Plastic Deformation in Nanoindentation of a BCC Metal: A New Mechanism for Prismatic Loop Formation

REMINGTON TP; CARLOS J. RUESTES; EDUARDO M. BRINGA; REMINGTON BA; LU C-H; KAD B; M.A. MEYERS

ACTA MATERIALIA

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014

1359-6454

The mechanisms of deformation under a nanoindentation in tantalum, chosen as a model BCCmetal, are indentified and quantified. Molecular dynamics calculations are conducted for themonocrystal 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 stackingfaults, which evolve into shear dislocation loops. It is shown through a dislocation analysis that anelementary twin (3 layers) is energetically favorable for a diameter below 7.2 nm, at which point ashear loop comprised of a perfect dislocation is formed. The shear loops expand into the material bythe advance of the edge components. Simultaneously with this advance, the screw components of theloop cross slip and generate a cylindrical surface. When the opposite segments approach, theyeventually cancel by virtue of the attraction between them, forming a quasi-circular prismatic loopcomposed of edge dislocation segments. This 'lasso' like mechanism by which a shear loop transitionsto a prismatic loop is identified for the first time. The prismatic loops advance into the material alongthe <111> directions, transporting material away from the nucleation site. Analytical calculationssupplement the molecular dynamics and experimental observations and provide a framework for theimproved understanding of the evolution of plastic deformation under a nanoindenter. Dislocationdensities under the indenter are estimated experimentally (~1.2 x10^15 m^-2), by moleculardynamics (7x10^15 m^-2), and through an analytical calculation (2.6x10^15-1.9x10^16 m^-2).Considering the assumptions and simplifications, this agreement is considered satisfactory.