Sudden displacement excursions during load-controlled nanoindentation of relatively dislocation-free surfaces of metals are frequently associated with dislocation nucleation, multiplication, and propagation. Insight into the nanomechanical origins of plasticity in metallic crystals may be gained through estimation of the stresses that nucleate dislocations. An assessment of the potential errors in the experimental measurement of nucleation stresses, especially in materials that exhibit the elastic-plastic transition at very small indentation depths, is critical. In this work, the near-apex shape of a Berkovich probe was measured by scanning probe microscopy. Then, this shape was used as a virtual indentation probe in a 3-dimensional finite-element analysis (FEA) of indentation on <100>-oriented single-crystal tungsten. Simultaneously, experiments were carried out with the real indenter, also on <100>-oriented single-crystal tungsten. There is good agreement between the FEA and experimental load-displacement curves. The Hertzian estimate of the radius of curvature was significantly larger than that directly measured from the scanning-probe experiments. This effect was also replicated in FEA simulation of indentation by a sphere. These results suggest that Hertzian estimates of the maximum shear stresses in the target material at the point of dislocation nucleation are a conservative lower bound. This is not only due to stress concentration by asperities or an aspheric probe, and use of the Hertzian approximation outside of its range of validity.
Citation: Journal of Materials Research
Pub Type: Journals
nanoindentation, Hertzian contact, finite element modeling, dislocation nucleation