Atomic-Scale Study of Plastic-Yield Criterion in Nanocrystalline Cu at High Strain Rates
NORTH CAROLINA STATE UNIV AT RALEIGH DEPT OF MECHANICAL AND AEROSPACE ENGINEERING
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Large-scale molecular dynamics MD simulations are used to understand the macroscopic yield behavior of nanocrystalline Cu with an average grain size of 6 nm at high strain rates. The MD simulations at strain rates varying from 10exp 9sec to 8 x 9 10exp9sec suggest an asymmetry in the flow stress values in tension and compression, with the nanocrystalline metal being stronger in compression than in tension. The tension-compression strength asymmetry is very small at 10exp 9sec, but increases with increasing strain rate. The calculated yield stresses and flow stresses under combined biaxial loading conditions X-Y gives a locus of points that can be described with a traditional ellipse. An asymmetry parameter is introduced that allows for the incorporation of the small tension-compression asymmetry. The biaxial yield surface X-Y is calculated for different values of stress in the Z direction, the superposition of which gives a full threedimensional 3-D yield surface. The 3-D yield surface shows a cylinder that is symmetric around the hydrostatic axis. These results suggest that a von Mises-type yield criterion can be used to understand the macroscopic deformation behavior of nanocrystalline Cu with a grain size in the inverse Hall-Petch regime at high strain rates.
- Physical Chemistry
- Properties of Metals and Alloys
- Test Facilities, Equipment and Methods
- Atomic and Molecular Physics and Spectroscopy