"Plastic deformation in nanoindentation of tantalum: A new mechanism for prismatic loop formation"

T. REMINGTON; C.J. RUESTES; E.M. BRINGA

ACTA MATERIALIA

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2014 vol. 78 p. 378 - 393

1359-6454

The mechanisms of deformation under a nanoindentation in tantalum, chosen as a model BCC metal, are indentified and quantified. Molecular dynamics calculations are conducted for the monocrystal orientation [100] and indentation experiments for the three orientations [100], [110], and [111]. The evolution of plastic deformation proceeds by the initiation through nanotwins and stacking faults, which evolve into shear dislocation loops. It is shown through a dislocation analysis that an elementary twin (3 layers) is energetically favorable for a diameter below 7.2 nm, at which point a shear loop comprised of a perfect dislocation is formed. The shear loops expand into the material by the advance of the edge components. Simultaneously with this advance, the screw components of the loop cross slip and generate a cylindrical surface. When the opposite segments approach, they eventually cancel by virtue of the attraction between them, forming a quasi-circular prismatic loop composed of edge dislocation segments. This ´lasso´ like mechanism by which a shear loop transitions to a prismatic loop is identified for the first time. The prismatic loops advance into the material along the <111> directions, transporting material away from the nucleation site. Analytical calculations supplement the molecular dynamics and experimental observations and provide a framework for the improved understanding of the evolution of plastic deformation under a nanoindenter.  Dislocation densities under the indenter are estimated experimentally (~1.2 x10^15 m^-2), by molecular dynamics (7x10^15 m^-2), and through an analytical calculation (1.5x10^15-1.1x10^16 m^-2). Considering the assumptions and simplifications, this agreement is considered satisfactory.