Discrete dislocation dynamics simulations of nanoindentation with pre-stress: Hardness and statistics of abrupt plastic events

Nanoindentation of crystalline materials has been thought as a primarily surface-driven technique that is not able to probe bulk mechanical properties directly, such as material yield strength or bulk plastic flow rates. We elucidate this question through extensive discrete dislocation plasticity simulations of nanoindentation on a single crystal. We consider the competition between nanoindentation and tensile loading (pre-stress) towards crystal plasticity. For this purpose, we study a two-dimensional discrete dislocation model where indentation is performed by using cylindrical indentation with varying radius under both displacement and load control. We focus on the behavior of the hardness and pop-in event statistics during nanoindentation under various pre-stress levels and we correlate them to the spatially correlated dislocation microstructure behavior. At small indentation depths (relative to other microstructural or tip length scales), we find that the hardness is inversely dependent on the plastic strain/dislocation density induced by the applied tensile pre-stress; consequently, we argue that small-depth indentation may be useful for identifying bulk plastic yielding behavior prior to indentation. In contrast, for larger indentation depths, pre-stress has no effect on hardness. However, effect of pre-stress can be revealed through plastic events statistics, both in load and displacement controlled protocols. Moreover, post-indentation surface morphology clearly shows the effect of the pre-stress.