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

Abstract

Here, the mechanisms of deformation under a nanoindentation in tantalum, chosen as a model body-centered cubic (bcc) metal, are identified and quantified. Molecular dynamics (MD) simulations and indentation experiments are conducted for [1 0 0], [1 1 0] and [1 1 1] normals to surface orientations. The simulated plastic deformation proceeds by the formation of nanotwins, which rapidly evolve into shear dislocation loops. It is shown through a dislocation analysis that an elementary twin (three layers) is energetically favorable for a diameter below ~7 nm, at which point a shear loop comprising a perfect dislocation is formed. MD simulations show that shear loops expand into the material by the advancement of their edge components. Simultaneously with this advancement, screw components of the loop cross-slip and generate a cylindrical surface. When 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 both [0 0 1] and [1 1 1] indentations. The prismatic loops advance into the material along $$\langle$$1 1 1$$\rangle$$ directions, transporting material away from the nucleation site. Analytical calculations supplement MD 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 × 10¹⁵ m^-2), by MD (~7 × 10¹⁵ m^-2) and through an analytical calculation (2.6–19 × 10¹⁵ m^-2). Considering the assumptions and simplifications, this agreement is considered satisfactory. MD simulations also show expected changes in pile-up symmetry after unloading, compatible with crystal plasticity.

Authors:

Remington, T. P. ^[1]; Ruestes, C. J. ^[2]; Bringa, E. M. ^[3]; Remington, B. A. ^[4]; Lu, C. H. ^[1]; Kad, B. ^[1]; Meyers, M. A. ^[1]

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+ Show Author Affiliations

1. Univ. of California, San Diego, CA (United States)
2. Univ. Nacional de Cuyo, Mendoza (Argentina)
3. Univ. Nacional de Cuyo, Mendoza (Argentina); CONICET, Mendoza (Argentina)
4. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Publication Date:

Sat Aug 02 00:00:00 EDT 2014

Research Org.:

Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States); Univ. of California, San Diego, CA (United States); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

USDOE National Nuclear Security Administration (NNSA); University of California; USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities Division

OSTI Identifier:

1808763

Alternate Identifier(s):

OSTI ID: 1462255; OSTI ID: 1556483

Report Number(s):

LLNL-JRNL-824531
Journal ID: ISSN 1359-6454; 1036748

Grant/Contract Number:  

AC52-07NA27344; 09-LR-06-118456-MEYM; PE-FG52-09NA-29043; PICT-PRH-0092; NA0002080; SeCTyP-UN Cuyo; 0092

Resource Type:

Accepted Manuscript

Journal Name:

Acta Materialia

Additional Journal Information:

Journal Volume: 78; Journal Issue: N/A; Journal ID: ISSN 1359-6454

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE; plasma physics; nanoindentation; molecular dynamics; dislocations; shear loops; prismatic loops; 77 NANOSCIENCE AND NANOTECHNOLOGY