finally, the kwinking mechanism will be shown to explain the (411)-twin patterns observed in NiTi austenite, which so far have not been sufficiently explained by the classical concepts of deformation twinning.