Is stress concentration relevant for nanocrystalline metals?

Area of Science:

• Materials Science
• Mechanical Engineering
• Nanotechnology

Background:

• Classical fracture mechanics and strain gradient plasticity predict stress concentration at notch tips.
• Ductile materials exhibit some relaxation of this stress concentration.
• Nanocrystalline materials present unique mechanical behaviors due to their small grain sizes.

Purpose of the Study:

• To experimentally investigate stress concentration at notch tips in nanocrystalline aluminum.
• To identify the underlying mechanisms responsible for observed stress homogenization.
• To compare experimental findings with theoretical predictions.

Main Methods:

• In situ uniaxial tension tests on freestanding, single edge notched aluminum specimens (80 nm thick, 50 nm average grain size) inside a transmission electron microscope.
• Molecular dynamics simulations on scaled-down samples to explore mechanisms.
• Analysis of electron diffraction patterns to assess stress distribution.

Main Results:

• Experimental evidence showed an immeasurable amount of stress concentration at the notch tip, despite theoretical predictions of a stress concentration factor as high as 8.
• Molecular dynamics simulations revealed extensive grain rotation and grain boundary diffusion, including Ashby-Verrall grain switching, at the notch tip.
• These microstructural dynamics were observed to effectively relieve stress concentration.

Conclusions:

• Nanocrystalline metals can exhibit extreme stress homogenization at notch tips.
• Grain realignment and rotation, driven by grain boundary diffusion, are critical mechanisms for accommodating strain and neutralizing stress concentration in the absence of dislocations.
• This suggests a significant deviation from classical mechanical theories in nanocrystalline materials.