Spherical indentation of copper: Crystal plasticity vs experiment

Alexandra J Cackett,Chris D Hardie,Joven J.H Lim,Edmund Tarleton

doi:10.1016/j.mtla.2019.100368

Alexandra J Cackett, Chris D Hardie + Show 2 more

Open Access

https://doi.org/10.1016/j.mtla.2019.100368

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Abstract

We analyse spherical nanoindentation of single crystal copper using two different indenter tips of radii 7.4 and 27 µm. The surface deformation surrounding the indents was measured using atomic force microscopy and the elastic rotation fields under the indents were measured using electron diffraction and transmission techniques. Using the measured load-displacement, surface relief, plastic zone size, and elastic rotation field removed the ambiguity in the optimal slip and hardening law parameters in a three parameter strain gradient crystal plasticity model. In addition to geometrically necessary dislocations, other hardening mechanisms were found to contribute to the size effect.

Highlights

Nanoindentation is a popular technique to measure the mechanical properties of materials, especially for thin films [1] or where sample size is limited [2]
A small pop-in event can be seen in the R = 7.4 μm indent experimental data, where a sudden increase in displacement at constant load occurs due to a plastic strain burst, indicating a low initial dislocation source density [32]
The elastic zone size induced by the larger tip extends much further at a given strain, dislocation source activation is likely to occur at a lower indentation strain/displacement, there is no influence of source activation in the larger tip [33]

Summary

Introduction

Nanoindentation is a popular technique to measure the mechanical properties of materials, especially for thin films [1] or where sample size is limited [2]. Initial contact using spherical indenters is usually elastic, transitioning to elastic-plastic and fully plastic as the indent progresses. This provides the ability to measure the full stress-strain response and observe a range of material behaviour that is inaccessible with other tip geometries. From these curves it is possible to infer conventional properties such as yield stress and work hardening with correlation to uniaxial tensile test results [5,6]. The spherical geometry lends itself to modelling since it avoids the high strains (and numerical instabilities) associated with pyramidal tips and enables the reduction of the model by use of symmetry boundary conditions

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