Regensburg 2007 – wissenschaftliches Programm

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MM: Fachverband Metall- und Materialphysik

MM 2: SYM Micro- and Nanomechanics I

MM 2.2: Vortrag

Montag, 26. März 2007, 10:45–11:15, H16

Size effects observed during indentation testing — •Karsten Durst — Institut für Werkstoffwissenschaften 1, Universität Erlangen-Nürnberg, Erlangen

Nanoindentation allows probing the mechanical response of materials from the nanoscale to the macroscale. It is found that strength of crystalline materials is size dependent and the size dependence is influenced by i.e. grain size, dislocation density and solid solution strengthening. Initially, materials deform purely elastically, supporting loads up to their theoretical strength. The nucleation and multiplication of dislocations leads to a discontinuity in the load displacement data, marking the transition from elastic to plastic deformation. After pop-in, a high hardness is observed, which decreases with increasing indentation depth until a constant hardness is found for large indentation depths. The experimental observations can be modeled within the framework of Taylor hardening, considering geometrically necessary dislocations (GND) and statistically stored dislocations (SSD). The statistically stored dislocation density is derived from uniaxial stress-strain data applying the Tabor concept of the representative strain. The GNDs are necessary for providing the lattice rotation underneath the indenter and for forming the residual impression. Their density is calculated for conical and spherical indenters, using the storage volume of GNDs as a fitting factor for describing the experimental observations. Hertzian contact theory is used to describe the initial elastic deformation of the material, whereas the critical pop-in load is derived from the theoretical strength of the material.