Nano-indentation was extensively used to measure the mechanical behaviour of materials at the micro- and nanoscale. However, the material response of nano/micro-indentation and its intrinsic mechanisms were very complicated; especially when considering heterogeneous polycrystalline materials. Nano-indentation in polycrystals, performed with a circular indenter, was studied by numerical modelling based on the two-dimensional discrete dislocation plasticity of Van der Giessen and Needleman (1995). The dependence of indentation hardness was investigated with respect to four typical characteristic dimensions: indenter radius, grain size, indentation depth, and the distance between the grain boundary and the indenter. Results show that these characteristic dimensions had considerable influence on the nano-indentation hardness. Further investigation showed that their influence had an effect in two main ways, i.e. via strain hardening and the indentation size effect. Although both effects were size dependent, their underlying mechanisms were clearly different. For the present polycrystal case, the strain hardening effect was mainly associated with the constraints of grain boundaries and dislocation obstacles to the dislocation glide, while the indentation size effect was related to the average strain gradient beneath the circular indenter.

Discrete Dislocation Analyses of Circular Nanoindentation and Its Size Dependence in Polycrystals. C.Ouyang, Z.Li, M.Huang, C.Hou: Acta Materialia, 2008, 56[12], 2706-17