Nanoindentation and incipient plasticity
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Nanoindentation and incipient plasticity E. B. Tadmor,a) R. Miller,b) and R. Phillipsc) Division of Engineering, Brown University, Providence, Rhode Island 02912
M. Ortiz Department of Aeronautics, California Institute of Technology, Pasadena, California 91125 (Received 19 August 1998; accepted 1 March 1999)
This paper presents a large-scale atomic resolution simulation of nanoindentation into a thin aluminum film using the recently introduced quasicontinuum method. The purpose of the simulation is to study the initial stages of plastic deformation under the action of an indenter. Two different crystallographic orientations of the film and two different indenter geometries (a rectangular prism and a cylinder) are studied. We obtain both macroscopic load versus indentation depth curves, as well as microscopic quantities, such as the Peierls stress and density of geometrically necessary dislocations beneath the indenter. In addition, we obtain detailed information regarding the atomistic mechanisms responsible for the macroscopic curves. A strong dependence on geometry and orientation is observed. Two different microscopic mechanisms are observed to accommodate the applied loading: (i) nucleation and subsequent propagation into the bulk of edge dislocation dipoles and (ii) deformation twinning.
I. INTRODUCTION
As mechanical systems continue to decrease in size and begin to approach atomic length scales, it is becoming important to develop experimental and corresponding theoretical tools to characterize material properties at these scales. One such experimental technique which has become popular due to its relative simplicity is nanoindentation. In this procedure an indenter with dimensions of the order of tens of nanometers is pressed into the surface of a solid. Nanoindentation has now become a standard technique for evaluating the mechanical properties of thin films.1 It can also be a useful tool for studying the onset of plastic flow in small volumes, a phenomenon which can play a significant role in macroscopic deformation processes such as adhesion, friction, and fracture.2 The nanoindentation test is basically an extension of traditional hardness and microhardness tests to very small scales. The classical tests offer a reasonably unambiguous measure of the hardness or mean pressure beneath the indenter for a given load which can then be related to the yield strength of the material through semiempirical relations.3,4 The assumption here is that a large plastic region forms beneath the indenter which can be treated approximately through plastic slip line theories a)
Present address: Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, 32000 Haifa, Israel. b) Present address: Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5A9, Canada. c) Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 6, Jun 1999
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