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Abstract

In order to model the mechanical performance of a heterogeneous (multiphase) material on a microstructural length scale we need to know the mechanical properties of all individual phases. Nanoindentation has been established as a valuable tool to measure such local material properties. However, it has been shown that nanoindentation results cannot be correlated directly to macroscopic material properties, because length scale effects in plasticity render the material apparently harder than measured in deformation experiments on larger scales. In order to understand and describe such length scale effects in plasticity strain gradient theories have been developed that are based on the evolution of geometrically necessary dislocation densities during nanoindentation [1]. However, the length scale needed as a model parameter in all strain gradient theories is not uniquely defined and moreover it is difficult to be measured experimentally. To overcome this problem we studied nanoindentation in large-scale atomistic simulations. With newly developed analysis tools we are able to calculate the line direction and Burgers vectors of all dislocations inside the deformed atomistic sample. From this data, dislocation density tensors and hence geometrically necessary dislocation densities can be calculated. With the possibility to calculate such quantities we investigate the role of geometrically necessary dislocations and lattice rotations on length scale and rate effects during nanoindentation. Furthermore, we study the implications of dislocation nucleation and multiplication on the measured hardness to obtain a better understanding of nanoindentation as a method to assess scale-independent material properties. [1] WD. Nix and H. Gao “Indentation size effects in crystalline materials: A law for strain gradient plasticity” J. Mech. Phys. Solids 46 (1997) 411-425.