SMST2019: Prediction of NiTi B19' Martensite Twin Activation Below a Spherical Indenter Tip

摘要

The mechanical behavior of pseudoelastic NiTi is strongly affected by the properties of its twinned martensitic microstructures, which has been recently demonstrated by observations of an anisotropic material response during nanoindentation. In this paper, we apply the phenomenological theory of martensitic transformations to the multi-axial load case under a spherical indenter tip. We first calculate the elastic stress fields that trigger the martensitic transformation in anisotropic, linear-elastic finite element simulations. For <100>, <110>, and <111> surface orientations of a single NiTi austenite grain, likely nucleation sites and the activated martensite correspondence variant pairs are then predicted. We further estimate the transformed martensitic volume and the resulting surface topography. In excellent agreement with recent experiments, four-, two-, and three-fold symmetries of the indents are observed; the <111> orientation shows the largest and the <110> orientation the smallest activated martensite volume. The tilt angles of the martensitic surface facets are about 1°, 5°, and 7° for the <100>, <110>, and <111> orientations. Our results clearly highlight that dedicated anisotropic calculations in combination with a simple martensite twin selection criterion can be used to characterize the properties of small sample volumes subjected to the stress-induced martensitic transformation even under complex loading.