Indentation crystal plasticity: experiments and multiscale simulations
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1224-GG02-05
Indentation crystal plasticity: experiments and multiscale simulations Hyung-Jun Chang , Marc Verdier, Marc Fivel Université de Grenoble, Lab. SIMaP-CNRS, BP 75, St Martin d'Hères ,F-38402 Cedex,France
ABSTRACT This work aims at a quantitative simulation of instrumented indentation test based on physics of crystal plasticity. Indentation loading is associated with a complex 3D deformation path: it can be viewed as an ideal benchmark to test various crystal plasticity assumptions. For large scale indentation (micron size), a 3D numerical simulation using finite element crystal plasticity (FEM) is setup and quantitatively compared to experimental results using critical constraints: the load/stiffness-displacement curves and the surface displacements. Various set of parameters obtained from Dislocation Dynamics (DD) are used. A comparison with experiments shows the dominant effect of initial dislocation density and slip system interactions. For smaller depth (maximum 100 nm), Dislocation Dynamics coupled simulations to FEM are setup. Since this approach does not provide defect nucleation rules, several strategies are implemented and tested: fitting to Molecular Dynamics (MD) load-depth curve for spherical tip, or automatic generation of deformation accommodating dislocation (GND) for conical tip geometry for example. In this framework, size effects show up in the modification of the dislocation structures with depth through critical expansion of dislocation loops and junction formation.
INTRODUCTION Mechanics of indentation display a very intense research activity, both in experimental and modeling approaches [1]. With the development of instrumented indentation and scanning probe microscopies, it has become a major tool for small scale mechanical testing (in particular thin film technologies). For metallic single crystal, particularly Cu, extracted mechanical properties from load-depth recordings exhibit several length scale effects (at room temperature, or T/Tmelting ~1/3 for fcc metals): in the sub-micron depth range (typically sub-200 nm with a sharp apex tip with tip radius around 50 nm, say for a Berkovich tip geometry), the load-depth curves show pop-in phenomena such as displacement bursts in load control mode of operation, along with characteristic 3/2 exponent “elastic Hertzian-like” power law segments [2-3]. In the absence of oxide or brittle coating layers, a required condition for such plastic dynamic instabilities is that the indented volume of the crystal be relatively free of structural defects such as dislocations [4]. Otherwise load-depth curves have a continuous behavior, with the power law exponent evolving towards an expected value of 2 for a conical indenter deforming an elastoplastic medium. However, this regime which extends to depths of several tens of microns, the hardness as defined by the load divided by the area of contact under load is experimentally not constant (the so called Indentation Size Effect ISE). In terms of defect microstructure accompanying the plastic flow in
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