In 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 lattice1,2,3,4, a trend also observed in idealized molecular dynamics simulations5,6,7,8,9. 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 first 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 load–displacement 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 m−2 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 material5,6,7,8,9. Moreover, our results in entirety are at odds with the prevalent notion that the first obvious displacement excursion in a nanoindentation test is indicative of the onset of plastic deformation.
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The authors acknowledge that the research was supported in part by a US Department of Energy SBIR grant (DE-FG02-04ER83979) awarded to Hysitron, which does not constitute an endorsement by DOE of the views expressed in the article. This work was also supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231.
The authors declare no competing financial interests.
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Minor, A., Syed Asif, S., Shan, Z. et al. A new view of the onset of plasticity during the nanoindentation of aluminium. Nature Mater 5, 697–702 (2006). https://doi.org/10.1038/nmat1714
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