Microstructure Evolution-Dominated Mechanical Properties of Pure Titanium under Nanoindentation

While dislocation activity and grain refinement are the two deformation mechanisms governing plastic deformation of pure titanium, their interaction has a strong impact on mechanical properties of deformed surface. In the current work, the microstructure evolution accommodated by dislocation activity and grain refinement under Berkovich nanoindentation of pure titanium is theoretically and experimentally investigated, with an emphasis on the strengthening mechanisms of indented surface. A 3D finite element model of pure titanium nanoindentation is established using a modified Johnson-Cook constitutive law to describe the elastoplastic deformation of the material. In particular, an ETMB model is employed to quantitatively assess dislocation density and grain size around the indentation impression, and then, the hardness of indented surface is calculated using a strengthening model that considers both dislocation activity and grain refinement based on the obtained dislocation density and grain size. Corresponding nanoindentation experiments are performed to calibrate the parameters of the constitutive law, which results into well agreement in load-displacement curves and surface pile-up topography. FE simulations reveal a strong correlation of indented surface strengthening with dislocation activity and grain refinement during nanoindentation of pure titanium. The proportional contribution to surface hardness increase by fine grain strengthening is more pronounced than that by dislocation strengthening. The microstructural evolution-based surface strengthening mechanism is also theoretically analyzed.