Meso-scale interface propagation in NiTi shape memory alloys – understanding experimental results from a thermo-mechanical perspective

This contribution examines experimental results on the thermo-mechanical behavior of NiTi in a wide range of testing conditions – from uniaxial tensile and compressive loading to compression-shear testing, tension-torsion experiments, and indentation. Special emphasis is placed on the distinction between deformation modes associated with localized transformation (where fully transformed martensite bands form and grow into austenitic regions) as opposed to situations where the stress-induced martensitic transformation proceeds homogeneously on the macro-scale. Localized deformation is characterized by the observation of distinct boundary regions (“meso-scale” interfaces) that separate martensitic and austenitic parts of a sample. We analyse a simple stability criterion for uniaxial tension and we argue that strain-softening associated with the stress-induced transformation in tension is a necessary condition for localized deformation in NiTi. We then discuss results of simple finite element simulations using a constitutive law that allows to model tension-compression asymmetry in terms of transformation stresses and strains. We analyze the deformation modes occurring during tension vs. compression and compression-shear testing. We demonstrate that the stress and strain distributions in the meso-scale interfaces that are formed in tension agree very well with experimental surface (strain) and bulk (phase volume fraction) measurements, and we discuss the special role of the corresponding stress state at the interface in facilitating its stable propagation. Finally, we derive key features that a material model of NiTi needs to possess in order to fully capture the different types of localized/homogeneous deformation modes observed, e.g., in combined tension/torsion, indentation, or biaxial load cases.