A new view of the onset of plasticity during the nanoindentation of aluminium

LETTERSA new view of the onset of plasticity during

the nanoindentation of aluminiumANDREW M. MINOR1,S.A.SYEDASIF2, ZHIWEI SHAN1,ERICA.STACH3, EDWARD CYRANKOWSKI2,

THOMAS J. WYROBEK2 AND ODEN L. WARREN2*1National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA2Hysitron Incorporated, 10025 Valley View Road, Minneapolis, Minnesota 55344, USA3School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA

*e-mail: [email protected] online: 13 August 2006; doi:10.1038/nmat1714In nanoscale contact experiments, it is generally believed

that the shear stress at the onset of plasticity can approach

the theoretical shear strength of an ideal, defect-free

lattice14, a trend also observed in idealized molecular dynamics

simulations59. Here we report direct evidence that plasticity in a

dislocation-free volume of polycrystalline aluminium can begin

at very small forces, remarkably, even before the rst sustained

rise in repulsive force. However, the shear stresses associated

with these very small forces do approach the theoretical shear

strength of aluminium (2.2 GPa). Our observations entail

correlating quantitative loaddisplacement measurements with

individual video frames acquired during in situ nanoindentation

experiments in a transmission electron microscope. We also

report direct evidence that a submicrometre grain of aluminium

plastically deformed by nanoindentation to a dislocation density

of 1014 m2 is also capable of supporting shear stresses close

to the theoretical shear strength. This result is contrary to

earlier assumptions that a dislocation-free volume is necessary

to achieve shear stresses near the theoretical shear strength of

the material59. Moreover, our results in entirety are at odds

with the prevalent notion that the rst obvious displacement

excursion in a nanoindentation test is indicative of the onset of

plastic deformation.The yield strength of a material is one of the most fundamental

concepts in materials science, and is frequently used in designing

materials for engineering applications. However, yield strength

is not an inherent material property, and depends on the

internal structure of the material and the loading conditions.

Conventionally, this strength is dened by the point at which the

material response deviates signicantly from elastic deformation

under applied load10. In bulk materials, because of their high

concentration of defects, a simple numerical value for yield

strength often suces. However, matters are greatly complicated

in nanoscale materials, as dierences in specic defect distribution

or even the complete lack of pre-existing dislocations can greatly

alter the...