On the prediction of activated NiTi B19' martensite twins beneath a spherical indenter

The complex thermo-mechanical behavior of NiTi is strongly affected by the distinct properties of its twinned martensitic microstructures. Micromechanical models have previously been developed to predict which twinning system is activated during the stress-induced formation of B19’ martensite under uniaxial loading. In this contribution we apply the phenomenological theory of martensitic transformations to the multi-axial load case under a spherical nanoindenter tip. We first calculate the elastic stress field that triggers the martensitic transformation, considering the elastic constants of B2 austenite in an elastic-anisotropic FE simulation. For <100>, <110> and <111> surface orientations of NiTi austenite, nucleation sites and the activated martensite twins are then predicted based on the stress field below the indenter tip. We further calculate the transformed martensite volume and the resulting surface topography. Four-, two- and threefold symmetries of the indents are predicted, respectively. The <111> orientation shows the largest and the <110> orientation the smallest activated martensite volume. The tilt angles between the undeformed austenitic surface and the martensitic surfaces is about 1°, 5° and 7° for the <100>, <110> and <111> orientations. The results for the <111> orientation are in excellent agreement with recent experiments on coarse-grained NiTi, demonstrating that dedicated anisotropic calculations can be used to characterize the properties of small sample volumes subjected to the stress-induced martensitic transformation even under complex load cases.