Defect generation and pileup of atoms during nanoindentation of Fe single crystals

Complementary large scale molecular-dynamics simulations and experiments have been carried out to determine the atomistic mechanisms of the nanoindentation process in single crystal $\mathrm{Fe}{110},$ ${100},$ and ${111}.$ The defect formation and motion causes the complex mechanisms of plastic and elastic deformation which is reflected in the pileup patterns. The experimental results show distinct patterns of pileup material which are dependent on the individual crystal faces and the superposition of the stress field of the indenter. The highest pileup around the indenter hole occurs on the ${100}$ surface and the shallowest on ${111}.$ The least symmetric surface is ${110}$ which produces an experimental pileup pattern displaying only twofold symmetry with the axially symmetric indenter. The pyramidal indenter produces an asymmetric pattern which changes as the crystal is rotated with respect to the tip but repeats with threefold rotational symmetry. Material displacement occurs primarily in planes of the ${110}$ family. Pileup is formed by cross slip between planes of the same family which intersect in $〈111〉$ directions. For the ${110}$ surface, dislocation loops propagate in the four in-plane $〈111〉$ directions and the two inclined $〈111〉$ directions. The loops that propagate in the in-plane directions are terminated by edge dislocations at the surface. These transport material away from the tip but cannot produce pileup. The loops that propagate in the inclined direction cross slip and cause the observed pileup. The ${100}$ surface has fourfold rotational symmetry and all the $〈111〉$ directions are inclined. The dislocation loops propagate in these directions and cross slip readily occurs, leading to a large pileup. The ${111}$ face shows the least pileup which is more spread out over the surface. In this case the dislocation loops propagate in shallow slip planes and do not readily cross slip. Experimentally determined force-depth curves show distinct ``pop-ins'' which correspond to the formation of dislocations. The contact pressure (nanohardness) is not a constant and increases with decreasing indentation depth. It also changes with crystal face. Calculated force-depth curves match the experimental trend but give estimates of the nanohardness and Young's modulus higher than those values experimentally determined.