Depinning-controlled plastic deformation during nanoindentation of BCC iron thin films and nanoparticles

Nanoindentation is commonly employed to study mechanical properties of metallic specimen. In recent years, the use of this technique has been extended to study how mechanical properties of sub-micrometer scale specimens depend on size and dimensionality. In this work, we explore this dependence for α-Fe in molecular dynamics simulations of nanoindentation of thin films and nanoparticles. The simulations show that the strength poorly depends on the dimensions in the lateral directions and that the indentation curves of thin films and nanoparticles are alike. This is in contrast with recent observation in FCC metals. A detailed analysis of the dislocation mechanisms during the deformation reveals that ½ 〈111〉{110} dislocations are nucleated in six possible directions, but these remain pinned to their nucleation sites after nucleation. Load drops in the indentation curve, which corresponds to strain bursts, are attributed to depinning of these dislocations as full or half prismatic loops, i.e., the deformation is depinning-controlled. In nanoparticles, some of the dislocations depin partially, resulting in pure screw dislocations between the indenter and the lateral facet. The locality of the depinning-controlled deformation rationalizes the lack of size effect. Finally, we relate the dislocation mechanisms during the depinning-controlled deformation to the frequency and extent of strain bursts during nanoindentation of BCC specimens.