Complementary large scale molecular-dynamics simulations and experiments were carried out to determine the atomistic mechanisms of the nano-indentation process in single crystal Fe{110}, {100}, and {111}. The defect formation and motion caused the complex mechanisms of plastic and elastic deformation which was reflected in the pile-up patterns. The experimental results exhibited distinct patterns of pile-up material which were dependent on the individual crystal faces and the superposition of the stress field of the indenter. The highest pile-up around the indenter hole occurred on the {100} surface and the shallowest on {111}. The least symmetric surface was {110} which produced an experimental pile-up pattern displaying only 2-fold symmetry with the axially symmetrical indenter. The pyramidal indenter produced an asymmetric pattern which

changed as the crystal was rotated with respect to the tip but repeated with 3-fold rotational symmetry. Material displacement occurred primarily in planes of the {110} family. Pile-ups were formed by cross-slip between planes of the same family which intersected in <111> directions. For the {110} surface, dislocation loops propagated in the 4 in-plane <111> directions and the 2 inclined <111> directions. The loops that propagated in the in-plane directions were terminated by edge dislocations at the surface. These transported material away from the tip but could not produce pile-up. The loops that propagated in the inclined direction cross-slipped and caused the observed pile-up. The {100} surface had 4-fold rotational symmetry and all of the <111> directions were inclined. The dislocation loops propagated in these directions and cross-slip readily occurred, leading to a large pile-up. The {111} face exhibited the least pile-up which was more spread out over the surface. In this case, the dislocation loops propagated in shallow slip planes and did not readily cross slip. Experimentally determined force-depth curves exhibited distinct so-called pop-ins which corresponded to the formation of dislocations. The contact pressure (nano-hardness) was not a constant, and increased with decreasing indentation depth. It also changed with crystal face. Calculated force-depth curves matched the experimental trends but gave estimated of the nano-hardness and Young's modulus that were higher than the experimentally determined values.

Defect Generation and Pile-Up of Atoms during Nano-Indentation of Fe Single Crystals. R.Smith, D.Christopher, S.D.Kenny, A.Richter, B.Wolf: Physical Review B, 2003, 67[24], 245405 (10pp)